#1634: Type signature normalization
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Reporter: [EMAIL PROTECTED] | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 6.6.1
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Os: Linux | Testcase:
Architecture: x86 |
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Old description:
> {{{
> t1 :: a->(forall b. b->(a,b)) -- isn't this the same as forall a b.
> a->b->(a,b)
> t1 = (,)
> }}}
> causes
> {{{
> Inferred type is less polymorphic than expected
> Quantified type variable `b' escapes
> Probable cause: `(,)' is applied to too few arguments
> In the expression: (,)
> In the definition of `t1': t1 = (,)
> }}}
>
> The problem goes away without the explicit signature.
> -- Kefer
New description:
{{{
t1 :: a->(forall b. b->(a,b))
-- isn't this the same as forall a b. a->b->(a,b)
t1 = (,)
}}}
causes
{{{
Inferred type is less polymorphic than expected
Quantified type variable `b' escapes
Probable cause: `(,)' is applied to too few arguments
In the expression: (,)
In the definition of `t1': t1 = (,)
}}}
The problem goes away without the explicit signature.
-- Kefer
Comment (by simonpj):
Currently, the two types are not the same. They '''used''' to be the
same; see Section 4.6 of [http://research.microsoft.com/~simonpj/papers
/higher-rank/index.htm Practical type inference for higher rank types];
but adding inference for impredicative types meant we had to remove deep
skolemisation.
See also #1123.
I'm interested in gathering evidence for whether this change really
matters to people. Did this arise in a real program?
Incidentally, another fix is just to eta-expand:
{{{
t1 :: a->(forall b. b->(a,b))
t1 x y = (,) x y
}}}
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1634>
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