Safe Haskell	Unsafe
Language	Haskell2010

GHC.Magic.Dict.Compat

Description

This module provides a compatibility layer of withDict and WithDict for GHC <9.4. For GHC <9.4, the definitions of WithDict and withDict are slightly different from those of GHC >= 9.4 to prevent user-defined instances.

To actually make withDict work, you have to invoke the accompanying GHC Plugin exposed from GHC.Magic.Dict.Plugin. For example:

{-# LANGUAGE RankNTypes, ScopedTypeVariables, TypeApplications, ConstraintKinds #-}
{-# GHC_OPTIONS -fplugin GHC.Magic.Dict.Plugin #-}
module MyModule where
import GHC.Magic.Dict.Compat

class Given a where
  given :: a

give :: a -> (Given a => r) -> r
give = withDict @(Given a) @a

For GHC >=9.4, this module just re-exports the module GHC.Magic.Dict and the plugin is just a no-op - so you can safely use this package without concerning break anything in newer GHCs.

Synopsis

class WithDict cls meth
withDict :: forall cls meth {rr :: RuntimeRep} (r :: TYPE rr). WithDict cls meth => meth -> (cls => r) -> r

Documentation

class WithDict cls meth Source #

withDict :: forall cls meth {rr :: RuntimeRep} (r :: TYPE rr). WithDict cls meth => meth -> (cls => r) -> r Source #

withDict d f provides a way to call a type-class–overloaded function f by applying it to the supplied dictionary d.

withDict can only be used if the type class has a single method with no superclasses. For more (important) details on how this works, see Note [withDict] in GHC.Tc.Instance.Class in GHC.