#11350: Allow visible type application in patterns -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: | TypeApplications PatternSynonyms Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #11385 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Iceland_jack): Use case: with [https://hackage.haskell.org/package/array-0.5.1.1/docs /Data-Array-MArray.html#v:newListArray newListArray] {{{#!hs newListArray :: (MArray a e m, Ix i) => (i, i) -> [e] -> m (a i e) }}} we want to pick an instance of `MArray` many of whom have this form {{{#!hs instance MArray (STUArray s) Word (ST s) instance MArray (STUArray s) Int (ST s) instance MArray (STUArray s) Float (ST s) instance MArray (STUArray s) Bool (ST s) }}} so you try {{{#!hs newListArray @(STArray _s1) @Bool @(ST _s2) @Int :: MArray (STArray t) Bool (ST t1) => (Int, Int) -> [Bool] -> ST t1 (STArray t Int Bool) }}} but we need to unify `_s1` and `_s2` if we want to discharge the constraint, using syntax from ticket:11385#comment:2 {{{#!hs \ @s -> newListArray @(STArray s) @Bool @(ST s) @Int }}} the alternative being `newListArray @(STArray s) @Bool @(ST s) @Int :: forall s. _` (which isn't that bad because there are no additional constraints: if we had left `@Int` off it would not work and we would have to write {{{#!hs newListArray @(STArray s) @Bool @(ST s) :: forall s i. Ix i => (i, i) -> _ }}} instead) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11350#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler