Move code to Internal module and add tests

hcat.cabal

@@ -6,7 +6,7 @@ cabal-version: 1.12
 
 name:           hcat
 version:        0.0.1.0
-description:    Hcat example from the book 'Effective Haskell'
+description:    HCat example from the book 'Effective Haskell'
 author:         Alexander Kobjolke
 maintainer:     alex@jakalx.net
 copyright:      Alexander Kobjolke 2023
@@ -18,11 +18,16 @@ extra-source-files:
 library
   exposed-modules:
       HCat
+      HCat.Internal
   other-modules:
       Paths_hcat
   hs-source-dirs:
       src
-  ghc-options: -Wall -Wunused-packages
+  default-extensions:
+      GHC2021
+      BlockArguments
+      OverloadedStrings
+  ghc-options: -Wall -Wunused-packages -fdefer-typed-holes
   build-depends:
       base >=4.13 && <5
     , text
@@ -34,9 +39,39 @@ executable hcat
       Paths_hcat
   hs-source-dirs:
       app
-  ghc-options: -Wall -Wunused-packages
+  default-extensions:
+      GHC2021
+      BlockArguments
+      OverloadedStrings
+  ghc-options: -Wall -Wunused-packages -fdefer-typed-holes
   build-depends:
       base >=4.13 && <5
     , hcat
     , text
   default-language: GHC2021
+
+test-suite spec
+  type: exitcode-stdio-1.0
+  main-is: Spec.hs
+  other-modules:
+      HCatSpec
+      Paths_hcat
+  hs-source-dirs:
+      test
+  default-extensions:
+      GHC2021
+      BlockArguments
+      OverloadedStrings
+  ghc-options: -Wall -Wunused-packages -fdefer-typed-holes
+  cpp-options: -DTEST
+  build-tool-depends:
+      hspec-discover:hspec-discover
+  build-depends:
+      QuickCheck
+    , base >=4.13 && <5
+    , hcat
+    , hspec
+    , quickcheck-instances
+    , quickcheck-text
+    , text
+  default-language: Haskell2010

package.yaml

@@ -4,7 +4,7 @@ license: MIT
 author: "Alexander Kobjolke"
 maintainer: "alex@jakalx.net"
 copyright: "Alexander Kobjolke 2023"
-description: "Hcat example from the book 'Effective Haskell'"
+description: "HCat example from the book 'Effective Haskell'"
 
 extra-source-files:
   - README.org
@@ -20,6 +20,12 @@ dependencies:
 ghc-options:
   - -Wall
   - -Wunused-packages
+  - -fdefer-typed-holes
+
+default-extensions:
+  - GHC2021
+  - BlockArguments
+  - OverloadedStrings
 
 library:
   source-dirs: src
@@ -34,3 +40,17 @@ executables:
       - hcat
     verbatim:
       default-language: GHC2021
+
+tests:
+  spec:
+    cpp-options: -DTEST
+    main: Spec.hs
+    source-dirs:
+      - test
+    dependencies:
+      - hcat
+      - hspec
+      - QuickCheck
+      - quickcheck-instances
+      - quickcheck-text
+    build-tools: hspec-discover

src/HCat.hs

@@ -9,27 +9,22 @@ import System.IO (hPutStrLn, stderr)
 import Control.Exception qualified as Exception
 import System.IO.Error qualified as IOError
 
-import Data.Text qualified as Text
 import Data.Text.IO qualified as TextIO
 
-parseArgs :: [String] -> Either String FilePath
-parseArgs args = case args of
-  [] -> Left "No filename given!"
-  [arg] -> Right arg
-  _ -> Left "Only a single file is supported"
+import HCat.Internal (parseArgs)
 
 runHCat :: IO ()
 runHCat = do
-  fileNameOrError <-  parseArgs <$> Env.getArgs
-  fileName <- eitherToError fileNameOrError
-  TextIO.readFile fileName >>= TextIO.putStr
+    fileNameOrError <- parseArgs <$> Env.getArgs
+    fileName <- eitherToError fileNameOrError
+    TextIO.readFile fileName >>= TextIO.putStr
 
 eitherToError :: Show a => Either a b -> IO b
 eitherToError = either (Exception.throwIO . IOError.userError . show) return
 
 handleError :: IOError -> IO ()
 handleError e = do
-  hPutStrLn stderr $ "I ran into an issue: " <> show e
-      
+    hPutStrLn stderr $ "I ran into an issue: " <> show e
+
 defaultMain :: IO ()
 defaultMain = Exception.catch runHCat handleError

src/HCat/Internal.hs

@@ -0,0 +1,8 @@
+-- | Internal module in order to facilitate testability.
+module HCat.Internal where
+
+parseArgs :: [String] -> Either String FilePath
+parseArgs args = case args of
+    [] -> Left "No filename given!"
+    [arg] -> Right arg
+    _ -> Left "Only a single file is supported"

test/HCatSpec.hs

@@ -0,0 +1,15 @@
+{-# LANGUAGE GHC2021 #-}
+
+module HCatSpec (spec) where
+
+import HCat.Internal (parseArgs)
+
+import Test.Hspec
+
+spec :: Spec
+spec = do
+  describe "parseArgs" do
+    it "returns the filename if given a single argument" do
+      parseArgs ["foo"] `shouldBe` Right "foo"
+      parseArgs [] `shouldBe` Left "No filename given!"
+      parseArgs ["foo", "bar"] `shouldBe` Left "Only a single file is supported"

test/Spec.hs

@@ -0,0 +1 @@
+{-# OPTIONS_GHC -F -pgmF hspec-discover #-}