GADTs: (.text+0xc6): undefined reference to `Main_IsEqual_con_info'

[igloo]

With this module:

{-# LANGUAGE ScopedTypeVariables, GADTs #-}

import Unsafe.Coerce

data IsEqual a b where
    IsEqual :: IsEqual a a

contextsEqual :: cxt1 -> cxt2 -> IsEqual cxt1 cxt2
contextsEqual = unsafeCoerce IsEqual

data Seq from to where
    Nil :: Seq here here

foo :: forall from to . Seq from to
foo = f undefined undefined
    where f :: from -> to -> Seq from to
          f x y = case contextsEqual x y of
                  IsEqual -> Nil

main :: IO ()
main = case foo :: Seq () () of
           Nil -> return ()

I get:

$ ghc --make z -fforce-recomp -O
[1 of 1] Compiling Main             ( z.hs, z.o )
Linking z ...
z.o: In function `smk_info':
(.text+0xc6): undefined reference to `Main_IsEqual_con_info'
collect2: ld returned 1 exit status

with the HEAD and 6.8.2.

[simonpj]

My compiler, compiled with -DDEBUG, gets an assert error.

What do you expect? You're doing

(unsafeCoerce IsEqual) undefined undefined

that is, applying a non-function as a function. Would you prefer a seg fault? Unsafe coerce is, as it says, unsafe.

What would be the right behaviour? A type error? Presumably not, since you have consciously suppressed that.

Simon

[igloo]

Whoops, you're quite right! I meant to say

contextsEqual _ _ = unsafeCoerce IsEqual

which works fine; sorry for the noise!