simonpj 2004/05/12 05:55:14 PDT
Modified files:
ghc/compiler/typecheck TcSimplify.lhs
Log:
---------------------------------
Another delicate fix to the way that the
Dreaded Monomorpism Restriction is handled
---------------------------------
I think this should merge nicely to the STABLE branch
In TcSimplify 1.130 I changed tcSimplifyRestricted (used for
declarations that fall under the monomorphism restriction) to use Plan
C. Unfortunately, it later transpired that George Russel and Serge
Mechveliani both made somewhat-dubious use of overlapping instances
that conflicted with this change. Here's the example
instance (HasBinary ty IO) => HasCodedValue ty
foo :: HasCodedValue a => String -> IO a
doDecodeIO :: HasCodedValue a => () -> () -> IO a
doDecodeIO codedValue view = let { act = foo "" } in act
You might think this should work becuase the call to 'foo' in the last
line gives rise to a constraint (HasCodedValue t), which can be
satisfied by the type sig for doDecodeIO. But the restricted binding
act = ... calls tcSimplifyRestricted, and Plan C simplifies the
constraint using the (rather bogus) instance declaration, and now we
are stuffed.
This commit implements Plan D, which is like plan B except that it does no
improvement, and hence avoids plan B's flaw. See the comments with Plan D.
The only criticism one might make of Plan D is that it may sometimes quantify
a restricted binding over "too few" type variables; but one can solve that
by adding a type sig. So this seems better than the very subtle problems
with Plan C.
All of this is very much at the margin: George and Sergey are pushing
their luck.
Revision Changes Path
1.134 +87 -18 fptools/ghc/compiler/typecheck/TcSimplify.lhs
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