#11995: Can't infer type -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: TypeInType Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by thomie): * keywords: => TypeInType @@ -2,0 +2,5 @@ + {-# LANGUAGE RankNTypes, LambdaCase, TypeOperators, + TypeInType, UnicodeSyntax, GADTs #-} + + module T11995 where + New description: {{{#!hs {-# LANGUAGE RankNTypes, LambdaCase, TypeOperators, TypeInType, UnicodeSyntax, GADTs #-} module T11995 where import Data.Kind data NP :: forall k. (ĸ → Type) → ([k] → Type) where Nil :: NP f '[] (:*) :: f x → NP f xs → NP f (x:xs) newtype K a b = K a deriving Show unK (K a) = a h'collapse :: NP (K a) xs -> [a] h'collapse = \case Nil -> [] K x:*xs -> x : h'collapse xs }}} if we replace `xs` by an underscore: {{{ tJN0.hs:13:29-30: error: … • Could not deduce: (xs :: [ĸ]) ~~ ((':) ĸ x xs :: [ĸ]) from the context: ((k :: *) ~~ (ĸ :: *), (t :: [k]) ~~ ((':) ĸ x xs :: [ĸ])) bound by a pattern with constructor: :* :: forall k (f :: k -> *) (x :: k) (xs :: [k]). f x -> NP k k f xs -> NP k k f ((':) k x xs), in a case alternative at /tmp/tJN0.hs:13:3-9 ‘xs’ is a rigid type variable bound by a pattern with constructor: :* :: forall k (f :: k -> *) (x :: k) (xs :: [k]). f x -> NP k k f xs -> NP k k f ((':) k x xs), in a case alternative at /tmp/tJN0.hs:13:3 Expected type: NP ĸ k (K ĸ a) t Actual type: NP ĸ ĸ (K ĸ a) xs • In the first argument of ‘h'collapse’, namely ‘xs’ In the second argument of ‘(:)’, namely ‘h'collapse xs’ In the expression: x : h'collapse xs • Relevant bindings include xs :: NP ĸ ĸ (K ĸ a) xs (bound at /tmp/tJN0.hs:13:8) h'collapse :: NP ĸ k (K ĸ a) t -> [a] (bound at /tmp/tJN0.hs:11:1) Compilation failed. }}} Should it not be able to infer that? The Glorious Glasgow Haskell Compilation System, version 8.1.20160419 -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11995#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler