#12018: Equality constraint not available in pattern type signature (GADTs/ScopedTypeVariables) -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: lowest | Milestone: Component: Compiler (Type | Version: 7.10.3 checker) | Keywords: GADTs, Resolution: | ScopedTypeVariables 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 RyanGlScott): Replying to [comment:3 simonpj]:
Yes, it is. The type equalities are available only "after" the match.
I must admit that I also find this behavior quite counterintuitive. I stumbled into this when I tried typechecking this code: {{{#!hs {-# LANGUAGE DataKinds #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeOperators #-} module Bug where data family Sing (a :: k) data instance Sing (z :: [a]) where SNil :: Sing '[] SCons :: Sing x -> Sing xs -> Sing (x ': xs) fl :: forall (l :: [a]). Sing l -> Sing l fl SNil = SNil fl (SCons (x :: Sing x) (xs :: Sing xs) :: Sing (x ': xs)) = SCons x xs }}} {{{ GHCi, version 8.2.0.20170623: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Bug.hs:17:5: error: • Couldn't match type ‘l’ with ‘x : xs’ ‘l’ is a rigid type variable bound by the type signature for: fl :: forall a1 (l :: [a1]). Sing l -> Sing l at Bug.hs:15:1-41 Expected type: Sing l Actual type: Sing (x : xs) • When checking that the pattern signature: Sing (x : xs) fits the type of its context: Sing l In the pattern: SCons (x :: Sing x) (xs :: Sing xs) :: Sing (x : xs) In an equation for ‘fl’: fl (SCons (x :: Sing x) (xs :: Sing xs) :: Sing (x : xs)) = SCons x xs • Relevant bindings include fl :: Sing l -> Sing l (bound at Bug.hs:16:1) | 17 | fl (SCons (x :: Sing x) (xs :: Sing xs) :: Sing (x ': xs)) = SCons x xs | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Bug.hs:17:62: error: • Could not deduce: l ~ (x1 : xs1) from the context: (x : xs) ~ (x1 : xs1) bound by a pattern with constructor: SCons :: forall a (x :: a) (xs :: [a]). Sing x -> Sing xs -> Sing (x : xs), in an equation for ‘fl’ at Bug.hs:17:5-39 ‘l’ is a rigid type variable bound by the type signature for: fl :: forall a1 (l :: [a1]). Sing l -> Sing l at Bug.hs:15:1-41 Expected type: Sing l Actual type: Sing (x : xs) • In the expression: SCons x xs In an equation for ‘fl’: fl (SCons (x :: Sing x) (xs :: Sing xs) :: Sing (x : xs)) = SCons x xs • Relevant bindings include xs :: Sing xs (bound at Bug.hs:17:26) x :: Sing x (bound at Bug.hs:17:12) fl :: Sing l -> Sing l (bound at Bug.hs:16:1) | 17 | fl (SCons (x :: Sing x) (xs :: Sing xs) :: Sing (x ': xs)) = SCons x xs | ^^^^^^^^^^ }}} That doesn't typecheck, and yet this does: {{{#!hs fl :: forall (l :: [a]). Sing l -> Sing l fl SNil = SNil fl (SCons (x :: Sing x) (xs :: Sing xs) :: Sing l) = SCons x xs }}} Why not? After all, GHC is smart enough to know how to bind the existentially quantified variables `x` and `xs`, so why can't it conclude that `l ~ x ': xs` from the GADT pattern match? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12018#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler