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:info IsList

#12697: Improve output of pattern synonym info -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: | PatternSynonyms Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by Iceland_jack: @@ -59,1 +59,1 @@ - pattern List :: () => [] ~~ b => T b + pattern IsList :: () => [] ~~ b => T b New description: {{{#!hs import GHC.Prim import Data.Kind data TypeRep :: forall k. k -> Type where TI :: TypeRep Int TB :: TypeRep Bool TL :: TypeRep [] TA :: TypeRep f -> TypeRep x -> TypeRep (f x) data (a::k1) :~~: (b::k2) where HRefl :: a :~~: a deriving instance Show (a :~~: b) eqTT :: TypeRep (a::k1) -> TypeRep (b::k2) -> Maybe (a :~~: b) eqTT TI TI = Just HRefl eqTT TB TB = Just HRefl eqTT TL TL = Just HRefl pattern IsList <- (eqTT TL -> Just HRefl) where IsList = TL }}} What is the type of `IsList`? The result of `:type` is only accurate when used as an expression (`IsList :: TypeRep []`) and the result of `:info` is uses `~#` {{{ pattern IsList :: forall k2 (b :: k2). () => ((* -> *) ~# k2, [] ~# b) => TypeRep b }}} which won't parse (`parse error on input ‘~#’`). {{{#!hs -- t9R4.hs:20:67: error: … -- • Expected kind ‘* -> *’, but ‘b’ has kind ‘k2’ -- • In the second argument of ‘~’, namely ‘b’ -- Compilation failed. pattern IsList :: forall k2 (b :: k2). () => ((* -> *) ~ k2, [] ~ b) => TypeRep b pattern IsList <- (eqTT TL -> Just HRefl) where IsList = TL }}} The user may try to use `~` but we need [https://hackage.haskell.org/package/ghc-prim-0.5.0.0/docs/GHC- Types.html#t:-126--126- heterogeneous equality from GHC.Types] (`~~`) {{{#!hs pattern IsList :: forall k2 (b :: k2). () => ((* -> *) ~ k2, [] ~~ b) => TypeRep b }}} and in the end, we only need {{{#!hs pattern IsList :: () => [] ~~ b => T b }}} `:info` should show something closer to that! Copy-pasting should work -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12697#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler