Extra.hs

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-- | Auxiliary functions for working with values of type 'String'.
module Data.String.Extra
    (
      -- * Safe I/O
      safeReadFile

      -- * String sanitization
    , sanitizeLCIdentifier
    , sanitizeUCIdentifier
    )
  where

-- External imports
import Control.Exception ( catch )
import Data.Char         ( toLower, toUpper )
import System.IO.Error   ( isDoesNotExistError )

-- * Safe I/O

-- | Safely read a file into a 'String', returning a 'Left' error message if
-- the file cannot be opened.

-- This function could also be placed in System.IO. However, we decide to
-- include it in this module for symmetry with an analogous function for
-- ByteString, included in Data.ByteString.Extra.
safeReadFile :: FilePath -> IO (Either String String)
safeReadFile fp =
  catch (return <$> readFile fp) $ \e ->
    if isDoesNotExistError e
      then return $ Left $ strStringFileNotFound fp
      else return $ Left $ strStringCannotOpenFile fp

-- ** Error messages

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

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

-- * Sanitization

-- | Remove extraneous characters from an identifier and make the starting
-- character lowercase.
--
-- This function currently replaces hyphens with underscores.
sanitizeLCIdentifier :: String -> String
sanitizeLCIdentifier = headToLower . map sanitizeCharacter
  where
    sanitizeCharacter '-' = '_'
    sanitizeCharacter x   = x

    headToLower :: [Char] -> [Char]
    headToLower []     = []
    headToLower (x:xs) = toLower x : xs

-- | Remove extraneous characters from an identifier and make the starting
-- character uppercase.
--
-- This function currently replaces hyphens with underscores.
sanitizeUCIdentifier :: String -> String
sanitizeUCIdentifier = headToUpper . map sanitizeCharacter
  where
    sanitizeCharacter '-' = '_'
    sanitizeCharacter x   = x

    headToUpper :: [Char] -> [Char]
    headToUpper []     = []
    headToUpper (x:xs) = toUpper x : xs