Repository : ssh://darcs.haskell.org//srv/darcs/ghc On branch : master
http://hackage.haskell.org/trac/ghc/changeset/10f83429ba493699af95cb8c3b16d179d78b7749 >--------------------------------------------------------------- commit 10f83429ba493699af95cb8c3b16d179d78b7749 Author: Simon Peyton Jones <simo...@microsoft.com> 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(-) Diff suppressed because of size. To see it, use: git show 10f83429ba493699af95cb8c3b16d179d78b7749 _______________________________________________ Cvs-ghc mailing list Cvs-ghc@haskell.org http://www.haskell.org/mailman/listinfo/cvs-ghc
