install the correct version of cabal (#90)

* two mismatched GHC version numbers were listed in .travis.yml
* bump CI to ghc-8.8.2
* bump CI to cabal-install-3.2 even for older GHCs
* build dependencies in dist-newstyle, do not install them in ~/.ghci
* move the cd command to example.hs
* validate the output of example.hs

Fixes #89

Author: gelisam

.travis.yml

@@ -3,30 +3,28 @@ language: c
 
 matrix:
   include:
-    - env: CABALVER=2.2  GHCVER=8.4.4
-      addons: {apt: {packages: [cabal-install-2.2,  ghc-8.4.4], sources: [hvr-ghc]}}
-    - env: CABALVER=2.4  GHCVER=8.6.5
-      addons: {apt: {packages: [cabal-install-2.4,  ghc-8.6.5], sources: [hvr-ghc]}}
-    - env: CABALVER=2.6  GHCVER=8.8.1
-      addons: {apt: {packages: [cabal-install-2.4,  ghc-8.8.1], sources: [hvr-ghc]}}
-    - env: CABALVER=2.6  GHCVER=8.8.1 CONFIG_OPTS=--enable-executable-dynamic
-      addons: {apt: {packages: [cabal-install-2.4,  ghc-8.8.1], sources: [hvr-ghc]}}
+    - env: CABALVER=3.2  GHCVER=8.4.4
+      addons: {apt: {packages: [cabal-install-3.2,  ghc-8.4.4], sources: [hvr-ghc]}}
+    - env: CABALVER=3.2  GHCVER=8.6.5
+      addons: {apt: {packages: [cabal-install-3.2,  ghc-8.6.5], sources: [hvr-ghc]}}
+    - env: CABALVER=3.2  GHCVER=8.8.2
+      addons: {apt: {packages: [cabal-install-3.2,  ghc-8.8.2], sources: [hvr-ghc]}}
+    - env: CABALVER=3.2  GHCVER=8.8.2 CONFIG_OPTS=--enable-executable-dynamic
+      addons: {apt: {packages: [cabal-install-3.2,  ghc-8.8.2], sources: [hvr-ghc]}}
 
 cache:
   directories:
     - $HOME/.cabal/
     - $HOME/.ghc/
-    - dist/
+    - dist-newstyle/
 
 before_install:
   - export PATH=/opt/ghc/$GHCVER/bin:/opt/cabal/$CABALVER/bin:$PATH
 
 install:
   - travis_retry cabal update
-  - cabal install --only-dependencies --enable-tests
+  - cabal v2-build --only-dependencies --enable-tests
 
 script:
-  - cabal configure --enable-tests $CONFIG_OPTS
-  - cabal test
-  - cabal install
-  - (cd examples && runghc example.hs)
+  - cabal v2-test
+  - cabal v2-exec -- runghc examples/example.hs | diff examples/expected.txt -

examples/example.hs

@@ -2,9 +2,11 @@ import Data.List
 
 import Control.Monad
 import Language.Haskell.Interpreter
+import System.Directory
 
 main :: IO ()
-main = do r <- runInterpreter testHint
+main = do setCurrentDirectory "examples"
+          r <- runInterpreter testHint
           case r of
             Left err -> putStrLn $ errorString err
             Right () -> return ()

examples/expected.txt

@@ -0,0 +1,34 @@
+Load SomeModule.hs
+
+Put the Prelude, Data.Map and *SomeModule in scope
+Data.Map is qualified as M!
+
+Now we can query the type of an expression
+e.g. typeOf M.singleton (f, g, h, 42)
+Num d => a1 -> M.Map ([a2] -> a2, [a3] -> a3, [a4] -> a4, d) a1
+
+Observe that f, g and h are defined in SomeModule.hs, but f is not exported. Let's check it...
+[Fun "g",Fun "h"]
+
+We can also evaluate an expression; the result will be a string
+e.g. eval "length $ concat [[f,g],[h]]"
+"3"
+
+Or we can interpret it as a proper, say, int value!
+3
+
+This works for any monomorphic type, even for function types
+e.g. we interpret \(Just x) -> succ x with type Maybe Int -> Int and apply it on Just 7
+8
+
+And sometimes we can even use the type system to infer the expected type (eg Maybe Bool -> Bool)!
+False
+
+Here we evaluate an expression of type string, that when evaluated (again) leads to a string
+Worked!
+
+We can also execute statements in the IO monad and bind new names, e.g.
+    x <- return 42
+    print x
+42
+