#7264: Adding GHC's inferred type signatures to a working program can make it
fail
with Rank2Types
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Reporter: guest | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.4.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Comment(by simonpj@…):
commit 10f83429ba493699af95cb8c3b16d179d78b7749
{{{
Author: Simon Peyton Jones <[email protected]>
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).
compiler/typecheck/TcCanonical.lhs | 21 +++---
compiler/typecheck/TcErrors.lhs | 62 +++++++++++++------
compiler/typecheck/TcExpr.lhs | 9 ++-
compiler/typecheck/TcHsType.lhs | 15 +++--
compiler/typecheck/TcMType.lhs | 10 +++-
compiler/typecheck/TcType.lhs | 118
++++++++++++++++++++++++-----------
compiler/typecheck/TcUnify.lhs | 79 +++++++++++++++++++-----
7 files changed, 223 insertions(+), 91 deletions(-)
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7264#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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