Converting From/To String

Haskell has three dominant types of the implementations of strings; String, ByteString and Text(*1). Since String is the default literal of strings, most libraries only support String as strings, even though you can write ByteString or Text directly in a Haskell code with OverloadedStrings pragma.

Haskell has IsString class that means you can convert the type to String, using fromString function. IsString only requires the type to have the fromString function.

-- in Data.String
class IsString a where
  fromString :: String -> a

ToString

I implemented ToString class that you can convert from the type to String by toString function, oppositely to the IsString class and its fromString function.

{-# LANGUAGE TypeSynonymInstances #-}
import qualified Data.Text as T
import qualified Data.ByteString.Char8 as C
import qualified Data.ByteString as B

class ToString a where
  toString :: a -> String

instance ToString String where
  toString = id

instance ToString T.Text where
  toString = T.unpack

instance ToString B.ByteString where
  toString = C.unpack

You can use them like the following way.

{-# LANGUAGE OverloadedStrings #-}
main = do
  putStrLn $ toString ("hello world!" :: String)
  putStrLn $ toString ("hello world!" :: T.Text)
  putStrLn $ toString ("hello world!" :: B.ByteString)

It might remind you about show :: a -> String. show and the toString I defined here have the same type but the purpose and the behaviour are different. show is to give human-readable and machine-parsable String represantation while toString is just to convert from a type which is an implementation of the concept of strings to String.

If you are familiar with Ruby, this may make sense; show in Haskell is inspect in Ruby while toString in Haskell is to_str in Ruby. Note that that's not to_s whom Integer or some other non-string classes also have.

Use case

An example is Network.HTTP.urlEncode function; the function is String -> String but you may want to use it for Text values.

import qualified Data.Text as T
import qualified Network.HTTP as H
urlEncodeText :: T.Text -> T.Text
urlEncodeText = T.pack . H.urlEncode . T.unpack

You can define Text-specific version of urlEncode easily. You can define ByteString-specific version as well in the same way. But you cannot have general one.

It's turn to use the ToString we saw here.

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TypeSynonymInstances #-}
import qualified Data.Text as T
import qualified Data.Text.IO as T
import qualified Data.ByteString.Char8 as C
import qualified Data.ByteString as B
import qualified Network.HTTP as H
import Data.String (fromString, IsString)

main = do
  putStrLn $ urlEncode ("hello world!" :: String)
  T.putStrLn $ urlEncode ("hello world!" :: T.Text)
  B.putStrLn $ urlEncode ("hello world!" :: B.ByteString)

class ToString a where
  toString :: a -> String

instance ToString String where
  toString = id

instance ToString T.Text where
  toString = T.unpack

instance ToString B.ByteString where
  toString = C.unpack

urlEncode :: IsString a => ToString a => a -> a
urlEncode = fromString . H.urlEncode . toString

This urlEncode function defined here can use String, ByteString and Text just by the single function! I just combined IsString and ToString there :)

Footnote

• *1 see also: http://www.slideshare.net/ujihisa/text-manipulation-withwithout-parsec