[Haskell-cafe] GHCi infers a type but refuses it as type signature
Brandon S. Allbery KF8NH
allbery at ece.cmu.edu
Tue Jun 23 20:33:26 EDT 2009
On Jun 22, 2009, at 20:02 , Eric wrote:
> Non type-variable argument in the constraint: MonadState [a] m
> (Use -XFlexibleContexts to permit this)
> In the type signature for `play3':
> play3 :: (MonadState [a] m, Eq a) => a -> m Bool
> So how is it possible that GHCi can infer (and use) a type that you
> cannot use as signature?
This is an infelicity in the GHC type checker: if a library uses an
extension, inferred types involving things in the library will
silently use the extension even though you haven't declared it --- but
if you try to specify the type manually, you have to explicitly
declare the extension. I think there's a bug report open on it,
because it's poor form (not to mention confusing).
I think this is the right bug.
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university KF8NH
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