Extra.hs

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-- | Auxiliary functions for working with values of type 'ByteString'.
module Data.ByteString.Extra where

import           Control.Exception    ( catch )
import qualified Data.ByteString.Lazy as B
import           System.IO.Error      ( isDoesNotExistError )

-- * Safe I/O

-- | Safely read a file into a lazy 'ByteString', returning a 'Left' error
-- message if the file cannot be opened.
safeReadFile :: FilePath -> IO (Either String B.ByteString)
safeReadFile fp =
  catch (return <$> B.readFile fp) $ \e ->
    if isDoesNotExistError e
      then return $ Left $ strByteStringFileNotFound fp
      else return $ Left $ strByteStringCannotOpenFile fp

-- ** Error messages

-- | File-not-found message.
strByteStringFileNotFound :: FilePath -> String
strByteStringFileNotFound fp = "File not found: " ++ fp

-- | Cannot-open-file message.
strByteStringCannotOpenFile :: FilePath -> String
strByteStringCannotOpenFile fp = "Error opening file " ++ fp