6.9.7. Overloaded labels¶
-
OverloadedLabels
¶ Since: 8.0.1 Enable use of the
#foo
overloaded label syntax.
GHC supports overloaded labels, a form of identifier whose interpretation may
depend both on its type and on its literal text. When the
OverloadedLabels
extension is enabled, an overloaded label can be written
with a prefix hash, for example #foo
. The type of this expression is
IsLabel "foo" a => a
.
The class IsLabel
is defined as:
class IsLabel (x :: Symbol) a where
fromLabel :: a
This is rather similar to the class IsString
(see
Overloaded string literals), but with an additional type parameter that makes the
text of the label available as a type-level string (see
Type-Level Literals). Note that fromLabel
had an extra Proxy# x
argument in GHC 8.0, but this was removed in GHC 8.2 as a type application (see
Visible type application) can be used instead.
There are no predefined instances of this class. It is not in scope by default,
but can be brought into scope by importing
GHC.OverloadedLabels. Unlike
IsString
, there are no special defaulting rules for IsLabel
.
During typechecking, GHC will replace an occurrence of an overloaded label like
#foo
with fromLabel @"foo"
. This will have some type alpha
and
require the solution of a class constraint IsLabel "foo" alpha
.
The intention is for IsLabel
to be used to support overloaded record fields
and perhaps anonymous records. Thus, it may be given instances for base
datatypes (in particular (->)
) in the future.
If RebindableSyntax
is enabled, overloaded
labels will be desugared using whatever fromLabel
function is in scope,
rather than always using GHC.OverloadedLabels.fromLabel
.
When writing an overloaded label, there must be no space between the hash sign
and the following identifier. The MagicHash
extension makes use
of postfix hash signs; if OverloadedLabels
and
MagicHash
are both enabled then x#y
means x# y
, but if
only OverloadedLabels
is enabled then it means x #y
. The
UnboxedTuples
extension makes (#
a single lexeme, so when
UnboxedTuples
is enabled you must write a space between an opening
parenthesis and an overloaded label. To avoid confusion, you are strongly
encouraged to put a space before the hash when using
OverloadedLabels
.
When using OverloadedLabels
(or other extensions that make use of
hash signs) in a .hsc
file (see Writing Haskell interfaces to C code: hsc2hs), the hash signs must be
doubled (write ##foo
instead of #foo
) to avoid them being treated as
hsc2hs
directives.
Here is an extension of the record access example in Type-Level Literals showing how an overloaded label can be used as a record selector:
{-# LANGUAGE DataKinds, KindSignatures, MultiParamTypeClasses,
FunctionalDependencies, FlexibleInstances,
OverloadedLabels, ScopedTypeVariables #-}
import GHC.OverloadedLabels (IsLabel(..))
import GHC.TypeLits (Symbol)
data Label (l :: Symbol) = Get
class Has a l b | a l -> b where
from :: a -> Label l -> b
data Point = Point Int Int deriving Show
instance Has Point "x" Int where from (Point x _) _ = x
instance Has Point "y" Int where from (Point _ y) _ = y
instance Has a l b => IsLabel l (a -> b) where
fromLabel x = from x (Get :: Label l)
example = #x (Point 1 2)