confusing type error

[Yuras Shumovich]

It seems to be an instance of
https://ghc.haskell.org/trac/ghc/ticket/7869

But it is fixed (both in HEAD and 7.8). Probably the fix is partial?

On Thu, 2014-12-04 at 14:53 -0500, Richard Eisenberg wrote:
> This seems straightforwardly to be a bug, to me. HEAD gives the same behavior you report below. Please post on the bug tracker at https://ghc.haskell.org/trac/ghc/newticket
> 
> Thanks!
> Richard
> 
> On 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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> > 
> 
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