6.5.11. Overloaded record update

OverloadedRecordUpdate

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 Overloaded record dot.

Example:

{-# 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.