#8053: unification error with ghc head
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Reporter: carter | Owner:
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 7.7
Resolution: invalid | Keywords:
Operating System: Unknown/Multiple | Architecture:
Type of failure: None/Unknown | Unknown/Multiple
Test Case: | Difficulty: Unknown
Blocking: | Blocked By:
| Related Tickets:
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Changes (by thoughtpolice):
* status: new => closed
* resolution: => invalid
Comment:
This is not a bug. It is the expected behaviour in HEAD. The relevant
commit is 10f83429ba493699af95cb8c3b16d179d78b7749.
{{{
commit 10f83429ba493699af95cb8c3b16d179d78b7749
Author: Simon Peyton Jones
Date: Wed Oct 31 09:08:39 2012 +0000
Do not instantiate unification variables with polytypes
Without -XImpredicativeTypes, the typing rules say that a function
should be instantiated only at a *monotype*. In implementation terms,
that means that a unification variable should not unify with a type
involving foralls. But we were not enforcing that rule, and that
gave rise to some confusing error messages, such as
Trac #7264, #6069
This patch adds the test for foralls. There are consequences
* I put the test in occurCheckExpand, since that is where we see if a
type can unify with a given unification variable. So
occurCheckExpand has to get DynFlags, so it can test for
-XImpredicativeTypes
* We want this to work
foo :: (forall a. a -> a) -> Int
foo = error "foo"
But that means instantiating error at a polytype! But error is
special
already because you can instantiate it at an unboxed type like
Int#.
So I extended the specialness to allow type variables of
openTypeKind
to unify with polytypes, regardless of -XImpredicativeTypes.
Conveniently, this works in TcUnify.matchExpectedFunTys, which
generates
unification variable for the function arguments, which can be
polymorphic.
* GHC has a special typing rule for ($) (see Note [Typing rule
for ($)] in TcExpr). It unifies the argument and result with a
unification variable to exclude unboxed types -- but that means I
now need a kind of unificatdion variable that *can* unify with a
polytype.
So for this sole case I added PolyTv to the data type
TcType.MetaInfo.
I suspect we'll find mor uses for this, and the changes are tiny,
but it still feel a bit of a hack. Well the special rule for ($)
is a hack!
There were some consequential changes in error reporting (TcErrors).
}}}
The fix is simple, and you don't need ImpredicativeTypes: change the
definition of anyContM in the instances from
{{{
anyContToM = lift . anyContToM
}}}
to:
{{{
anyContToM x = lift $ anyContToM x
}}}
--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8053#comment:2
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler
