confusing type error

Richard Eisenberg eir at
Fri Dec 5 14:49:02 UTC 2014

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.


On Dec 4, 2014, at 4:23 PM, Evan Laforge <qdunkan at> wrote:

> On Thu, Dec 4, 2014 at 12:59 PM, migmit <migmit at> 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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