Add Grains example

Author: koladilip

grains/README.md

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+# Grains
+
+Calculate the number of grains of wheat on a chessboard given that the number
+on each square doubles.
+
+There once was a wise servant who saved the life of a prince. The king
+promised to pay whatever the servant could dream up. Knowing that the
+king loved chess, the servant told the king he would like to have grains
+of wheat. One grain on the first square of a chess board, with the number
+of grains doubling on each successive square.
+
+There are 64 squares on a chessboard (where square 1 has one grain, square 2 has two grains, and so on).
+
+Write code that shows:
+- how many grains were on a given square, and
+- the total number of grains on the chessboard
+
+## For bonus points
+
+Did you get the tests passing and the code clean? If you want to, these
+are some additional things you could try:
+
+- Optimize for speed.
+- Optimize for readability.
+
+Then please share your thoughts in a comment on the submission. Did this
+experiment make the code better? Worse? Did you learn anything from it?
+
+
+## Getting Started
+
+Please refer to the [installation](https://exercism.io/tracks/haskell/installation)
+and [learning](https://exercism.io/tracks/haskell/learning) help pages.
+
+## Running the tests
+
+To run the test suite, execute the following command:
+
+```bash
+stack test
+```
+
+#### If you get an error message like this...
+
+```
+No .cabal file found in directory
+```
+
+You are probably running an old stack version and need
+to upgrade it.
+
+#### Otherwise, if you get an error message like this...
+
+```
+No compiler found, expected minor version match with...
+Try running "stack setup" to install the correct GHC...
+```
+
+Just do as it says and it will download and install
+the correct compiler version:
+
+```bash
+stack setup
+```
+
+## Running *GHCi*
+
+If you want to play with your solution in GHCi, just run the command:
+
+```bash
+stack ghci
+```
+
+## Feedback, Issues, Pull Requests
+
+The [exercism/haskell](https://github.com/exercism/haskell) repository on
+GitHub is the home for all of the Haskell exercises.
+
+If you have feedback about an exercise, or want to help implementing a new
+one, head over there and create an issue.  We'll do our best to help you!
+
+## Source
+
+JavaRanch Cattle Drive, exercise 6 [http://www.javaranch.com/grains.jsp](http://www.javaranch.com/grains.jsp)
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.

grains/grains.cabal

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+cabal-version: 1.12
+
+-- This file has been generated from package.yaml by hpack version 0.33.0.
+--
+-- see: https://github.com/sol/hpack
+--
+-- hash: 88f82193be09582c3e69242198a92f693c1cd8ea6328c5dd88f0438ebd446d54
+
+name:           grains
+version:        1.2.0.6
+build-type:     Simple
+
+library
+  exposed-modules:
+      Grains
+  other-modules:
+      Paths_grains
+  hs-source-dirs:
+      src
+  ghc-options: -Wall
+  build-depends:
+      base
+  default-language: Haskell2010
+
+test-suite test
+  type: exitcode-stdio-1.0
+  main-is: Tests.hs
+  other-modules:
+      Paths_grains
+  hs-source-dirs:
+      test
+  build-depends:
+      base
+    , grains
+    , hspec
+  default-language: Haskell2010

grains/package.yaml

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+name: grains
+version: 1.2.0.6
+
+dependencies:
+  - base
+
+library:
+  exposed-modules: Grains
+  source-dirs: src
+  ghc-options: -Wall
+  # dependencies:
+  # - foo       # List here the packages you
+  # - bar       # want to use in your solution.
+
+tests:
+  test:
+    main: Tests.hs
+    source-dirs: test
+    dependencies:
+      - grains
+      - hspec

grains/src/Grains.hs

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+module Grains (square, total) where
+
+square' :: Integer -> Integer
+square' n
+    | n > 1 = 2 * square' (n-1)
+    | otherwise = 1
+
+square :: Integer -> Maybe Integer
+square n
+    | n > 64 || n < 1 = Nothing
+    | otherwise = Just (square' n)
+
+
+total :: Integer
+-- 2^0 + 2^1 + 2^2 .. + 2^63 = 2^64 - 1 
+total = (2 * square' 64) - 1

grains/stack.yaml

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+resolver: lts-15.8

grains/stack.yaml.lock

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+# This file was autogenerated by Stack.
+# You should not edit this file by hand.
+# For more information, please see the documentation at:
+#   https://docs.haskellstack.org/en/stable/lock_files
+
+packages: []
+snapshots:
+- completed:
+    size: 492015
+    url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/15/8.yaml
+    sha256: 926bc3d70249dd0ba05277ff00943c0addb35b627cb641752669e7cf771310d0
+  original: lts-15.8

grains/test/Tests.hs

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+{-# OPTIONS_GHC -fno-warn-type-defaults #-}
+
+import Data.Foldable     (for_)
+import Test.Hspec        (Spec, describe, it, shouldBe)
+import Test.Hspec.Runner (configFastFail, defaultConfig, hspecWith)
+
+import Grains (square, total)
+
+main :: IO ()
+main = hspecWith defaultConfig {configFastFail = True} specs
+
+specs :: Spec
+specs = do
+          describe "square" $ for_ squareCases squareTest
+          describe "total"  $ totalTest totalCase
+  where
+
+    squareTest (description, n, expected) = it description assertion
+      where
+        assertion  = expression `shouldBe` expected
+        expression = fmap fromIntegral . square . fromIntegral $ n
+
+    totalTest (description, expected) = it description assertion
+      where
+        assertion = fromIntegral total `shouldBe` expected
+
+squareCases :: [(String, Integer, Maybe Integer)]
+squareCases =
+    [ ("square 1"             ,  1, Just                   1)
+    , ("square 2"             ,  2, Just                   2)
+    , ("square 3"             ,  3, Just                   4)
+    , ("square 4"             ,  4, Just                   8)
+    , ("square 16"            , 16, Just               32768)
+    , ("square 32"            , 32, Just          2147483648)
+    , ("square 64"            , 64, Just 9223372036854775808)
+    , ("square negative"      , -1, Nothing                 )
+    , ("square 0"             ,  0, Nothing                 )
+    , ("square bigger than 64", 65, Nothing                 ) ]
+
+totalCase :: (String, Integer)
+totalCase = ("total grains", 18446744073709551615)