I'm using GHC 7.0.2 and running into a compiler error that I cannot
understand. Can anyone shed light on the issue for me? The code does
not make use of GADTs and compiles just fine without them. But when I
add a {-# LANGUAGE GADTs #-} pragma, it fails to compile.
Here is the code:
====
{-# LANGUAGE Rank2Types #-}
{-# LANGUAGE GADTs #-}
mkUnit :: (forall t. t -> m t) -> m ()
mkUnit f = f ()
data Const b a = Const { unConst :: b }
sillyId :: r -> r
sillyId r = unConst (mkUnit mkConst_r) -- This line can't compile with GADTs
where mkConst_r t = mkConst r t
mkConst :: b -> t -> Const b t
mkConst b _ = Const b
====
And the error I get is:
Couldn't match type `t0' with `t'
because type variable `t' would escape its scope
This (rigid, skolem) type variable is bound by
a type expected by the context: t -> Const r t
The following variables have types that mention t0
mkConst_r :: t0 -> Const r t0
(bound at /u/dm/hack/hs/gadt.hs:11:11)
In the first argument of `mkUnit', namely `mkConst_r'
In the first argument of `unConst', namely `(mkUnit mkConst_r)'
In the expression: unConst (mkUnit mkConst_r)
I've found several workarounds for the issue, but I'd like to
understand what the error message means and why it is caused by GADTs.
Thanks in advance for any help.
David
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