Re: [GHC] #11010: Untouchable type, pattern synonyms

23 Oct 2015


      #11010: Untouchable type, pattern synonyms
---------------------------------+------------------------------
        Reporter:  Iceland_jack  |                Owner:
            Type:  bug           |               Status:  new
        Priority:  normal        |            Milestone:
       Component:  Compiler      |              Version:  7.10.2
      Resolution:                |             Keywords:
Operating System:  Linux         |         Architecture:  x86
 Type of failure:  None/Unknown  |            Test Case:
      Blocked By:                |             Blocking:
 Related Tickets:                |  Differential Rev(s):
       Wiki Page:                |
---------------------------------+------------------------------
Description changed by Iceland_jack:

Old description:
...
GHC panicked while experimenting
{{{#!hs
{-# LANGUAGE PatternSynonyms, ExistentialQuantification, GADTSyntax #-}
module Test where
data Expr a where
  Fun :: String -> (a -> b) -> (Expr a -> Expr b)
pattern IntFun :: () => (a ~ Int) => String -> (a -> b) -> (Expr a ->
Expr b)
pattern IntFun str f x = Fun str f x
pattern Suc :: () => (a ~ Int) => Expr a -> Expr Int
pattern Suc n <- IntFun _     _     n where
         Suc n =  IntFun "suc" (+ 1) n
}}}
{{{
% ghc Test.hs
[1 of 1] Compiling Test             ( Test.hs, Test.o )
Test.hs:12:18:
    Couldn't match expected type ‘Int’ with actual type ‘a1’
      ‘a1’ is untouchable
        inside the constraints (a ~ Int)
        bound by the type signature for Suc :: Expr a -> Expr Int
        at Test.hs:12:9-11ghc: panic! (the 'impossible' happened)
  (GHC version 7.10.2 for i386-unknown-linux):
        No skolem info: a1_anA[ssk]
Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
}}}
New description:

 GHC panicked while experimenting

 {{{#!hs
 {-# LANGUAGE PatternSynonyms, ExistentialQuantification, GADTSyntax #-}

 module Test where

 data Expr a where
   Fun :: String -> (a -> b) -> (Expr a -> Expr b)

 pattern IntFun :: () => (a ~ Int) => String -> (a -> b) -> (Expr a -> Expr
 b)
 pattern IntFun str f x = Fun str f x

 -- Alternative syntax for pattern synonyms:
 --   pattern
 --     Suc :: () => (a ~ Int) => Expr a -> Expr Int
 --     Suc n <- IntFun _     _     n where
 --     Suc n =  IntFun "suc" (+ 1) n
 pattern Suc :: () => (a ~ Int) => Expr a -> Expr Int
 pattern Suc n <- IntFun _     _     n where
          Suc n =  IntFun "suc" (+ 1) n
 }}}

 {{{
 % ghc Test.hs
 [1 of 1] Compiling Test             ( Test.hs, Test.o )

 Test.hs:12:18:
     Couldn't match expected type ‘Int’ with actual type ‘a1’
       ‘a1’ is untouchable
         inside the constraints (a ~ Int)
         bound by the type signature for Suc :: Expr a -> Expr Int
         at Test.hs:12:9-11ghc: panic! (the 'impossible' happened)
   (GHC version 7.10.2 for i386-unknown-linux):
         No skolem info: a1_anA[ssk]

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

--

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11010#comment:2
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