Add Readable typeclass to simplify safe reading of files

Why?
====

After the introduction of:

    safeReadFile :: FilePath -> IO (Either E.IOException String)

There were places who needed to repack data into the appropriate type.
This is an extra step and, while in these situations not necessarily
*slow*, should be removed.

This introduces a Readable typeclass which dictates implementation of
`readFile'` based on the resulting requested type. This means `safeReadFile`
can be used in various situations without having to `pack`.

src/Unused/Cache.hs

@@ -9,7 +9,6 @@ import Data.Csv (FromRecord, ToRecord, HasHeader(..), encode, decode)
 import Data.Vector (toList)
 import System.Directory (createDirectoryIfMissing)
 import qualified Data.ByteString.Lazy as BS
-import qualified Data.ByteString.Lazy.Char8 as C
 import Unused.Cache.DirectoryFingerprint
 import Unused.Util (safeReadFile)
 
@@ -36,7 +35,7 @@ readCache = do
 
     either
         (const Nothing)
-        (processCsv . decode NoHeader . C.pack)
+        (processCsv . decode NoHeader)
         <$> (liftIO $ safeReadFile fileName)
   where
     processCsv = either (const Nothing) (Just . toList)

src/Unused/ResultsClassifier/Config.hs

@@ -4,7 +4,6 @@ module Unused.ResultsClassifier.Config
     ) where
 
 import qualified Data.Yaml as Y
-import qualified Data.ByteString.Char8 as C
 import qualified Data.Either as E
 import qualified Data.Bifunctor as B
 import System.FilePath ((</>))
@@ -19,7 +18,7 @@ loadConfig = do
 
     either
         (const $ Left "default config not found")
-        (Y.decodeEither . C.pack)
+        Y.decodeEither
         <$> safeReadFile configFileName
 
 loadAllConfigurations :: IO (Either [ParseConfigError] [LanguageConfiguration])
@@ -38,7 +37,7 @@ loadAllConfigurations = do
 
 loadConfigFromFile :: String -> IO (Either ParseConfigError [LanguageConfiguration])
 loadConfigFromFile path = do
-    file <- fmap C.pack <$> safeReadFile path
+    file <- safeReadFile path
     return $ case file of
         Left _ -> Right []
         Right body -> addSourceToLeft path $ Y.decodeEither body

src/Unused/Util.hs

@@ -1,3 +1,6 @@
+{-# LANGUAGE TypeSynonymInstances #-}
+{-# LANGUAGE FlexibleInstances #-}
+
 module Unused.Util
     ( groupBy
     , stringToInt
@@ -10,6 +13,8 @@ import qualified Control.Exception as E
 import qualified Data.List as L
 import Data.Function
 import Data.Char (digitToInt, isDigit)
+import qualified Data.ByteString.Lazy.Char8 as Cl
+import qualified Data.ByteString.Char8 as C
 
 groupBy :: (Ord b) => (a -> b) -> [a] -> [(b, [a])]
 groupBy f = map (f . head &&& id)
@@ -27,5 +32,17 @@ stringToInt xs
   where
     loop = foldl (\acc x -> acc * 10 + digitToInt x)
 
-safeReadFile :: FilePath -> IO (Either E.IOException String)
-safeReadFile = E.try . readFile
+class Readable a where
+    readFile' :: FilePath -> IO a
+
+instance Readable String where
+    readFile' = readFile
+
+instance Readable C.ByteString where
+    readFile' = C.readFile
+
+instance Readable Cl.ByteString where
+    readFile' = Cl.readFile
+
+safeReadFile :: Readable s => FilePath -> IO (Either E.IOException s)
+safeReadFile = E.try . readFile'