6.9.6. Overloaded string literals


Enable overloaded string literals (e.g. string literals desugared via the IsString class).

GHC supports overloaded string literals. Normally a string literal has type String, but with overloaded string literals enabled (with OverloadedStrings) a string literal has type (IsString a) => a.

This means that the usual string syntax can be used, e.g., for ByteString, Text, and other variations of string like types. String literals behave very much like integer literals, i.e., they can be used in both expressions and patterns. If used in a pattern the literal will be replaced by an equality test, in the same way as an integer literal is.

The class IsString is defined as:

class IsString a where
    fromString :: String -> a

The only predefined instance is the obvious one to make strings work as usual:

instance IsString [Char] where
    fromString cs = cs

The class IsString is not in scope by default. If you want to mention it explicitly (for example, to give an instance declaration for it), you can import it from module Data.String.

Haskell’s defaulting mechanism (Haskell Report, Section 4.3.4) is extended to cover string literals, when OverloadedStrings is specified. Specifically:

  • Each type in a default declaration must be an instance of Num or of IsString.
  • If no default declaration is given, then it is just as if the module contained the declaration default( Integer, Double, String).
  • The standard defaulting rule is extended thus: defaulting applies when all the unresolved constraints involve standard classes or IsString; and at least one is a numeric class or IsString.

So, for example, the expression length "foo" will give rise to an ambiguous use of IsString a0 which, because of the above rules, will default to String.

A small example:

module Main where

import Data.String( IsString(..) )

newtype MyString = MyString String deriving (Eq, Show)
instance IsString MyString where
    fromString = MyString

greet :: MyString -> MyString
greet "hello" = "world"
greet other = other

main = do
    print $ greet "hello"
    print $ greet "fool"

Note that deriving Eq is necessary for the pattern matching to work since it gets translated into an equality comparison.