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.. _overloaded-record-update:
Overloaded record update
------------------------
.. extension:: OverloadedRecordUpdate
:shortdesc: Record '.' syntax record updates
:since: 9.2.0
Provides record '.' syntax in record updates e.g. ``x{foo.bar = 1}``.
**EXPERIMENTAL**
*This design of this extension may well change in the future. It would be inadvisable to start using this extension for long-lived libraries just yet.*
It's usual (but not required) that this extension be used in conjunction with :ref:`overloaded-record-dot`.
Example:
.. code-block:: haskell
{-# LANGUAGE AllowAmbiguousTypes, FunctionalDependencies, ScopedTypeVariables, PolyKinds, TypeApplications, DataKinds, FlexibleInstances #-}
{-# LANGUAGE NamedFieldPuns, RecordWildCards #-}
{-# LANGUAGE OverloadedRecordDot, OverloadedRecordUpdate, RebindableSyntax #-}
import Prelude
class HasField x r a | x r -> a where
hasField :: r -> (a -> r, a)
getField :: forall x r a . HasField x r a => r -> a
getField = snd . hasField @x -- Note: a.x = is getField @"x" a.
setField :: forall x r a . HasField x r a => r -> a -> r
setField = fst . hasField @x -- Note : a{x = b} is setField @"x" a b.
data Person = Person { name :: String } deriving Show
instance HasField "name" Person String where
hasField r = (\x -> case r of Person { .. } -> Person { name = x, .. }, name r)
data Company = Company { company :: String, owner :: Person } deriving Show
instance HasField "company" Company String where
hasField r = (\x -> case r of Company { .. } -> Company { company = x, .. }, company r)
instance HasField "owner" Company Person where
hasField r = (\x -> case r of Company { .. } -> Company { owner = x, .. }, owner r)
main = do
let c = Company {company = "Acme Corp.", owner = Person { name = "Wile E. Coyote" }}
-- Top-level update
print $ c{company = "Acme United"} -- Company {company = "Acme United", owner = Person {name = "Wile E. Coyote"}}
-- Nested update
print $ c{owner.name = "Walter C. Johnsen"} -- Company {company = "Acme Corp.", owner = Person {name = "Walter C. Johnsen"}}
-- Punned update
let name = "Walter C. Johnsen"
print $ c{owner.name} -- Company {company = "Acme Corp.", owner = Person {name = "Walter C. Johnsen"}}
``OverloadedRecordUpdate`` works by desugaring record ``.`` update expressions to expressions involving the functions ``setField`` and ``getField``. Note that all record updates will be desugared to ``setField`` expressions whether they use ``.`` notation or not.
At this time, ``RebindableSyntax`` must be enabled when ``OverloadedRecordUpdate`` is and users are required to provide definitions for ``getField`` and ``setField``. We anticipate this restriction to be lifted in a future release of GHC with builtin support for ``setField``.
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