#5927: A type-level "implies" constraint on Constraints -------------------------------------+------------------------------------- Reporter: illissius | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.4.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Iceland_jack): Use case from [http://ozark.hendrix.edu/~yorgey/pub/twisted.pdf How to Twist Pointers without Breaking Them]:

{{{#!hs class Monoid m => Action m a where act :: m -> a -> a }}} ''[...]''

We first formalise the action of monoids on functors. Intuitively, for a monoid `m` to act on the functor `f`, it should act in a uniform way on all the types `f a`. Therefore, we would like to assert a constraint something like `forall a. Action m (f a)`. Unfortunately, Haskell does not allow the use of universally quantified constraints and hence we need a new type class.

{{{#!hs class (Monoid m, Functor f) => ActionF m f where act' :: m -> f a -> f a }}}

-- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/5927#comment:18 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler