Overloaded string literals

In Haskell, the type of a string literal "Hello World" is always String which is defined as [Char] though there are other textual data types such as ByteString and Text. To put it another way, string literals are monomorphic.

GHC provides a language extension called OverloadedStrings. When enabled, literal strings have the type IsString a => a. IsString moudle is defined in Data.String module of base package:

class IsString a where
    fromString :: String -> a

ByteString and Text are examples of IsString instances, so you can declare the type of string literals as ByteString or Text when OverloadedStrings is enabled.

{-# LANGUAGE OverloadedStrings #-}
a :: Text
a = "Hello World"

b :: ByteString
b = "Hello World"

Of course, String is also an instance of IsString. So you can declare the type of a string literal as String as usual.

c :: String
c = "Hello World"