confusing type error

[Dr. ÉRDI Gergő]

But it says `expected Char -> Bool, got Char -> m Bool', note the `m' type
constructor in the second one. So it's not `n' vs. `n'.
On Dec 5, 2014 10:50 PM, "Richard Eisenberg" <eir at cis.upenn.edu> wrote:

> The reason I said "That's a bug!" so confidently is because of the
> "expected n but got n" part. Even if everything else is OK, we need to fix
> that one bit.
>
> And I tend to agree about using heuristics to report better error messages
> in the presence of instantiating a type variable with (->). I've been
> caught and confused by that, too.
>
> Richard
>
> On Dec 4, 2014, at 4:23 PM, Evan Laforge <qdunkan at gmail.com> wrote:
>
> > On Thu, Dec 4, 2014 at 12:59 PM, migmit <migmit at gmail.com> wrote:
> >> It tries to get `m Bool` by applying f1 to three arguments: 0, 0, and
> 'a'. Now, since `f2` has the type `Int -> Float -> n Bool`, where `n` is of
> kind `* -> *` (and an instance of `Monad` class, but it's not yet the time
> to look for instances), we have `f2 0 :: Float -> n Bool` and `f2 0 0 :: n
> Bool`. Since that is applied to 'a', Haskell deduces that the last type
> should be something like `Char -> Something` — or, equivalently, `(->) Char
> Something`. Therefore, it can see that `n` is in fact `(->) Char` and
> `Something` is `Bool`. Therefore, `f2 0 0 'a' :: Bool`. But it is expecting
> `m Bool`, not `Bool` — which is exactly what an error message says.
> >
> > Right, that's what I suspected was happening.  The confusion arrises
> > because it guesses that 'm' should be (->), and that deduction then
> > leads to a dead-end.  But when it reports the problem, it uses its
> > guessed 'm', rather that backing up to the declared value.
> >
> > But surely always backing up to the declared unspecialized value is no
> > good either, because then you get vague errors.  All the compiler
> > knows is that when it simplifies as far as it can, it winds up with a
> > /= b, it doesn't know that I would have been surprised by its path a
> > few steps back.
> >
> > But arity errors are common, and intentionally instantiating a prefix
> > type constructor like 'm a' as (->) is probably much less common.  So
> > perhaps there could be a heuristic that treats (->) specially and
> > includes an extra clause in the error if it unified a type variable to
> > (->)?
> >
> > I suspect the "expected n but got n" error is also due to the same
> > thing, it counts arrows on one side but inferred arrows on the other?
> > Or something?  In any case, it seems like the two sides are counting
> > inconsistently.
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