Bugs item #1231273, was opened at 2005-07-01 22:01
Message generated for change (Settings changed) made by simonpj
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Category: Compiler (Type checker)
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Andrew Pimlott (pimlott)
>Assigned to: Simon Peyton Jones (simonpj)
Summary: confusing error
Initial Comment:
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?)".
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