On Fri, Apr 25, 2008 at 11:49 PM, Brandon S. Allbery KF8NH
On Apr 26, 2008, at 2:36 , Ken Takusagawa wrote:
But this does not:
foo::IO a; foo = do{ (x::a) <- bar; return x;};
Error message: A pattern type signature cannot bind scoped type variables `a' unless the pattern has a rigid type context.
Yeah, using the "forall" is exactly what you want to fix this problem. It puts the type variable in scope throughout the definition of the function.
This works for me (in a slightly out of date HEAD) if I explicitly forall the declaration as per the ghc manual (see section 8.7.6.3):
bar :: forall b. IO b bar = return undefined -- just want a type for now foo :: forall a. IO a
foo = do { (x :: a) <- bar; return x; }
-- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH
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