Haskell Bytestrings: How to pattern match?

Question

I\'m a Haskell newbie, and having a bit of trouble figuring out how to pattern match a ByteString. The [Char] version of my function looks like:

Answer 1

Patterns use data constructors. http://book.realworldhaskell.org/read/defining-types-streamlining-functions.html

Your empty is just a binding for the first parameter, it could have been x and it would not change anything.

You can't reference a normal function in your pattern so (x cons empty) is not legal. Note: I guess (cons x empty) is really what you meant but this is also illegal.

ByteString is quite different from String. String is an alias of [Char], so it's a real list and the : operator can be used in patterns.

ByteString is Data.ByteString.Internal.PS !(GHC.ForeignPtr.ForeignPtr GHC.Word.Word8) !Int !Int (i.e. a pointer to a native char* + offset + length). Since the data constructor of ByteString is hidden, you must use functions to access the data, not patterns.

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Here a solution (surely not the best one) to your UTF-16 filter problem using the text package:

module Test where

import Data.ByteString as BS
import Data.Text as T
import Data.Text.IO as TIO
import Data.Text.Encoding

removeAll :: Char -> Text -> Text
removeAll c t =  T.filter (/= c) t

main = do
  bytes <- BS.readFile "test.txt"
  TIO.putStr $ removeAll 'c' (decodeUtf16LE bytes)