Extra.hs

-- Copyright 2020 United States Government as represented by the Administrator
-- of the National Aeronautics and Space Administration. All Rights Reserved.
--
-- Disclaimers
--
-- Licensed under the Apache License, Version 2.0 (the "License"); you may
-- not use this file except in compliance with the License. You may obtain a
-- copy of the License at
--
--      https://www.apache.org/licenses/LICENSE-2.0
--
-- Unless required by applicable law or agreed to in writing, software
-- distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
-- WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
-- License for the specific language governing permissions and limitations
-- under the License.
--
-- | 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