#5650: Type error when annotating a let binding with a forall type
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Reporter: pelotom | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.2.1 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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I'm not sure if this is a bug or not, but it's certainly counterintuitive
to me.
Why does this example fail to type check?
{{{
Prelude> let (x :: forall a. a -> a) = id in x 3
<interactive>:0:31:
Couldn't match expected type `forall a. a -> a'
with actual type `a0 -> a0'
In the expression: id
In a pattern binding: (x :: forall a. a -> a) = id
}}}
I have {{{RankNTypes}}} and {{{ScopedTypeVariables}}} enabled.
I asked this question on StackOverflow:
http://stackoverflow.com/questions/8190431/type-error-when-ascribing-a
-valid-forall-type-to-a-let-bound-variable
User ibid's theory there was this:
"In a let binding let x = id in ..., what happens is that id's type forall
a. a->a gets instantiated into a monotype, say a0 -> a0, which is then
assigned as x's type and is then generalized as forall a0. a0 -> a0. If,
as I think, the pattern type signature is checked before generalization,
it's essentially asking the compiler to verify that the monotype a0 -> a0
is more general than the polytype forall a. a -> a, which it isn't."
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5650>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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