#4347: Asymmetry of (impredicative) instantiation/higher rank types
-------------------------------+--------------------------------------------
Reporter: dolio | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 7.1 | Keywords:
Testcase: | Blockedby:
Os: Linux | Blocking:
Architecture: x86_64 (amd64) | Failure: GHC rejects valid program
-------------------------------+--------------------------------------------
The new type checker in GHC 7 seems to reject some cases of impredicative
instantiation that prior versions allowed. I was initially alerted to this
by Simon Marlow, who sent a patch for vector-algorithms removing a use of
{{{($)}}} where it would have to be instantiated impredicatively.
Initially, I thought this was due to a planned removal of impredicativity,
but this cannot be the case, because:
{{{
const :: a -> (forall b. b) -> a
}}}
is accepted by the type checker. However, the simple:
{{{
id :: (forall a. a) -> (forall b. b)
}}}
is not, giving an error message:
{{{
Couldn't match type `b' with `forall a. a'
`b' is a rigid type variable bound by
an expression type signature at <interactive>:1:32
In the expression: id :: (forall a. a) -> (forall b. b)
}}}
This would seem to indicate that the type is being rewritten to:
{{{
forall b. (forall a. a) -> b
}}}
and then the {{{forall a. a}}} matched with the bare {{{b}}}. It is, of
course, fine to rewrite the type this way, since the two are isomorphic,
but it is unfortunate that it causes the checker to reject what would
otherwise be a valid instantiation.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4347>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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