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GHC panic with ViewPatterns and GADTs in a proc pattern

    • Closed Issue created by Rafbill @trac-Rafbill

    The following code causes a ghc panic with versions 7.8.4 and 7.9.

    module A where

{-# LANGUAGE GADTs, ViewPatterns, PatternSynonyms, DataKinds, Arrows, TypeOperators, TypeFamilies, UndecidableInstances #-}

import Control.Arrow

data Nat = Z | S Nat
data Vec n a where
  VNil  :: Vec Z a
  VCons :: a -> Vec n a -> Vec (S n) a
viewVNil :: Vec Z a -> ()
viewVNil VNil = ()
viewVCons :: Vec (S n) a -> (a, Vec n a)
viewVCons (VCons a as) = (a, as)
pattern (:>) :: a -> Vec n a -> Vec (S n) a
pattern a :> as <- (viewVCons -> (a, as))
pattern VNil' <- (viewVNil -> ())

type family n + m where
  n + Z   = n
  n + S m = S (n + m)

type family P2 n where
  P2 Z     = S Z
  P2 (S n) = P2 n + P2 n

class A n where
  a :: Arrow b => b (Vec (P2 n) a) a
instance A Z where
  a = proc (a :> VNil) -> returnA -< a

    If the pattern (a :> VNil) is changed to (a :> _) or (a :> VNil'), the code compiles.

    GADTs pattern are not allowed to appear in proc patterns, but view patterns seems to be able to bypass this restriction, #9953 (closed), and the check on subpatterns.

    Trac metadata
    Trac field Value
    Version 7.8.4
    Type Bug
    TypeOfFailure OtherFailure
    Priority normal
    Resolution Unresolved
    Component Compiler
    Test case
    Differential revisions
    BlockedBy
    Related
    Blocking
    CC
    Operating system
    Architecture
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