6.9.6. Overloaded string literals¶
-
OverloadedStrings
¶ Since: 6.8.1 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 ofNum
or ofIsString
. - If no
default
declaration is given, then it is just as if the module contained the declarationdefault( 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 orIsString
.
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.