id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
409	confusing error	pimlott	simonpj	"{{{
I got a perplexing error message.  Here is a concise
example:

    t = ((\Just x -> x) :: Maybe a -> a) (Just 1)
    
    Try.hs:1:6:
        Couldn't match the rigid variable `a' against
`t -> t1'
          `a' is bound by the polymorphic type `forall
a. Maybe a -> a' at Try.hs:1:5-34
          Expected type: a
          Inferred type: t -> t1
        In the expression: (\ Just x -> x) :: Maybe a -> a
        In the definition of `t': t = ((\ Just x -> x)
:: Maybe a -> a) (Just 1)
    Failed, modules loaded: none.

It seems to be telling me that the whole expression ""(\
Just x -> x) ::
Maybe a -> a"" was expected to have type a, in
contradiction to the explicit
type annotation it prints out!  In the context of a
larger program, this
threw me for a loop.  I would have expected

          Expected type: Maybe -> a
          Inferred type: Maybe -> t -> t1

Even better, if I change the code, I get a helpful
diagnostic:

    t = (\Just x -> x) (Just 1)

    Try.hs:1:6:
        Constructor `Just' should have 1 argument, but
has been given 0
        When checking the pattern: Just
        In a lambda abstraction: \ Just x -> x
        In the definition of `t': t = (\ Just x -> x)
(Just 1)
    Failed, modules loaded: none.

Could I get that error in the first example?  You could
probably go even further: ""(did you forget parentheses
around the pattern?)"".
}}}"	bug	closed	normal		Compiler (Type checker)	None	Fixed