[GHC] #8422: type nats solver is too weak!

[GHC]

#8422: type nats solver is too weak!
-------------------------------------+-------------------------------------
        Reporter:  carter            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  7.7
  checker)                           |
      Resolution:                    |             Keywords:  TypeLits
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by thomie):

 * keywords:   => TypeLits


@@ -5,0 +5,24 @@
+
+ {{{#!hs
+ {-# LANGUAGE DataKinds #-}
+ {-# LANGUAGE GADTs #-}
+ {-# LANGUAGE KindSignatures #-}
+ {-# LANGUAGE TypeOperators #-}
+ module Fancy where
+
+ import GHC.TypeLits
+
+ infixr 3 :*
+
+ data Shape (rank :: Nat) where
+     Nil  :: Shape 0
+     (:*) :: {-# UNPACK #-} !(Int :: *) -> !(Shape r) -> Shape (1 + r)
+
+ reverseShape :: Shape r -> Shape r
+ reverseShape Nil = Nil
+ reverseShape shs = go shs Nil
+     where
+         go :: Shape a -> Shape b -> Shape (a+b)
+         go Nil res = res
+         go (ix :* more ) res = go more (ix :* res)
+ }}}

New description:

 I just built ghc HEAD today, and the type nat solver can't handle the
 attached program, which *should* be simple to check!  (and while I could
 use unsafeCoerce to "prove" it correct, that defeats the purpose of having
 the type nat solver!)

 {{{#!hs
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE TypeOperators #-}
 module Fancy where

 import GHC.TypeLits

 infixr 3 :*

 data Shape (rank :: Nat) where
     Nil  :: Shape 0
     (:*) :: {-# UNPACK #-} !(Int :: *) -> !(Shape r) -> Shape (1 + r)

 reverseShape :: Shape r -> Shape r
 reverseShape Nil = Nil
 reverseShape shs = go shs Nil
     where
         go :: Shape a -> Shape b -> Shape (a+b)
         go Nil res = res
         go (ix :* more ) res = go more (ix :* res)
 }}}

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8422#comment:6>
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