#3043: An unrelated definition monomorphizes a type even without the MR
----------------------------------------+-----------------------------------
Reporter: Deewiant | Owner:
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 6.10.1
Severity: normal | Resolution: invalid
Keywords: | Difficulty: Unknown
Testcase: | Os: Unknown/Multiple
Architecture: Unknown/Multiple |
----------------------------------------+-----------------------------------
Changes (by simonpj):
* status: new => closed
* difficulty: => Unknown
* resolution: => invalid
Comment:
Your final remark isn't right in general. Consider:
{{{
f x = let y = x::Int
in x && True
}}}
This is rejected, because `x` can't be both an `Int` and a `Bool`. But
it'd be accepted if the binding for `y` was dropped. The type checker
takes no notice of whether `y` is mentioned or not. (You might want it
to, but I don't think it matters one way or the other in practice.)
In your example, since `a` and `b` are both mentioned in the right hand
side of `x`, the type of `x` can't be generalised. Suppose `a :: ta` and
`b :: tb`. Then the inferred type for `x` will be something like
{{{
x :: forall c. Id ta c => (c, tb)
}}}
Now the type sig in `y` instantiates `c` to `String`, forces `tb=Int`, and
generates a constraint `(Id ta String)`. Now the fundep on `Id` forces
`ta=String`.
That's as far as I have time to go today. I don't think this has anything
to do with the MR.
Reopen if you disagree.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3043#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler_______________________________________________
Glasgow-haskell-bugs mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
