#5595: Type error: expected type = actual type ???
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Reporter: basvandijk | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.2.1
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: GHC rejects valid program
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Comment(by simonpj):
Thank you for reporting this. In `TcUnify` there is this comment:
{{{
-- Check for escape; e.g. (forall a. a->b) ~ (forall a. a->a)
-- VERY UNSATISFACTORY; the constraint might be fine, but
-- we fail eagerly because we don't have any place to put
-- the bindings from an implication constraint
-- This only works because most constraints get solved on the
fly
-- See Note [Avoid deferring]
}}}
You are the first to trip over this place, where we took a short cut in
the implementation. It's really a bug -- admittedly to do with matching
polymorphic types -- and we think we know how to fix it. It it's not just
a quick fix (else we'd have done it before now).
The problem is the type function under a higher-rank forall. I don't know
a good workaround. We'll get to it, but not instantly. Is it ruining your
life?
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5595#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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