OverloadedStrings Language Pragma

The OverloadedStrings language pragma can be enabled either by passing the -XOverloadedStrings switch to GHC or you can just add the following to the top of your Haskell source:

{-# LANGUAGE OverloadedStrings #-}

The OverloadedStrings language pragma changes the way that literal strings identify themselves, in a way that favours performance. [Char] is a rather cumbersome type to be used when dealing with something as primitive as a string.

Prelude> :t "Hello, world"
"Hello, world" :: [Char]
Prelude> :set -XOverloadedStrings
Prelude> :t "Hello, world"
"Hello, world" :: Data.String.IsString t => t

The literal string "Hello, world" now identifies as a call to the fromString function out of the IstString type class. You can define instances like so:

import GHC.Exts ( IsString(..) )

data Colour = Red | Green | Blue | Other String deriving Show

instance IsString Colour where
  fromString "Red" = Red
  fromString "Green" = Green
  fromString "Blue" = Blue 
  fromString xs = Other xs

Now we just cast our strings to our type, and the fromString functions are invoked for us:

Prelude GHC.Exts> "Red" :: Colour
Red
Prelude GHC.Exts> "Yellow" :: Colour
Other "Yellow"