haskell-mode.texi

\input texinfo          @c -*-texinfo-*-
@c %**start of header
@setfilename haskell-mode.info
@documentencoding UTF-8
@settitle Haskell Mode 16.1-git
@c %**end of header

@c Macro used to mark references of defcustom variables
@macro defcustom{var}
@vrindex \var\
@code{\var\}
@end macro

@dircategory Emacs
@direntry
* Haskell Mode: (haskell-mode).             Haskell Development Environment for Emacs(en)
@end direntry

@copying
This manual is for Haskell mode, version 16.1-git

Copyright @copyright{} 2013-2017 Haskell Mode contributors.

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the @uref{http://www.gnu.org/licenses/fdl.html,GNU
Free Documentation License}, Version 1.3 or any later version published
by the Free Software Foundation; with no Invariant Sections, no
Front-Cover Texts and no Back-Cover Texts.
@end quotation
@end copying

@iftex
@titlepage
@title Haskell Mode
@subtitle Haskell Development Environment for Emacs

@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@contents
@end iftex

@ifnottex
@node Top
@top Haskell Mode

Haskell Mode is an Haskell development Environment for GNU Emacs version
24.3 or later.  It provides syntax-based indentation, font locking,
editing cabal files, and supports running an inferior Haskell
interpreter (e.g. GHCi).

@end ifnottex

@menu
* Introduction::                         An introduction to Haskell Mode
* Installation::                         How to get started
* Editing Haskell Code::                 How to edit code
* Syntax highlighting::                  Haskell Mode has colors
* Completion support::                   Autocomplete
* Unicode support::                      How to use Unicode
* Indentation::                          Notes about indentation
* External indentation::                 Other ways to indent code
* Autoformating::                        Using external formatters
* Module templates::                     Module templates
* Declaration scanning::                 How to navigate in a source file
* Compilation::                          How to compile
* Interactive Haskell::                  How to interact with GHCi
* Editing Cabal files::                  Cabal support
* Browsing Haddocks::                    Using @code{w3m} to browse documentation
* Spell checking strings and comments::  Using @code{flyspell-prog-mode}
* Aligning code::                        Aligning code using @code{align-regexp}
* Rectangular commands::                 Manage indentation manually
* REPL::                                 GHCi REPL
* Collapsing Haskell code::              View more code on screen
* Getting Help and Reporting Bugs::      How to improve Haskell Mode
* Concept index::                        Index of Haskell Mode concepts
* Function index::                       Index of commands
* Variable index::                       Index of options and types
@end menu

@ifhtml
@insertcopying
@end ifhtml


@node Introduction
@chapter Introduction

@dfn{Haskell Mode} is a major mode providing a convenient environment
for editing @uref{http://www.haskell.org,Haskell} programs.

Some of its major features are:

@itemize
@item
Syntax highlighting (font lock),
@item
automatic semi-intelligent indentation,
@item
on-the-fly documentation,
@item
interaction with inferior GHCi/Hugs instance,
@item
project building with cabal and stack
@item
scanning declarations and placing them in a menu.
@end itemize

The name Haskell Mode refers to the whole collection of modules in this
package. There is specifically a file @file{haskell-mode.el} which
defines a major mode called @code{haskell-mode}. Generally, in this
documentation they will be distinguished by normal font and title case
(Haskell Mode) and code font (@code{haskell-mode}).

@section History

@code{haskell-mode} has a long history. It goes all the way back
to 1992. Since then, it has received many contributions in many
forms. Some design choices that remain in haskell-mode today are
historical. Some modules are outdated or no longer used, or are used
by a few people.

Historically there hasn't been a single individual or set of
individuals directing the package's architecture for a long period of
time, rather, patches and new modules were accepted in liberally and
we are left with a box full of interesting toys that may or may not
work.

As of 2016 Haskell Mode is coordinated using Github at
@uref{https://github.com/haskell/haskell-mode}.

@node Installation
@chapter Installation

Haskell Mode is distributed as a package in
@uref{https://melpa.org,MELPA repository}. To use MELPA as Emacs package
archive do the following:

@enumerate
@item
Customize @code{package-archives} using
@example
M-x customize-option RET package-archives
@end example
@item
Use @kbd{INS} to add new archive, use:
@example
Archive name:          melpa-stable
URL or directory name: http://stable.melpa.org/packages/
@end example
@item
Fetch new packages using:
@example
M-x package-refresh-contents
@end example
@item
Install Haskell Mode using:
@example
M-x package-install RET haskell-mode RET
@end example
@end enumerate

Voila! @code{haskell-mode} is installed! You should be able to edit Haskell
source code in color now.

The above steps should result in the following snippet in your @file{.emacs}:

@lisp
(require 'package)
(custom-set-variables
 ;; custom-set-variables was added by Custom.
 ;; If you edit it by hand, you could mess it up, so be careful.
 ;; Your init file should contain only one such instance.
 ;; If there is more than one, they won't work right.
 '(package-archives
   (quote
    (("gnu" . "http://elpa.gnu.org/packages/")
     ("melpa-stable" . "http://stable.melpa.org/packages/")))))
@end lisp


Haskell Mode supports GNU Emacs versions 24.3+, including 25
(snapshot).

Haskell Mode is available from
@uref{http://stable.melpa.org,melpa-stable (releases)} and
@uref{http://melpa.org, melpa (git snapshots)}.

Other means of obtaining @code{haskell-mode} include
@uref{https://github.com/dimitri/el-get, el-get},
@uref{https://github.com/bbatsov/prelude, Emacs Prelude} and
@uref{https://packages.debian.org/search?keywords=haskell-mode, Debian package}.

Last version of @code{haskell-mode} that supported Emacs 23, 24.1, and 24.2 is
@code{haskell-mode} 13.16 available at
@uref{https://github.com/haskell/haskell-mode/releases/tag/v13.16}.

@section Customizing

@cindex customizing
Most of Haskell Mode's settings are configurable via customizable
variables (@pxref{Easy Customization,,,emacs}, for details). You can use
@kbd{M-x customize-group @key{RET} haskell} to browse the @code{haskell}
customization sub-tree.

@vindex haskell-mode-hook
One of the important setting you should customize is the
@code{haskell-mode-hook} variable (@pxref{Hooks,,,emacs}) which gets run
right after the @code{haskell-mode} major mode is initialized for a
buffer. You can customize @code{haskell-mode-hook} by

@example
M-x customize-variable RET haskell-mode-hook
@end example

There you can enable or disable a couple of predefined options or add
any function to the list.

@node Editing Haskell Code
@chapter Editing Haskell Code

@findex haskell-mode
@cindex @code{haskell-mode}

Haskell Mode as one of its components provides a major mode for editing
Haskell source code called @code{haskell-mode}, which gave the name to
the whole project. There is a derived mode provided called
@code{literate-haskell-mode} that support Literate Haskell source code
both in Bird and in Latex forms.

Haskell Mode supports files with the following extensions:

@table @file
@item .hs
official file extension for Haskell files. Haskell Mode out of the box
supports most of GHC extensions.
@item .lhs
official file extension for Literate Haskell files. Both Bird and Latex
styles are supported.
@item .hsc
Haskell interfaces to C code used by @uref{http://www.haskell.org/ghc/docs/latest/html/users_guide/hsc2hs.html,hsc2hs}
pre-processor.
@item .cpphs
Haskell source with CPP pragmas used with @uref{http://projects.haskell.org/cpphs,cpphs}
pre-processor.
@item .c2hs
Haskell FFI bindings to C libraries used with @uref{https://github.com/haskell/c2hs,c2hs}
pre-processor.
@end table

Haskell Mode offers many productivity tools described in following
chapters in this manual.

@section Managing imports

There are a few functions for managing imports.

@subsection Jump to imports

To jump to your import list, run

    @kbd{M-x} @code{haskell-navigate-imports}

It's nicer to have a keybinding to do this, for example:

@lisp
(define-key haskell-mode-map (kbd "<f8>") 'haskell-navigate-imports)
@end lisp

You can hit it repeatedly to jump between groups of imports. It will
cycle.

@subsection Format imports

To generally format (sort, align) your imports, you can run

    @kbd{M-x} @code{haskell-mode-format-imports}

Or @kbd{C-c C-,}.

@subsection Sort imports

To just sort imports, jump to an import section and run

    @kbd{M-x} @code{haskell-sort-imports}

@subsection Align imports

To just align imports, jump to an import section and run

    @kbd{M-x} @code{haskell-align-imports}

@subsection stylish-haskell

As an alternative to the elisp functions described above, haskell-mode
can use the program
@url{http://hackage.haskell.org/package/stylish-haskell,
stylish-haskell} to format imports.  You can set this behavior by
typing: @kbd{M-x} @code{customize-variable} @kbd{RET}
@code{haskell-stylish-on-save}.  You can install
@code{stylish-haskell} by running @code{stack install
stylish-haskell}, or if you have not installed @code{stack},
@code{cabal install stylish-haskell}.

@section Haskell Tags

@code{haskell-mode} can generate tags when saving source files.  To
generate tags @code{haskell-mode} uses external program —
@url{https://github.com/MarcWeber/hasktags, Hasktags}
(@url{https://wiki.haskell.org/Tags, wiki-article}).  To turn on tags
generatation customize or set to @code{t} @code{haskell-tags-on-save}
variable.  Also, you may find useful to revert tags tables
automatically, this can be done by customizing
@code{tags-revert-without-query} variable (either globally or for
Haskell buffers only).

@section Profiling and Debugging support

When profiling code with GHC, it is often useful to add
@uref{https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/profiling.html#cost-centres,
cost centres} by hand.  These allow finer-grained information about
program behavior.  @code{haskell-mode} provides the function
@code{haskell-mode-toggle-scc-at-point} to make this more convenient.
It will remove an SCC annotation at point if one is present, or add
one if point is over whitespace.  By default it is bound to @kbd{C-c C-s}.

@node Syntax highlighting
@chapter Syntax highlighting

@code{haskell-mode} supports @dfn{syntax highlighting} via Emacs' Font
Lock minor mode which should be enabled by default in current
Emacsen. @xref{Font Lock,,,emacs}, for more information on how to
control @code{font-lock-mode}.

@ifhtml
@image{anim/font-lock}
@end ifhtml

Syntax highlighting facilities parse strings and string escape sequences
and are able to highlight unrecognized constructs.

@ifhtml
@image{anim/string-escape-highlight}
@end ifhtml

Haskell Mode shows keywords, identifiers, operators, constructors and
types in different colors.

@ifhtml
@image{anim/font-lock-types}
@end ifhtml

There is also support to use mode-specific syntax highlighing for
quasiquotes.

@ifhtml
@image{anim/font-lock-quasi-quotes}
@end ifhtml

At this point quasi quotes for HTML, XML, shell scripts, Hamlet
templates and SQL are supported out of the box. Customize
@code{haskell-font-lock-quasi-quote-modes} to make sure your quoters are
supported.

The following customization variables are responsible for faces applied:

@itemize @code
@item @defcustom{haskell-keyword-face}: for keywords
@item @defcustom{haskell-type-face}: for type names and type class names
@item @defcustom{haskell-constructor-face}: for constructors
@item @defcustom{haskell-definition-face}: function and operator name at the definition place
@item @defcustom{haskell-operator-face}: operators
@item @defcustom{haskell-pragma-face}: GHC pragmas
@item @defcustom{haskell-literate-comment-face}: literate comments
@item @defcustom{haskell-quasi-quote-face}: quasi quotes unless using mode specific highlighting
@item @defcustom{haskell-c2hs-hook-pair-face}: c2hs hooks
@item @defcustom{haskell-c2hs-hook-name-face}: c2hs hook names
@end itemize

All the above are available for customization.

GHC quasi quote syntax is ambiguous with list comprehension therefore
syntax highlighting might get confused with situations like these:

@example
result = [html| html <- htmlList]
result = [html| <html><body>...</body></html> |]
@end example

Please use spaces around a list comprehension variable to make this
unambiguous. Any of the following will work:

@example
result = [ html| html <- htmlList]
result = [html | html <- htmlList]
@end example

GHC's ambiguity is an accident of the past but it is unlikely to be
fixed due to backward compatibility baggage.

@node Completion support
@chapter Completion support

@code{haskell-mode} can complete symbols, pragma directives, language
extensions, and language keywords out-of-box.  @code{haskell-mode}
completes identifiers (symbols) using tags (see ``Tags''), however you
can get more precise completions with @code{haskell-interactive-mode}.
In interactive mode completion candidates are produced by querying
GHCi REPL.

@ifhtml
@image{anim/company-mode-language-pragma}
@end ifhtml

If @code{haskell-interactive-mode} is enabled and working Haskell mode
provides completions for import statements taking into account
currently loaded and available packages.  Also it completes symbols
querying REPL with @code{:complete} command, hence completion
candidate list also includes symbols from imported modules.

@ifhtml
@image{anim/company-mode-import-statement}
@end ifhtml

Unfortunatelly, it is not possible to provide candidates for
identifiers defined locally in @code{let} and @code{where} blocks even
in interactive mode.  But if you're using
@url{http://company-mode.github.io/, company-mode} you can override
@code{company-backends} variable for Haskell buffers to combine
completion candidates from completion-at-point function
(@code{company-capf} backend) and dynamic abbrevs.
@code{company-mode} provides special backend for dabbrev code
completions, namely @code{company-dabbrev-code}.  To combine
completions from diffrent backends you can create grouped backends, it
is very easy — a grouped backend is just a list of backends, for
example:

@lisp
(add-hook 'haskell-mode-hook
          (lambda ()
            (set (make-local-variable 'company-backends)
                 (append '((company-capf company-dabbrev-code))
                         company-backends))))
@end lisp

If you use a GHCi version prior to 8.0.1 you might want to set
@code{haskell-completions-complete-operators} to @code{nil}, if you
experience major slowdown while trying to complete after an Haskell
operator (See @url{https://ghc.haskell.org/trac/ghc/ticket/10576, GHC-Bug 10576}).

@node Unicode support
@chapter Unicode support

@cindex Unicode

See the Haskell Wiki's entry on
@uref{http://www.haskell.org/haskellwiki/Unicode-symbols, Unicode
Symbols} for general information about Unicode support in Haskell.

As Emacs supports editing files containing Unicode out of the box, so
does Haskell Mode. As an add-on, Haskell Mode includes the
@code{haskell-unicode} input method which allows you to easily type a
number of Unicode symbols that are useful when writing Haskell code;
@xref{Input Methods,,,emacs}, for more details.

To automatically enable the @code{haskell-unicode} input method in
haskell-mode buffers use @kbd{M-x customize-variable @key{RET}
haskell-mode-hook} or put the following code in your @file{.emacs} file:

@lisp
(add-hook 'haskell-mode-hook 'turn-on-haskell-unicode-input-method)
@end lisp

@noindent
To temporarily enable this input method for a single buffer you can use
@kbd{M-x turn-on-haskell-unicode-input-method}.

When the @code{haskell-unicode} input method is active, you can simply
type @samp{->} and it is immediately replaced with @samp{→}. Use
@kbd{C-\} to toggle the input method. To see a table of all key
sequences use @kbd{M-x describe-input-method @key{RET}
haskell-unicode}. A sequence like @samp{<=} is ambiguous and can mean
either @samp{⇐} or @samp{≤}. Typing it presents you with a choice. Type
@kbd{1} or @kbd{2} to select an option or keep typing to use the default
option.

Currently defined sequences are listed in the following table:

@multitable @columnfractions .08 .15 .08 .15 .08 .15 .08 .15
@headitem Sequence @tab Unicode @tab Sequence @tab Unicode @tab Sequence @tab Unicode @tab Sequence @tab Unicode
@item alpha
@tab α
@tab Alpha
@tab Α
@tab beta
@tab β
@tab Beta
@tab Β
@item gamma
@tab γ
@tab Gamma
@tab Γ
@tab delta
@tab δ
@tab Delta
@tab Δ
@item epsilon
@tab ε
@tab Epsilon
@tab Ε
@tab zeta
@tab ζ
@tab Zeta
@tab Ζ
@item eta
@tab η
@tab Eta
@tab Η
@tab theta
@tab θ
@tab Theta
@tab Θ
@item iota
@tab ι
@tab Iota
@tab Ι
@tab kappa
@tab κ
@tab Kappa
@tab Κ
@item lambda
@tab λ
@tab Lambda
@tab Λ
@tab lamda
@tab λ
@tab Lamda
@tab Λ
@item mu
@tab μ
@tab Mu
@tab Μ
@tab nu
@tab ν
@tab Nu
@tab Ν
@item xi
@tab ξ
@tab Xi
@tab Ξ
@tab omicron
@tab ο
@tab Omicron
@tab Ο
@item pi
@tab π
@tab Pi
@tab Π
@tab rho
@tab ρ
@tab Rho
@tab Ρ
@item sigma
@tab σ
@tab Sigma
@tab Σ
@tab tau
@tab τ
@tab Tau
@tab Τ
@item upsilon
@tab υ
@tab Upsilon
@tab Υ
@tab phi
@tab φ
@tab Phi
@tab Φ
@item chi
@tab χ
@tab Chi
@tab Χ
@tab psi
@tab ψ
@tab Psi
@tab Ψ
@item omega
@tab ω
@tab Omega
@tab Ω
@tab digamma
@tab ϝ
@tab Digamma
@tab Ϝ
@item san
@tab ϻ
@tab San
@tab Ϻ
@tab qoppa
@tab ϙ
@tab Qoppa
@tab Ϙ
@item sampi
@tab ϡ
@tab Sampi
@tab Ϡ
@tab stigma
@tab ϛ
@tab Stigma
@tab Ϛ
@item heta
@tab ͱ
@tab Heta
@tab Ͱ
@tab sho
@tab ϸ
@tab Sho
@tab Ϸ
@item |A|
@tab 𝔸
@tab |B|
@tab 𝔹
@tab |C|
@tab ℂ
@tab |D|
@tab 𝔻
@item |E|
@tab 𝔼
@tab |F|
@tab 𝔽
@tab |G|
@tab 𝔾
@tab |H|
@tab ℍ
@item |I|
@tab 𝕀
@tab |J|
@tab 𝕁
@tab |K|
@tab 𝕂
@tab |L|
@tab 𝕃
@item |M|
@tab 𝕄
@tab |N|
@tab ℕ
@tab |O|
@tab 𝕆
@tab |P|
@tab ℙ
@item |Q|
@tab ℚ
@tab |R|
@tab ℝ
@tab |S|
@tab 𝕊
@tab |T|
@tab 𝕋
@item |U|
@tab 𝕌
@tab |V|
@tab 𝕍
@tab |W|
@tab 𝕎
@tab |X|
@tab 𝕏
@item |Y|
@tab 𝕐
@tab |Z|
@tab ℤ
@tab |gamma|
@tab ℽ
@tab |Gamma|
@tab ℾ
@item |pi|
@tab ℼ
@tab |Pi|
@tab ℿ
@tab ::
@tab ∷
@tab forall
@tab ∀
@item exists
@tab ∃
@tab ->
@tab →
@tab <-
@tab ←
@tab =>
@tab ⇒
@item ~>
@tab ⇝
@tab <~
@tab ⇜
@tab &&
@tab ∧
@tab ||
@tab ∨
@item ==
@tab ≡
@tab /=
@tab ≢, ≠
@tab <=
@tab ≤
@tab >=
@tab ≥
@item /<
@tab ≮
@tab />
@tab ≯
@tab  *
@tab  ⋅
@tab elem
@tab ∈
@item notElem
@tab ∉
@tab member
@tab ∈
@tab notMember
@tab ∉
@tab union
@tab ∪
@item intersection
@tab ∩
@tab isSubsetOf
@tab ⊆
@tab isProperSubsetOf
@tab ⊂
@tab <<<
@tab ⋘
@item >>>
@tab ⋙
@tab <|
@tab ⊲
@tab |>
@tab ⊳
@tab ><
@tab ⋈
@item mappend
@tab ⊕
@tab  .
@tab  ∘
@tab undefined
@tab ⊥
@tab :=
@tab ≔
@item =:
@tab ≕
@tab =def
@tab ≝
@tab =?
@tab ≟
@tab ...
@tab …
@item _0
@tab ₀
@tab _1
@tab ₁
@tab _2
@tab ₂
@tab _3
@tab ₃
@item _4
@tab ₄
@tab _5
@tab ₅
@tab _6
@tab ₆
@tab _7
@tab ₇
@item _8
@tab ₈
@tab _9
@tab ₉
@tab ^0
@tab ⁰
@tab ^1
@tab ¹
@item ^2
@tab ²
@tab ^3
@tab ³
@tab ^4
@tab ⁴
@tab ^5
@tab ⁵
@item ^6
@tab ⁶
@tab ^7
@tab ⁷
@tab ^8
@tab ⁸
@tab ^9
@tab ⁹
@end multitable

If you don't like the highlighting of partially matching tokens you can
turn it off by setting @code{input-method-highlight-flag} to @code{nil}
via @kbd{M-x customize-variable}.

@node Indentation
@chapter Indentation

@cindex indentation
@cindex layout rule
@cindex off-side rule

In Haskell, code indentation has semantic meaning as it defines the
block structure. Haskell also supports braces and semicolons
notation for conveying the block structure. However, most Haskell
programs written by humans use indentation for block structuring.

Haskell Mode ships with two indentation modes:

@itemize
@item @code{haskell-indentation-mode} (default).

This is a semi-intelligent indentation mode doing a decent job at
recognizing Haskell syntactical constructs.  It is based on a recursive
descent Haskell parser. @kbd{TAB} selects the next potential indentation
position, @kbd{S-TAB} selects the previous one. If a block is selected
you can use @kbd{TAB} to indent the block more and @kbd{S-TAB} to indent
the block less.

When @code{electric-indent-mode} is enabled or the variable
@code{haskell-indentation-electric-flag} is non-nil, the insertion of
some characters (by default @kbd{,} @kbd{;} @kbd{)} @kbd{@}} @kbd{]})
may trigger auto reindentation under appropriate conditions. See the
documentation of @code{haskell-indentation-common-electric-command} for
more details.

@item @code{haskell-indent-mode} (optional).

This is a semi-intelligent indentation mode doing a decent job at
recognizing Haskell syntactical constructs.  It is based on a decision
table. Sadly it is no longer developed and does not recognize newer
Haskell syntax. @kbd{TAB} cycles through all available indentation
positions.

To use @code{haskell-indent-mode}, add this to your @file{~/.emacs}
file:

@lisp
(add-hook 'haskell-mode-hook 'turn-on-haskell-indent)
@end lisp

Note that @code{turn-on-haskell-indent} will disable
@code{haskell-indentation-mode}.

@end itemize

For general information about indentation support in GNU Emacs,
@pxref{Indentation,,,emacs}.

@section Rectangle Commands

@cindex rectangle
@cindex CUA mode

GNU Emacs provides so-called @dfn{rectangle commands} which operate on
rectangular areas of text, which are particularly useful for languages
with a layout rule such as Haskell. @xref{Rectangles,,,emacs}, to learn
more about rectangle commands.

Moreover, CUA mode (@pxref{CUA Bindings,,,emacs}) provides enhanced
rectangle support with visible rectangle highlighting. When CUA mode is
active, you can initiate a rectangle selection by @kbd{C-RET} and extend
it simply by movement commands. You don't have to enable full CUA mode
to benefit from these enhanced rectangle commands; you can activate CUA
selection mode (without redefining @kbd{C-x},@kbd{C-c},@kbd{C-v}, and
@kbd{C-z}) by calling @kbd{M-x cua-selection-mode} (or adding
@code{(cua-selection-mode nil)} to your @code{haskell-mode-hook}).

@section Region indent is a no-op

There is a @code{indent-region} function that supposedly could be used
to indent code region without changing its semantics. Sadly it does not
work that way because usual use case for @code{indent-region} is:

@enumerate
@item
Alter first line of code in region.
@item
Call @code{indent-region} to fix indentation for remaining lines.
@end enumerate

Note that between 1 and 2 program is already semantically broken and
knowing how to indent it preserving semantic from before step 1 would
require time travel.

To stay on the safe side @code{indent-region-function} is bound to a
no-op in @code{haskell-mode}.

@node External indentation
@chapter Other ways to indent code

@section Indentation with tabs, not spaces

Some projects require indenting code with tabs and forbid indenting it
with spaces.  For hacking on such projects, check out
@uref{https://spwhitton.name/tech/code/haskell-tab-indent,haskell-tab-indent-mode}.

@section Structured indentation

Another alternative is to install
@uref{https://github.com/chrisdone/structured-haskell-mode,structured-haskell-mode}.
which indents code by parsing the code with a full Haskell parser and
deciding where to indent based on that.

@node Autoformating
@chapter Using external formatters

You can enable @uref{https://github.com/jaspervdj/stylish-haskell,stylish-haskell} by
installing it:

@example
$ cabal install stylish-haskell
@end example

And by enabling it with a customization

@lisp
(custom-set-variables
 '(haskell-stylish-on-save t))
@end lisp

Now when you run @code{save-buffer} (or @kbd{C-x C-s}) the module will
be automatically formatted.

Alternatively, you can run the function directly on demand with
@kbd{M-x} @code{haskell-mode-stylish-buffer}.

@node Module templates
@chapter Module templates

To enable auto-insertion of module templates, enable:

@lisp
(add-hook 'haskell-mode-hook 'haskell-auto-insert-module-template)
@end lisp

When you open a file called @file{Foo.hs}, it will auto-insert

@example
-- |

module Foo where
@end example

And put your cursor in the comment section.

@node Declaration scanning
@chapter Declaration scannning

@findex haskell-decl-scan-mode
@vindex haskell-decl-scan-mode-hook

@code{haskell-decl-scan-mode} is a minor mode which performs declaration
scanning and provides @kbd{M-x imenu} support (@pxref{Imenu,,,emacs} for
more information).

For non-literate and @TeX{}-style literate scripts, the common
convention that top-level declarations start at the first column is
assumed.  For Bird-style literate scripts, the common convention that
top-level declarations start at the third column, ie. after @samp{> },
is assumed.

When @code{haskell-decl-scan-mode} is active, the standard Emacs
top-level definition movement commands (@pxref{Moving by
Defuns,,,emacs}) are enabled to operate on Haskell declarations:

@table @kbd
@item C-M-a
Move to beginning of current or preceding declaration
(@code{beginning-of-defun}).

@item C-M-e
Move to end of current or following declaration (@code{end-of-defun}).

@item C-M-h
Select whole current or following declaration (@code{mark-defun}).
@end table

Moreover, if enabled via the option
@code{haskell-decl-scan-add-to-menubar}, a menu item ``Declarations'' is
added to the menu bar listing the scanned declarations and allowing to
jump to declarations in the source buffer.

It's recommended to have font lock mode enabled (@pxref{Font
Lock,,,emacs}) as @code{haskell-decl-scan-mode} ignores text highlighted
with @code{font-lock-comment-face}.

As usual, in order to activate @code{haskell-decl-scan-mode}
automatically for Haskell buffers, add @code{haskell-decl-scan-mode}
to @code{haskell-mode-hook}:

@lisp
(add-hook 'haskell-mode-hook 'haskell-decl-scan-mode)
@end lisp

@code{haskell-decl-scan-mode} enables the use of features that build
upon @code{imenu} support such as Speedbar Frames
(@pxref{Speedbar,,,emacs}) or the global ``Which Function'' minor mode
(@pxref{Which Function,,,emacs}).

In order to enable @code{which-function-mode} for Haskell buffers you
need to add the following to your Emacs initialization:

@lisp
(eval-after-load "which-func"
  '(add-to-list 'which-func-modes 'haskell-mode))
@end lisp

@section Speedbar

Haskell-mode comes with declaration scanning support. This means that if you enable Haskell support for speedbar:

@lisp
(speedbar-add-supported-extension ".hs")
@end lisp

And open speedbar with

@code{M-x speedbar}

It gives a listing of each module and under each module:

@example
    Imports
    Instances
    Data types
    Classes
    Bindings
@end example

You will get a bar that looks like this:

@verbatim
~/Projects/ace/src/ACE/
0:<+> Types
0:[+] Combinators.hs
0:[-] Datalog.hs
1:   {-} Classes
2:      > ToTerm
1:   {-} Imports
2:      > ACE.Types.Syntax
2:      > Database.Datalog
1:   {-} Instances
2:    {+} ToTerm A
2:    {+} ToTerm Co to ToTerm Gen
2:    {+} ToTerm Intransitive to ToTerm N
2:    {+} ToTerm P
2:    {+} ToTerm Quotation to ToTerm Un
2:    {+} ToTerm V
0:[-] Html.hs
1:   {+} Imports
1:   {+} Instances
1:     > mtoMarkup
1:     > toMarkupm
1:     > wrap
0:[-] Parsers.hs
1:   {+} Imports
1:   {-} Datatypes
2:      > ACEParser
0:[+] Pretty.hs
0:[+] Tokenizer.hs
@end verbatim

The hierarchy is expandable/collapsible and each entry will jump to the
line in the right file when clicked/selected.

@node Compilation
@chapter Compilation

Haskell Mode comes with an intelligent Major mode for compilation named
@code{HsCompilation} defined in @code{haskell-compile.el}. This mode is
derived from @code{compilation-mode} defined in @code{compile.el}.

First the compilation mode tries to find the project's root directory.
This is done by searching for files such as @code{cabal.sandbox.config},
@code{stack.yaml}, and @code{something.cabal} in that order.

If you have both @code{cabal.sandbox.config} and @code{stack.yaml} files
defined, in your project's root directory, @code{cabal.sandbox.config}
takes precedence over @code{stack.yaml} and similary @code{stack.yaml}
takes precedence over @code{something.cabal}.

You can start the compilation by @kbd{M-x haskell-compile}. The intelligent
thing about this mode is that like it sets the project's root directory.
It will run the appropriate compilation command from the project's root
directory.

@multitable @columnfractions 0.25 0.25 0.50
@headitem Dominant file @tab Build command @tab Commandline Options

@item @code{cabal.sandbox.config}
@tab @code{cabal build}
@tab @code{--ghc-options=\"-ferror-spans -Wall -fforce-recomp\"}

@item @code{stack.yaml}
@tab @code{stack build}
@tab @code{--ghc-options=\"-ferror-spans -Wall\"}

@item @code{something.cabal}
@tab @code{cabal build}

@item default
@tab @code{ghc <name-of-file-in-buffer>}
@tab @code{--make -ferror-spans -Wall -fforce-recomp}
@end multitable

@section Keybindings

@multitable @columnfractions 0.3 0.7
@headitem Key binding @tab Function
@item TAB
@tab compilation-next-error

@item RET
@tab compile-goto-error

@item C-o
@tab compilation-display-error

@item SPC
@tab scroll-up-command

@item -
@tab negative-argument

@item 0 .. 9
@tab digit-argument

@item <
@tab beginning-of-buffer

@item >
@tab end-of-buffer

@item ?
@tab describe-mode

@item g
@tab recompile

@item h
@tab describe-mode

@item q
@tab quit-window

@item DEL
@tab scroll-down-command

@item S-SPC
@tab scroll-down-command

@item <backtab>
@tab compilation-previous-error

@item <follow-link>
@tab mouse-face

@item <mouse-2>
@tab compile-goto-error

@item <remap>
@tab Prefix Command

@item M-n
@tab compilation-next-error

@item M-p
@tab compilation-previous-error

@item M-@{
@tab compilation-previous-file

@item M-@}
@tab compilation-next-file

@item C-c C-c
@tab compile-goto-error

@item C-c C-f
@tab next-error-follow-minor-mode

@item C-c C-k
@tab kill-compilation

@end multitable

@section Related Defcustoms

@multitable @columnfractions 0.3 0.7
@headitem Defcustom @tab Default Value

@item @code{haskell-process-path-ghc}
@tab @code{ghc}

@item @code{haskell-process-args-ghc}
@tab @code{--make -ferror-spans -Wall -fforce-recomp}

@item @code{haskell-process-path-cabal}
@tab @code{cabal}

@item @code{haskell-process-args-cabal-build}
@tab @code{--ghc-options=\"-ferror-spans -Wall -fforce-recomp\"}

@item @code{haskell-process-path-stack}
@tab @code{stack}

@item @code{haskell-process-args-stack-build}
@tab @code{--ghc-options=\"-ferror-spans -Wall\"}
@end multitable

@section Hooks

None

@node Interactive Haskell
@chapter Interactive Haskell

An alternative mode providing a @acronym{REPL,read–eval–print loop} via
GHCi sessions is called @code{haskell-interactive-mode}, which
effectively replaces @code{inferior-haskell-mode}, but comes with a
different set of features:

@itemize
@item
Separate sessions per Cabal project @file{haskell-session.el}.
@item
A new inferior Haskell process handling code @file{haskell-process.el}.
@item
New REPL implementation similiar to SLIME/IELM
@item
Navigatable error overlays
@file{haskell-interactive-mode.el}.
@end itemize

With @code{haskell-interactive-mode}, each Haskell source buffer is
associated with at most one GHCi session, so when you call
@code{haskell-process-load-file} for a Haskell source buffer which has
no session associated yet, you're asked which GHCi session to create or
associate with.

@section Goto Error

In a Haskell source buffer associated with a GHCi session, errors that
prevent the file from loading are highlighted with
@code{haskell-error-face}.  You can move between these error lines with

@table @kbd
@item M-n
is bound to @code{haskell-goto-next-error}
@item M-p
is bound to @code{haskell-goto-prev-error}
@item C-c M-p
is bound to @code{haskell-goto-first-error}
@end table

@section Using GHCi 8+ or GHCi-ng

If you use either of the above, then you can use these functions:

@lisp
(define-key interactive-haskell-mode-map (kbd "M-.") 'haskell-mode-goto-loc)
(define-key interactive-haskell-mode-map (kbd "C-c C-t") 'haskell-mode-show-type-at)
@end lisp

You have to load the module before it works, after that it will remember
for the current GHCi session.

@section Customizing

@cindex customizing
What kind of Haskell REPL @code{haskell-interactive-mode} will start up
depends on the value of @code{haskell-process-type}. This can be one of the
symbols @code{auto}, @code{ghci}, @code{cabal-repl}, @code{cabal-new-repl}, or
@code{stack-ghci}. If it's @code{auto}, the directory contents and available
programs will be used to make a best guess at the process type. The actual
process type will then determine which variables
@code{haskell-interactive-mode} will access to determine the program to start
and its arguments:

@itemize
@item
If it's @code{ghci}, @code{haskell-process-path-ghci} and
@code{haskell-process-args-ghci} will be used.
@item
If it's @code{cabal-repl}, @code{haskell-process-path-cabal} and
@code{haskell-process-args-cabal-repl}.
@item
If it's @code{cabal-new-repl}, @code{haskell-process-path-cabal} and
@code{haskell-process-args-cabal-new-repl}.
@item
If it's @code{stack-ghci}, @code{haskell-process-path-stack} and
@code{haskell-process-args-stack-ghci} will be used.
@end itemize

With each of these pairs, the the @code{haskell-process-path-...}
variable needs to be a string specifying the program path, or a list of
strings where the first element is the program path and the rest are
initial arguments.  The @code{haskell-process-args-...} is a list of
strings specifying (further) command-line arguments.

@vindex haskell-process-type
@vindex haskell-process-path-ghci
@vindex haskell-process-path-cabal
@vindex haskell-process-path-stack
@vindex haskell-process-args-ghci
@vindex haskell-process-args-cabal-repl
@vindex haskell-process-args-cabal-new-repl
@vindex haskell-process-args-stack-ghci

@section Haskell Interactive Mode Setup

The most straight-forward way to get setup with Interactive Mode is to
bind the right keybindings and set some customizations. This page
contains a good base setup.

To enable the minor mode which activates keybindings associated with interactive mode, use:

@lisp
(require 'haskell-interactive-mode)
(require 'haskell-process)
(add-hook 'haskell-mode-hook 'interactive-haskell-mode)
@end lisp

@subsection Customizations

This enables some handy and benign features.

@lisp
(custom-set-variables
  '(haskell-process-suggest-remove-import-lines t)
  '(haskell-process-auto-import-loaded-modules t)
  '(haskell-process-log t))
@end lisp


@subsection Haskell-mode bindings

This gives the basic ways to start a session. In a Haskell buffer:

@itemize
@item
Run @kbd{C-`} to make a REPL open, this will create a
session, start GHCi, and open the REPL.
@item
Or: run @kbd{C-c C-l} to load the file. This will first try to start a
session as the previous command does.
@item
Or: run any command which requires a running session. It will always
prompt to create one if there isn't one already for the current project.
@end itemize

@lisp
(define-key haskell-mode-map (kbd "C-c C-l") 'haskell-process-load-or-reload)
(define-key haskell-mode-map (kbd "C-`") 'haskell-interactive-bring)
(define-key haskell-mode-map (kbd "C-c C-t") 'haskell-process-do-type)
(define-key haskell-mode-map (kbd "C-c C-i") 'haskell-process-do-info)
(define-key haskell-mode-map (kbd "C-c C-c") 'haskell-process-cabal-build)
(define-key haskell-mode-map (kbd "C-c C-k") 'haskell-interactive-mode-clear)
(define-key haskell-mode-map (kbd "C-c c") 'haskell-process-cabal)
@end lisp


@subsection Cabal-mode bindings

The below commands pretty much match the ones above, but are handy to
have in cabal-mode, too:

@lisp
(define-key haskell-cabal-mode-map (kbd "C-`") 'haskell-interactive-bring)
(define-key haskell-cabal-mode-map (kbd "C-c C-k") 'haskell-interactive-mode-clear)
(define-key haskell-cabal-mode-map (kbd "C-c C-c") 'haskell-process-cabal-build)
(define-key haskell-cabal-mode-map (kbd "C-c c") 'haskell-process-cabal)
@end lisp

@subsection GHCi process type

By default @code{haskell-process-type} is set to @code{auto}. It is
smart enough to pick the right type based on your project structure and
installed tools, but in case something goes funky or you want to
explicitly set the process type and ignore the inferred type, you can
customize this setting by running @kbd{M-x} @code{customize-variable}
@kbd{RET} @code{haskell-process-type} @kbd{RET}, or by setting the code:

@lisp
(custom-set-variables
  '(haskell-process-type 'cabal-repl))
@end lisp

Here is a list of available process types:

@itemize
@item ghci
@item cabal-repl
@item cabal-new-repl
@item cabal-dev
@item cabal-ghci
@item stack-ghci
@end itemize

Please, check the documentation for @code{haskell-process-type} to see how
the real type is guessed, when it's set to @code{auto}.

@subsection Troubleshooting

Launching your GHCi process can fail when you're first getting setup,
depending on the type you choose. If it does fail to launch, switch to
the buffer @code{*haskell-process-log*} and see what's up. The buffer
contains a log of incoming/outgoing messages to the GHCi process.

@section Haskell Interactive Mode Tags Using GHCi

You can bind the following to use GHCi to find definitions of things:

@lisp
(define-key haskell-mode-map (kbd "M-.") 'haskell-mode-jump-to-def)
@end lisp

The one problem with this approach is that if your code doesn't compile,
GHCi doesn't give any location info. So you need to make sure your code
compiles and the modules you want to jump to are loaded byte-compiled.

Note: I think that when you restart GHCi you lose location
information, even if you have the @file{.o} and @file{.hi} files lying around. I'm not
sure. But sometimes @code{:i foo} will give @code{foo is defined in Bar} rather
than @code{foo is defined in /foo/Bar.hs:123:23}.

Alternatively, you can use tags generation, which doesn't require a
valid compile.

@subsection Tags Setup

Make sure to install @file{hasktags}.

@example
    $ cabal install hasktags
@end example

Then add the customization variable to enable tags generation on save:

@lisp
(custom-set-variables
  '(haskell-tags-on-save t))
@end lisp

And make sure @file{hasktags} is in your @code{$PATH} which Emacs can see.

@subsection Generating tags

Now, every time you run @code{save-buffer} (@kbd{C-x C-s}), there is a
hook that will run and generate Emacs @xref{Tags,,,emacs} for the whole
project directory. The resulting file will be called @file{TAGS}.

WARNING: You should be careful that your project root isn't your
home directory or something, otherwise it will traverse all the way
down and take an impossibly long time.

@subsection Jumping to tags

Bind the following keybinding:

@lisp
(define-key haskell-mode-map (kbd "M-.") 'haskell-mode-tag-find)
@end lisp

To jump to the location of the top-level identifier at point, run
@kbd{M-x} @code{haskell-mode-tag-find} or @kbd{M-.}.

@subsection Hybrid: GHCi and fallback to tags

To use GHCi first and then if that fails to fallback to tags for jumping, use:

@lisp
(define-key haskell-mode-map (kbd "M-.") 'haskell-mode-jump-to-def-or-tag)
@end lisp

@subsection Troubleshooting tags

Sometimes a @file{TAGS} file is deleted (by you or some other
process). Emacs will complain that it doesn't exist anymore. To
resolve this simply do @kbd{M-x} @code{tags-reset-tags-tables}.

@section Sessions

All commands in Haskell Interactive Mode work within a session. Consider
it like a “project” or a “solution” in popular IDEs. It tracks the root
of your project and an associated process and REPL.

@subsection Start a session

To start a session run the following steps:

@itemize
@item
Open some Cabal or Haskell file.
@item
Run @kbd{C-`} to make a REPL open, this will create a session, start
GHCi, and open the REPL.
@item
Or: run @kbd{C-c C-l} to load the file. This will first try to start a
session as the previous command does.
@item
Or: run any command which requires a running session. It will always
prompt to create one if there isn't one already for the current project.
@end itemize

It will prompt for a Cabal directory and a current directory. It figures
out where the cabal directory is and defaults for the current directory,
so you should be able to just hit RET twice.

@subsection Switch a session

Sometimes a particular file is used in two different
sessions/projects. You can run

@example
    M-x haskell-session-change
@end example

If it prompts you to make a new session, tell it no (that's a
bug). It will ask you to choose from a list of sessions.

@subsection Killing a session

To kill a session you can run

@example
    M-x haskell-session-kill
@end example

Which will prompt to kill all associated buffers, too. Hit `n` to
retain them.

Alternatively, you can switch to the REPL and just kill the buffer
normally with @kbd{C-x k RET}. It will prompt

@example
    Kill the whole session (y or n)?
@end example

You can choose @kbd{y} to kill the session itself, or @kbd{n} to just
kill the REPL buffer. You can bring it back with @kbd{M-x}
@code{haskell-interactive-bring}.

@subsection Menu

To see a list of all sessions you have open with some simple
statistics about memory usage, etc. run

@example
    M-x haskell-menu
@end example

For example:

@example
    foo  14648 08:21:42 214MB /path/to/fpco/foo/  /path/to/fpco/foo/ ghci
    bar  29119 00:22:03 130MB /path/to/bar/       /path/to/bar/      ghci
    mu   22575 08:48:20 73MB  /path/to/fpco/mu/   /path/to/fpco/mu/  ghci
@end example


@section Compiling

There are a bunch of ways to compile Haskell modules. This page covers
a few of them.

@subsection Load into GHCi

To compile and load a Haskell module into GHCi, run the following

@example
    M-x haskell-process-load
@end example

Or @kbd{C-c C-l}. You'll see any compile errors in the REPL window.

@subsection Build the Cabal project

To compile the whole Cabal project, run the following

@example
    M-x haskell-process-cabal-build
@end example

Or @kbd{C-c C-c}. You'll see any compile errors in the REPL window.

@subsection Reloading modules

To reload the current module, even when you're in other modules, you can
run @kbd{C-u M-x} @code{haskell-process-load-or-reload} or @kbd{C-u C-c
C-l}. It will now reload that module whenever you run @kbd{C-c C-l} in
the future from whatever module you're in. To disable this mode, just
run @kbd{C-u C-c C-l} again.

@subsection Jumping to compile errors

You can use the standard compile error navigation function @kbd{C-x `} —
jump to the next error.

Or you can move your cursor to an error in the REPL and hit @kbd{RET} to
jump to it.

@subsection Auto-removing imports

If the customization variable
@code{haskell-process-suggest-remove-import-lines} is enabled.

@lisp
(custom-set-variables
  '(haskell-process-suggest-remove-import-lines t))
@end lisp

Building and loading modules which output warnings like,

@example
    Warning: The import of `Control.Monad' is redundant
      except perhaps to import instances from `Control.Monad'
    To import instances alone, use: import Control.Monad()
@end example

will prompt the user with

@example
> The import line `Control.Monad' is redundant. Remove? (y, n, c: comment out)
@end example

If you answer

@itemize
@item
@kbd{y}: it will delete the import, but leave the empty line remaining
(this avoids messing with line positions in subsequent error messages).
@item
@kbd{n}: it will leave the import.
@item
@kbd{c}: it will comment out the import (this is handy for when you just
want to temporarily hide an import).
@end itemize

@subsection Auto-adding of modules to import

Enable the customization variable
@code{haskell-process-suggest-hoogle-imports}.

@lisp
(custom-set-variables
  '(haskell-process-suggest-hoogle-imports t))
@end lisp

Whenever GHC says something is not in scope, it will hoogle that
symbol. If there are results, it will prompt to add one of the modules
from Hoogle's results.

You need to make sure you've generated your Hoogle database properly.

@subsection Auto-adding of extensions

It you use an extension which is not enabled, GHC will often inform
you. For example, if you write:

@example
newtype X a = X (IO a)
  deriving (Monad)
@end example


Then you'll see a message like:

@example
    x.hs:13:13: Can't make a derived instance of `Monad X': …
          `Monad' is not a derivable class
          Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension
        In the newtype declaration for `X'
@end example

This @code{-XFoo} pattern will be picked up and you will be prompted:

@example
> Add `@{-# LANGUAGE GeneralizedNewtypeDeriving #-@}` to the top of the
> file? (y or n)
@end example

If you answer `y`, it will temporarily jump to the buffer and it to
the top of the file.

@subsection Orphan instances

If GHC complains about orphan instances, you usually are doing it
intentionally, so it prompts to add @code{-fno-warn-orphans} to the top of
the file with an @kbd{OPTIONS} pragma.

@subsection Auto-adding of dependencies

When doing a build, you will sometimes get a message from GHC like:

@example
    src/ACE/Tokenizer.hs:11:18: Could not find module `Data.Attoparsec.Text' …
        It is a member of the hidden package `attoparsec-0.11.1.0'.
@end example

This message contains all the necessary information to add this to
your .cabal file, so you will be prompted to add it to your .cabal
file:

@example
    Add `attoparsec' to ace.cabal? (y or n)  y
@end example

If you hit @kbd{y}, it will prompt with this:

@example
    attoparsec >= 0.11.1.0
@end example

Which you can edit (e.g. do some PVP decision or remove constraints
entirely), and then it will open up your @file{.cabal} file and go
through each section:

@example
    Add to library? (y or n)  y
@end example

This will add it to the top of the @code{build-depends} field in your
library section. If you have any executables, it will go through each
of those, prompting, too.

Now you can rebuild with @kbd{C-c C-c} again.

@section Haskell Interactive Mode REPL

When GHCi has been launched, it works on a read-eval-print basis. So
you will be presented with the prompt:

@example
    The lambdas must flow.
    Changed directory: /path/to/your/project/
    λ>
@end example

@subsection Changing REPL target

@findex haskell-session-change-target
@vindex haskell-interactive-mode-hook

With @code{haskell-session-change-target} you can change the target for
REPL session.


After REPL session started, in @code{haskell-interactive-mode} buffer invoke the
@code{haskell-session-change-target} and select from available targets for

@cindex testing
- Testing

@cindex benchmarking
- Benchmark

- Executable

- Library

Answer ``yes'' to restart the session and run your tests, benchmarks, executables.


TODO/WRITEME


@subsection Bringing the REPL

If you don't know where the REPL buffer is, you can always bring it
with:

@example
    M-x haskell-interactive-bring
@end example

Or @kbd{C-`}.

@subsection Evaluating expressions

To evaluate expressions, simply type one out and hit `RET`.

@example
    λ> 123
    123
@end example

@subsection Evaluating multiline expressions

GHCi features two ways to evaluate multiline expressions. You can use
@code{:set +m} to
enable @uref{https://www.haskell.org/ghc/docs/latest/html/users_guide/ghci.html#multiline-input,
multiline input} for all expressions, or you can wrap your expression in
@code{:@{} and @code{:@}} (they have to be on their own lines).

The prompt will change to indicate that you're inputting a multiline
expression:

@example
λ> :@{
λ| let a = 10
λ|     b = 20
λ|     c = 30
λ| :@}
@end example

You can also simulate multiline mode by having your input contain
newline characters. You can input a literal newline character with
@kbd{C-q C-j}, or you can use:

@example
    M-x haskell-interactive-mode-newline-indent
@end example

which is bound to @kbd{C-j}. This command indents after the newline. You
can simulate the above example like so:

@example
λ> let a = 10
       b = 20
       c = 30
@end example

@subsection Type of expressions

You can use normal @code{:type} which is part of GHCi to get the type of
something:

@example
    λ> :t id
    id :: a -> a
@end example

But you can also just write out the value directly,

@example
    λ> id
    id :: a -> a
@end example

and because there's no @code{Show} instance for @code{(a -> a)}. This would
normally yield a compile error:

@example
    No instance for (Show (a0 -> a0))
      arising from a use of `print'
    Possible fix: add an instance declaration for (Show (a0 -> a0))
    In a stmt of an interactive GHCi command: print it
@end example

It will run @code{:type id} in the background and print out the
result. The same is true for ambiguous things:

@example
    λ> :t read "a"
    read "a" :: Read a => a
@end example

Because this would normally be an ambiguous constraint:

@example
    Ambiguous type variable `a0' in the constraint:
      (Read a0) arising from a use of `read'
    Probable fix: add a type signature that fixes these type variable(s)
    In the expression: read \"a\"
    In an equation for `it': it = read \"a\"
@end example

Which is less useful than just printing the type out.

You can disable this behaviour by disabling the customization option:

@lisp
(custom-set-variables
  '(haskell-interactive-types-for-show-ambiguous nil))
@end lisp

@subsection Printing mode

You can choose between printing modes used for the results of
evaluating expressions. To do that, configure the variable
@code{haskell-interactive-mode-eval-mode}. Example:

@lisp
(setq haskell-interactive-mode-eval-mode 'haskell-mode)
@end lisp


A handy function you can use is:

@lisp
(defun haskell-interactive-toggle-print-mode ()
  (interactive)
  (setq haskell-interactive-mode-eval-mode
        (intern
         (ido-completing-read "Eval result mode: "
                              '("fundamental-mode"
                                "haskell-mode"
                                "espresso-mode"
                                "ghc-core-mode"
                                "org-mode")))))
@end lisp

(Add whichever modes you want to use.)

And then run

@example
    M-x haskell-interactive-toggle-print-mode
@end example

Or @kbd{C-c C-v}:

@lisp
(define-key haskell-interactive-mode-map (kbd "C-c C-v")
            'haskell-interactive-toggle-print-mode)
@end lisp

There you can choose `haskell-mode`, for example, to pretty print the
output as Haskell.

@subsection Presentations

If you have the @file{present} package installed, you can use the following
syntax to print anything which is an instance of @code{Data}:

@example
    λ> :present 123
    123
@end example

It will print data structures lazily:

@example
    λ> :present [1..]
    [1
    ,[Integer]]
@end example

It shows types when there is an unevaluated field in a constructor. You
can click the @code{[Integer]} or press @kbd{RET} on it to expand
further:

@example
    λ> :present [1..]
    [1
    ,2
    ,[Integer]]
@end example

Etc. Remember: this only works for instances of @code{Data.Data.Data}.

@subsection History

A history is maintained for the duration of the REPL buffer. To go up
and down in the history, run @kbd{M-p} for previous and @kbd{M-n} for
next.

@subsection Cancelling commands

To cancel a running REPL command, run @kbd{C-c C-c}.

@subsection Clear the REPL

Run @kbd{C-c C-k} to clear the REPL.


@subsection Trick: Put Interactive REPL in Separate Frame


The following @code{create-haskell-interactive-frame} is a quick hack to
move the repl to a separate frame, for those that want a more
predictable layout of windows in Emacs.

@lisp
(defun create-unfocused-frame ()
  (let*
    ((prv (window-frame))
     (created (make-frame)))
    (select-frame-set-input-focus prv) created))

(defun create-haskell-interactive-frame ()
  (interactive)
  (haskell-interactive-bring)
  (create-unfocused-frame)
  (delete-window))

@end lisp

@subsection Troubleshooting

If the REPL ever goes funny, you can clear the command queue via:

@example
    M-x haskell-process-clear
@end example

Alternatively, you can just restart the process:

@example
    M-x haskell-process-restart
@end example

You can also switch to the buffer @code{*haskell-process-log*}, which can
be enabled and disabled with the customization variable
`haskell-process-log`, to see what the cause of your troubles are.

If the process fails and nothing unusual is in the process log, the
following command can dump the @code{haskell-process} state:

@example
    M-: (haskell-process)
@end example

The output can be copied from the @code{*Messages*} buffer.

@section Haskell Interactive Mode Querying

There a few ways GHCi lets you query information about your code.

@subsection Get identifer type

To print the type of the top-level identifier at point in the REPL and
in the message buffer, run the following command:

@example
    M-x haskell-process-do-type
@end example

or @kbd{C-c C-t}.

@subsection Insert identifier's type as type signature

To print the type of the top-level identifier at point, run the
following command:

@example
    C-u M-x haskell-process-do-type
@end example

or @kbd{C-u C-c C-t}.

@subsection Get identifier info

To print the info of the identifier at point, run the following
command:

@example
    M-x haskell-process-do-info
@end example

or @kbd{C-c C-i}.

@subsection Presentation mode

When using @kbd{C-c C-i} or @kbd{C-c C-t} it will open a buffer in
haskell-presentation-mode. You can hit @kbd{q} to close the buffer.

But you can also continue to use @kbd{C-c C-i} inside the buffer to
drill further down data types and classes.

E.g. if you go to @code{Ord} in your code buffer and @kbd{C-c C-i}, it
will popup a buffer containing

@example
class Eq a => Ord a where
  compare :: a -> a -> Ordering
  (<) :: a -> a -> Bool
  (>=) :: a -> a -> Bool
  (>) :: a -> a -> Bool
  (<=) :: a -> a -> Bool
  max :: a -> a -> a
  min :: a -> a -> a
  	-- Defined in `GHC.Classes'
@end example

And all the instances of that class. But then you can also move your
cursor to @code{Ordering} and hit @kbd{C-c C-i} again to get another
popup:

@example
data Ordering = LT | EQ | GT 	-- Defined in `GHC.Types'
instance Bounded Ordering -- Defined in `GHC.Enum'
instance Enum Ordering -- Defined in `GHC.Enum'
instance Eq Ordering -- Defined in `GHC.Classes'
instance Ord Ordering -- Defined in `GHC.Classes'
instance Read Ordering -- Defined in `GHC.Read'
instance Show Ordering -- Defined in `GHC.Show'
@end example

And so on. It's a very good way of exploring a new codebase.

@subsection Browse import's module

To print all exported identifiers of the module imported by the import
line at point, run the following command:

@example
    M-x haskell-process-do-info
@end example

or @kbd{C-c C-i}. It will print all exports by running @code{:browse
The.Module} in the GHCi process.

@section Haskell Interactive Mode Cabal integration

There's some integration with Cabal in Haskell Interactive Mode. Once
you've started a session, the features below are available.

@subsection Cabal building

The most common Cabal action is building, so that has a specific command:

@example
    M-x haskell-process-cabal-build
@end example

Or @kbd{C-c C-c}. When building, it will hide unneccessary output.

For example, to build the `ace` package, the output is simply:

@example
    Compiling: ACE.Types.Tokens
    Compiling: ACE.Combinators
    Compiling: ACE.Tokenizer
    Compiling: ACE.Parsers
    Compiling: ACE.Pretty
    Compiling: ACE
    Complete: cabal build (0 compiler messages)
@end example

Whereas the complete output is normally:

@example
    Building ace-0.5...
    Preprocessing library ace-0.5...
    [4 of 9] Compiling ACE.Types.Tokens ( src/ACE/Types/Tokens.hs, dist/build/ACE/Types/Tokens.o )
    [5 of 9] Compiling ACE.Combinators  ( src/ACE/Combinators.hs, dist/build/ACE/Combinators.o ) [ACE.Types.Tokens changed]
    [6 of 9] Compiling ACE.Tokenizer    ( src/ACE/Tokenizer.hs, dist/build/ACE/Tokenizer.o ) [ACE.Types.Tokens changed]
    [7 of 9] Compiling ACE.Parsers      ( src/ACE/Parsers.hs, dist/build/ACE/Parsers.o )
    [8 of 9] Compiling ACE.Pretty       ( src/ACE/Pretty.hs, dist/build/ACE/Pretty.o )
    [9 of 9] Compiling ACE              ( src/ACE.hs, dist/build/ACE.o ) [ACE.Tokenizer changed]
    In-place registering ace-0.5...
@end example

Which is considerably more verbose but rarely useful or interesting.

@subsection Arbitrary cabal commands

To run an arbitrary Cabal command:

@example
    C-u M-x haskell-process-cabal
@end example

Or run @kbd{C-u C-c c}.

It will prompt for an input, so you can write @code{configure -fdev},
for example.

@subsection Completing cabal commands

To run some common Cabal commands, just run:

@example
    M-x haskell-process-cabal
@end example

Or @kbd{C-c c}. This is commonly used to do @code{install},
@code{haddock}, @code{configure}, etc.

@section Haskell Interactive Mode Debugger

There is limited support for debugging in GHCi. Haskell Interactive Mode
provides an interface for interacting with this.

@subsection Opening the debug buffer

To open the debug buffer run the following command from any buffer
associated with a session:

@example
    M-x haskell-debug
@end example

It will open a buffer that looks like this:

@example
    Debugging haskell

    You have to load a module to start debugging.

    g - refresh

    Modules

    No loaded modules.
@end example


@subsection Loading modules

To debug anything you need to load something into GHCi. Switch to a
normal file, for example:

@example
main = do putStrLn "Hello!"
          putStrLn "World"
@end example

and load it into GHCi (@kbd{C-c C-l}). Now when you hit @kbd{g}
(to refresh) in the debugging buffer, you'll see something like:

@example

    Debugging haskell

    b - breakpoint, g - refresh

    Context

    Not debugging right now.

    Breakpoints

    No active breakpoints.

    Modules

    Main - hello.hs
@end example

@subsection Setting a breakpoint

To set a breakpoint hit @kbd{b} in the debugger buffer. It will prompt
for a name. Enter @code{main} and hit @kbd{RET}.

Now the buffer will look like this:

@example
    Debugging haskell

    s - step into an expression, b - breakpoint
    d - delete breakpoint, g - refresh

    Context

    Not debugging right now.

    Breakpoints

    0 - Main (1:8)

    Modules

    Main - hello.hs
@end example

@subsection Start stepping

Hit @kbd{s} to step through an expression: it will prompt for an
expression to evaluate and step through. Enter @code{main} and hit
@kbd{RET}. Now the buffer will look like this:

@example
    Debugging haskell

    s - step into an expression, b - breakpoint
    d - delete breakpoint, a - abandon context, c - continue
    p - previous step, n - next step
    g - refresh

    Context

    main - hello.hs (stopped)

    do putStrLn "Hello!"
       putStrLn "World"

    _result :: IO () = _

       1 do putStrLn "Hello!" putStrLn "World"

    Breakpoints

    0 - Main (1:8)

    Modules

    Main - hello.hs
@end example

What we see here is the current expression being evaluated:

@example
do putStrLn "Hello!"
   putStrLn "World"
@end example

And we see the type of it:

@example
_result :: IO () = _
@end example

And we see a backtrace of steps so far:

@example
1 do putStrLn "Hello!" putStrLn "World"
@end example

@subsection Continue stepping

To continue stepping, just hit @kbd{s} again. Now the context will change
to:

@example
main - hello.hs (stopped)

putStrLn "Hello!"

_result :: IO () = _

   1 do putStrLn "Hello!" putStrLn "World"
@end example


Hitting @kbd{s} once more, we see the context change to:

@example
putStrLn "World"

_result :: IO () = _

   2 putStrLn "Hello!"
   1 do putStrLn "Hello!" putStrLn "World"
@end example

Finally hitting @kbd{s} again will say "Computation finished". Hitting
@kbd{s} a final time will change the display back to:

@example
    Debugging haskell

    s - step into an expression, b - breakpoint
    d - delete breakpoint, g - refresh

    Context

    Finished debugging.

       2 putStrLn "Hello!"
       1 do putStrLn "Hello!" putStrLn "World"

    Breakpoints

    1 - Main (1:8)

    Modules

    Main - hello.hs
@end example

And you're done debugging.


@node Editing Cabal files
@chapter Editing Cabal files

@findex haskell-cabal-mode
@vindex haskell-cabal-mode-hook

@code{haskell-cabal-mode} is a major mode for editing
@uref{http://www.haskell.org/cabal/users-guide/developing-packages.html,Cabal
package description files} and is automatically associated with files
having a @file{.cabal} extension.

@findex haskell-cabal-visit-file

For quickly locating and jumping to the nearest @file{.cabal} file from
a Haskell source buffer, you can use @kbd{M-x haskell-cabal-visit-file};
with a prefix argument (i.e. @kbd{C-u}) @code{find-file-other-window} is
used to visit the @file{.cabal} file.
@code{haskell-cabal-visit-file} is bound to the key sequence @kbd{C-c v c}.

TODO/WRITEME

@node Browsing Haddocks
@chapter Browsing Haddocks using @code{w3m}

An experimental feature is use of the w3m browser to browse Haddock docs
inside Emacs.

@section Get w3m

Most Linux distributions will have a package for the binary:

@example
$ sudo apt-get install w3m
@end example

Now grab @code{w3m.el} from:

@itemize
@item @url{http://emacs-w3m.namazu.org/}
@item @kbd{M-x} @code{package-install} @kbd{RET} @code{w3m} @kbd{RET}
@end itemize

Confirm installation by trying @kbd{M-x} @code{w3m-browse-url} @kbd{RET}
@code{haskell.org} @kbd{RET}.

If this works, you're good to go.

@section Configure w3m

Now that you have w3m, you probably want to configure it to be more of a
passive viewer than a full-fledged browser. For example:

@lisp
(setq w3m-mode-map (make-sparse-keymap))

(define-key w3m-mode-map (kbd "RET") 'w3m-view-this-url)
(define-key w3m-mode-map (kbd "q") 'bury-buffer)
(define-key w3m-mode-map (kbd "<mouse-1>") 'w3m-maybe-url)
(define-key w3m-mode-map [f5] 'w3m-reload-this-page)
(define-key w3m-mode-map (kbd "C-c C-d") 'haskell-w3m-open-haddock)
(define-key w3m-mode-map (kbd "M-<left>") 'w3m-view-previous-page)
(define-key w3m-mode-map (kbd "M-<right>") 'w3m-view-next-page)
(define-key w3m-mode-map (kbd "M-.") 'w3m-haddock-find-tag)

(defun w3m-maybe-url ()
  (interactive)
  (if (or (equal '(w3m-anchor) (get-text-property (point) 'face))
          (equal '(w3m-arrived-anchor) (get-text-property (point) 'face)))
      (w3m-view-this-url)))
@end lisp

@section Import w3m-haddock

It's not enabled by default in haskell-mode at present, so you need to
import it manually:

@lisp
(require 'w3m-haddock)
@end lisp

@section Add a hook for w3m

In order to make haddock pages a little more palatable (and add syntax
highlighting to source view), you can add this hook:

@lisp
(add-hook 'w3m-display-hook 'w3m-haddock-display)
@end lisp

It's a little rough around the edges, but it's a start.

@section Configure your package locations

By default, the package locations is set to:

@lisp
(defcustom haskell-w3m-haddock-dirs
  '("~/.cabal/share/doc/"))
@end lisp

If you are using an hsenv or a custom package directory, you should
configure this variable with M-x customize-variable or by writing the
custom-set-variables code for it.

@section Finally

You did all that! Now you're ready to bind a useful key:

@lisp
(define-key haskell-mode-map (kbd "C-c C-d") 'haskell-w3m-open-haddock)
@end lisp

Now when you press @kbd{C-c} @kbd{C-d} it will prompt for a package to
browse to.

This feature will be improved gradually as time goes on.


@node Spell checking strings and comments
@chapter Using with @code{flyspell-prog-mode}

Strings and comments can be checked for spelling mistakes. There is a
standard Emacs mode for this purpose, @code{flyspell-prog-mode}, that
can be enabled in Haskell buffers. Spelling errors are underlined using
squiggly red lines.

@ifhtml
@image{anim/flyspell-prog-mode}
@end ifhtml

Documentation for @code{flyspell-prog-mode} can be found in
@xref{Spelling,,,emacs}. Here we point to a couple of useful
keybindings:

@itemize
@item
@kbd{M-$} - Check and correct spelling of the word at point (@code{ispell-word}).

@item
@kbd{digit} - Replace the word, just this time, with one of the
displayed near-misses. Each near-miss is listed with a digit; type that
digit to select it.

@item
@kbd{SPC} - Skip this word—continue to consider it incorrect, but don’t
change it here.
@end itemize

To enable spell checking of strings and comments add this line to your
@code{~/.emacs} file:

@code{(add-hook 'haskell-mode-hook 'flyspell-prog-mode)}

@node Aligning code
@chapter Aligning code

Select a region you want to align text within, @kbd{M-x}
@code{align-regexp}, and type a regexp representing the alignment
delimiter.

For example, I often line up my Haddock comments:

@example
f :: a -- ^ does a
  -> Foo b -- ^ and b
  -> c -- ^ to c
@end example

Select the region, and let the regexp be @samp{--}:

@example
f :: a     -- ^ does a
  -> Foo b -- ^ and b
  -> c     -- ^ to c
@end example

Of course, this works for just about anything. Personally, I've globally
bound it to @kbd{C-x a r}:

@lisp
(global-set-key (kbd "C-x a r") 'align-regexp)
@end lisp

Note that you can also just use the rules below for telling the aligner
about Haskell. Once you evaluate this, you can just use @kbd{M-x}
@code{align}, which I like to bind to @kbd{M-[}.

@lisp
(add-to-list 'align-rules-list
             '(haskell-types
               (regexp . "\\(\\s-+\\)\\(::\\|∷\\)\\s-+")
               (modes quote (haskell-mode literate-haskell-mode))))
(add-to-list 'align-rules-list
             '(haskell-assignment
               (regexp . "\\(\\s-+\\)=\\s-+")
               (modes quote (haskell-mode literate-haskell-mode))))
(add-to-list 'align-rules-list
             '(haskell-arrows
               (regexp . "\\(\\s-+\\)\\(->\\|→\\)\\s-+")
               (modes quote (haskell-mode literate-haskell-mode))))
(add-to-list 'align-rules-list
             '(haskell-left-arrows
               (regexp . "\\(\\s-+\\)\\(<-\\|←\\)\\s-+")
               (modes quote (haskell-mode literate-haskell-mode))))
@end lisp

@node Rectangular commands
@chapter Using rectangular region commands

Emacs has a set of commands which operate on the region as if it were
rectangular. This turns out to be extremely useful when dealing with
whitespace sensitive languages.

@itemize
@item @kbd{C-x r o} is "Open Rectangle".

It will shift any text within the rectangle to the right side. Also see:

@item @kbd{C-x r t} is "String Rectangle".

It will replace any text within the rectangle with the given string on
all the lines in the region. If comment-region didn't already exist, you
could use this instead, for example.

@item @kbd{C-x r d} is "Delete Rectangle".

It will delete the contents of the rectangle and move anything on the
right over.

@item @kbd{C-x r r} is "Copy Rectangle to Register".

It will prompt you for a register number so it can save it for later.

@item @kbd{C-x r g} is "Insert register".

This will insert the contents of the given register, overwriting
whatever happens to be within the target rectangle. (So make room)

@item @kbd{C-x r k} is "Kill rectangle".

Delete rectangle and save contents for:

@item @kbd{C-x r y} is "Yank rectangle".

This will insert the contents of the last killed rectangle.
@end itemize

As with all Emacs modifier combos, you can type @kbd{C-x r C-h} to find
out what keys are bound beginning with the @kbd{C-x r} prefix.

@node REPL
@chapter Using GHCi REPL within Emacs

To start the REPL you can run one of the following:

@itemize
@item @kbd{M-x run-haskell}
@item @kbd{M-x switch-to-haskell}
@end itemize

This repl works with @uref{https://www.emacswiki.org/emacs/ComintMode, Comint}.
So you will feel at home if you are already using @kbd{M-x Shell} or @kbd{M-x ielm}.

@code{Inf-Haskell} is a Major mode for running GHCi, with comint.

Important key bindings in @code{Inf-haskell}:

@table @kbd
@item RET
invokes @kbd{comint-send-input}. Sends the input to the GHCi process, evaluates
the line and returns the output.

@item C-d or <delete>
deletes the forward character

@item <C-up> or M-p
invokes @kbd{comint-previous-input}. Cycle backwards through input history,
saving input.

@item <C-down> or M-n
invokes @kbd{comint-next-input}. Cycle forwards through input history.

@item C-c C-c
invokes @kbd{comint-interrupt-subjob}. Sends KeyboardInterrupt signal.

@item C-c C-\
invokes @kbd{comint-quit-subjob}. Sends KeyboardInterrupt signal.

@item C-c C-z
invokes @kbd{comint-stop-subjob}. Kills the GHCi process.

@item C-c M-r
invokes @kbd{comint-previous-matching-input-from-input}. If you are familiar
with @kbd{C-r} in bash. This is the same as that. Searches backwards through
input history for match for current input.

@item C-c M-s
invokes @kbd{comint-next-matching-input-from-input}. Searches forwards through
input history for match for current input.

@item C-c C-l
invokes @kbd{comint-dynamic-list-input-ring}. Displays a list of recent inputs
entered into the current buffer.

@item C-c M-o
invokes @kbd{comint-clear-buffer}. Clears the buffer (Only with Emacs 25.X and above)

@item C-c C-n
invokes @kbd{comint-next-prompt}. Goes to the start of the previous REPL prompt.

@item C-c C-p
invokes @kbd{comint-previous-prompt}. Goes to the start of the next REPL prompt.

@item C-c C-o
invokes @kbd{comint-delete-output}. Clears the output of the most recently evaluated
expression.

@item C-c C-e
invokes @kbd{comint-show-maximum-output}. Moves the point to the end of the buffer.

@item C-c C-u
invokes @kbd{comint-kill-input}. Kills backward, the line at point. (Use this when you have typed in an expression into the prompt
but you dont want to evaluate it.)

@item C-c C-w
invokes @kbd{backward-kill-word}. Kills backward, the word at point

@item C-c C-s
invokes @kbd{comint-write-output}. Write output from interpreter since last
input to FILENAME. Any prompt at the end of the output is not written.
@end table

@section Relevant defcustoms:

@multitable @columnfractions .40 .20 .40
@headitem Interpreter (defcustom) @tab Default Value @tab Possible Values
@item @code{haskell-process-type} @tab @code{'auto} @tab @code{'stack-ghci, 'cabal-repl, 'ghci, 'auto}
@item @code{inferior-haskell-hook} @tab @code{nil} @tab -
@item @code{haskell-process-path-ghci} @tab @code{ghci} @tab -
@item @code{haskell-process-args-ghci} @tab @code{-ferror-spans} @tab -
@item @code{haskell-process-path-cabal} @tab @code{cabal} @tab -
@item @code{haskell-process-args-cabal-repl} @tab @code{--ghc-option=-ferror-spans} @tab -
@item @code{haskell-process-path-stack} @tab @code{stack} @tab -
@item @code{haskell-process-args-stack-ghci} @tab @code{--ghci-options=-ferror-spans --no-build --no-load} @tab -
@end multitable

@section More on @code{haskell-process-type}

The Haskell interpreter used by @code{Inf-Haskell} is auto-detected by default,
but is customizable with defcustom @code{haskell-process-type}. The values
recognized by it are (default is 'auto):

@itemize
@item @code{'stack-ghci}
@item @code{'cabal-repl}
@item @code{'ghci}
@item @code{'auto}
@end itemize

if the @code{haskell-process-type} is @code{'auto}, the directories are searched for
@code{cabal.sandbox.config} or @code{stack.yaml} or  @code{*.cabal} file.
If the file is present, then appropriate process is started.

When @code{cabal.sandbox.config} is found @code{haskell-process-type} is @code{'cabal-repl}.
Similarly, when @code{stack.yaml} is found @code{haskell-process-type} is @code{'stack-ghci}.
Similarly, when @code{xyz.cabal} is found @code{haskell-process-type} is @code{'cabal-repl}.
When nothing is found @code{haskell-process-type} is @code{'ghci}. When more than one
file such as @code{cabal.sandbox.config} and @code{stack.yaml} are found the following
preference is followed.

@code{cabal.sandbox.config} > @code{stack.yaml} >  @code{*.cabal}

@node Collapsing Haskell code
@chapter Collapsing Haskell code

This is @code{hs-minor-mode} for @code{haskell-mode}. This module uses
hideshow module.

To activate this minor mode (haskell-collapse-mode)
@itemize
@item @kbd{M-x haskell-collapse-mode} is "To start haskell-collapse-mode".
@end itemize
This minor mode works with indentation.

In a quick glance:

@table @kbd
@item C-c @ C-c
is bound to @code{haskell-hide-toggle}
@item C-c @ C-M-c
@item C-c @ C-M-h
@item C-c @ C-M-s
are all bound to @code{haskell-hide-toggle-all}
@end table

How to use @code{M-x haskell-hide-toggle}?

Place your point on the code block that you want to collapse and hit the
keybinding. Now the code collapses and you can see the first line of the
block and elipsis.

Take this example code (example usage of @code{M-x haskell-hide-toggle}):
when you place the cursor here, like this (notice the thick block in the
first line):

@code{
f█x
  | rem i 3 == 0 = if i == 0 && (k /= 0) then (c k) else (h j)
  | otherwise = 0
  where i = sum x
        j = g x (div i 3) 0 0 []
        k = zeroes x 0
}

or

@code{
f x
  | rem i 3 == 0 = if i == 0 && (k /= 0) then (c k) else (h j)
█ | otherwise = 0
  where i = sum x
        j = g x (div i 3) 0 0 []
        k = zeroes x 0
}

then when you collapse it becomes something like this:

@code{
f█x@dots{}
}

It works in terms of (indentation) blocks.

One more example:

@code{
f x
  | rem i 3 == 0 = if i == 0 && (k /= 0) then (c k) else (h j)
  | otherwise = 0
  w█ere i = sum x
        j = g x (div i 3) 0 0 []
        k = zeroes x 0
}

or

@code{
f x
  | rem i 3 == 0 = if i == 0 && (k /= 0) then (c k) else (h j)
  | otherwise = 0
  where i = sum x
        j = g x (div i 3) 0 0 []
     █  k = zeroes x 0
}

this, will result in something like:

@code{
f x
  | rem i 3 == 0 = if i == 0 && (k /= 0) then (c k) else (h j)
  | otherwise = 0
  where i = sum █@dots{}
}

The other functionality @code{M-x haskell-hide-toggle-all} also
works only for indentation and it collapses all toplevel functions.

So a file that looks like this:

@example
main = interact $ show.f. map read .words
f (x:xs) = dp x xs

dp money a | money < 0 || null a = [1..1000]
dp 0 a = []
dp money a @atchar{} (coin:coins)
  | (length i) <= length j = i
  | otherwise = j
  where i = (coin:(dp (money-coin) a))
        j = (dp money coins)
@end example

will turn into this:

@example
main = interact $ show.f. map read .words
f (x:xs) = dp x xs

dp money a | money < 0 || null a = [1..1000]
dp 0 a = []
dp money a @atchar{} (coin:coins)@dots{}
@end example

@node Getting Help and Reporting Bugs
@chapter Getting Help and Reporting Bugs

Work on Haskell Mode is organized with Github @code{haskell-mode}
project.  To understand how the project is run please read the
information in the
@uref{https://github.com/haskell/haskell-mode/wiki,project wiki pages}.

To report any issues please use the Github's issue mechanism available from
@uref{https://github.com/haskell/haskell-mode,Haskell Mode's GitHub Home}.

For a quick question visit @code{#haskell-emacs} channel on IRC
@code{irc.freenode.net}.

There is also a (now defunct)
@uref{http://projects.haskell.org/cgi-bin/mailman/listinfo/haskellmode-emacs,
Haskellmode-emacs mailing list}, also available on
@uref{http://gmane.org/, Gmane} via the
@uref{http://dir.gmane.org/gmane.comp.lang.haskell.emacs,
gmane.comp.lang.haskell.emacs} newsgroup.

We welcome code and non-code contributions so that we can all enjoy
coding Haskell even more.

@node Concept index
@unnumbered Concept index

@printindex cp

@node Function index
@unnumbered Function index

@printindex fn

@node Variable index
@unnumbered Variable index

@printindex vr

@bye

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