id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
430	Result type signatures and lexically scoped type variables	nobody	nobody	"{{{
Dear ghc developers,

The ghc documentation (7.4.10.) states that
  ""A lexically scoped type variable can be bound by: 
  [...] A result type signature""

However, actually trying to use them causes some
unexpected behavior:

import Data.Typeable

foo :: Typeable b => b
foo :: a = typeOf (undefined :: a) `seq` (undefined :: a)

bar :: forall b. Typeable b => b
bar :: a = typeOf (undefined :: a) `seq` (undefined :: a)
         
baz :: forall a. Typeable a => a
baz :: a = typeOf (undefined :: a) `seq` (undefined :: a)

All three examples give rise to basically the same
error message, namely

    Inferred type is less polymorphic than expected
      Quantified type variable `b' is mentioned in the
environment:
        Scoped type variable `a' = b (bound at:
test135.hs:4:7)
    When trying to generalise the type inferred for `foo'
      Signature type:     forall b. (Typeable b) => b
      Type to generalise: b
    In the type signature for `foo'
    When generalising the type(s) for `foo'

If I understand the documentation correctly, they
should all compile. An especially interesting case is
'baz', where the 'a' from the result type annotation
seems to shadow the a from the type signature (that
doesn't happen with pattern type annotations).

Another curiosity happens if we alpha-rename foo:

qux :: Typeable a => a
qux :: a = typeOf (undefined :: a) `seq` (undefined :: a)

The error message becomes

    All of the type variables in the constraint
`Typeable a' are already in scope
        (at least one must be universally quantified here)
    In the type signature: qux :: (Typeable a) => a

This is just my wild speculation, but does this really
mean that the 'a' in qux's signature is bound by ""qux
:: a""?

ghc6.2 gives the same error messages, where of course
'bar' behaves like 'foo' and 'baz' like 'qux'.

Thanks you,

-- Thomas Jäger
}}}"	bug	closed	normal		Compiler (Type checker)	None	Invalid