confusing type error

[Brent Yorgey]

Int -> Float -> Char -> Bool  *is* in fact a valid type for f2, since ((->)
Char) is a Monad. However, I agree the error message is confusing,
especially the "expected n, but got n" part.

-Brent

On Thu, Dec 4, 2014 at 1:50 PM, Evan Laforge <qdunkan at gmail.com> wrote:

> I recently got a confusing error msg, and reduced it to a small case:
>
> f1 :: Monad m => m Bool
> f1 = f2 0 0 'a'
>
> f2 :: Monad m => Int -> Float -> m Bool
> f2 = undefined
>
> From this, it's clear that f2 is being given an extra Char argument it
> didn't ask for.  However, the error msg (ghc 7.8.3) is:
>
>     Couldn't match type ‘m Bool’ with ‘Bool’
>     Expected type: Char -> m Bool
>       Actual type: Char -> Bool
>     Relevant bindings include f1 :: m Bool (bound at Bug.hs:4:1)
>     The function ‘f2’ is applied to three arguments,
>     but its type ‘Int -> Float -> Char -> Bool’ has only three
>     In the expression: f2 0 0 'a'
>     In an equation for ‘f1’: f1 = f2 0 0 'a'
>
> The confusing part is that 'f2' was applied to three arguments, but
> it's type has only three.  It includes the Char in expected and actual
> types, and implies that the type of 'f2' includes the Char.  So I took
> quite a while to realize that the type of 'f2' in fact *didn't* expect
> a Char (and had an 'm'), so that the "but its type" is *not* in fact
> its declared type.
>
> I suppose it infers a type for 'f2' based on its use, and that then
> becomes the "actual" type, but it seems less confusing if it picked
> the declared type of 'f2' as its actual type.  Perhaps this is working
> as intended, but it it is confusing!  Especially the part about
> "expected three but got three".
>
> Ideally I'd like to see "too many arguments" or at least "expected
> (Char -> m Bool) but actually 'm Bool'".  Actually I'd expect the
> other way: "expected 'm Bool' but got (Char -> m Bool)' but I think
> ghc has always done it backwards from how I expect.  It looks like
> it's substituting (->) for 'm', so maybe it's one of those things
> where ((->) a) is also a monad.
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