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 <[email protected]> wrote: > On Thu, Dec 4, 2014 at 12:59 PM, migmit <[email protected]> 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. > _______________________________________________ > Glasgow-haskell-users mailing list > [email protected] > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > _______________________________________________ Glasgow-haskell-users mailing list [email protected] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
