Bug or feature?

[Iavor Diatchki]

Hi,

thanks for the detailed response---I thought something like that might
be happening, which is why I thought I'd better ask before reporting a
bug.

I wonder how we might make the error message a little more user
friendly.   One idea would be to compute a couple of examples to show,
after an ambiguity check fails.

-Iavor

On Sat, May 18, 2019 at 7:03 AM Ryan Scott <ryan.gl.scott at gmail.com> wrote:
>
> Hi Iavor,
>
> This is a feature in the sense that it is an expected outcome of the
> OutsideIn(X) type inference algorithm (in particular, Section 5 of the
> corresponding paper [1]). I'll try to explain the issue to the best of
> my understanding.
>
> The tricky thing about the Q3 data type:
>
>     data Q3 = forall a. Q3 (forall b. (F b ~ Int) => T a b)
>
> Is that there is a rank-n type with a /local/ equality constraint. In
> order to ensure that the type of the Q3 data constructor is never
> ambiguous, GHC tries to infer that Q3's type is always a subtype of
> itself. As a part of this, GHC attempts to solve the following
> implication constraint:
>
>     forall a. forall b. (F b ~ Int) => (a ~ alpha)
>
> Where `alpha` is a unification variable. GHC tries to find a unique,
> most-general substitution that solves this constraint but ultimately
> gives up and reports the "untouchable" error message you observed.
>
> This is a bit strange, however. Upon first glance, this constraint
> would appear to have a unique solution: namely, [alpha |-> a]. Why
> doesn't GHC just use this? Ultimately, it's because OutsideIn(X) is
> conservative: there may exist constraints that admit a unique solution
> which it may fail to solve. This is explained in greater depth in
> Section 5.2 of the paper, but the essence of this restriction is
> because of the open-world assumption for type families. Namely, it's
> possible that your Q3 data type might later be linked against code
> which defines the following type family:
>
>     type family G i a
>     type instance G Int a = a
>
> If that were the case, then the implication constraint above would no
> longer have a unique solution, since there would exist another
> substitution [alpha |-> G (F b) a] that is incomparable with [alpha
> |-> a]. OutsideIn(X) was designed to only pick unique solutions that
> remain unique solutions even if more axioms (i.e., type family
> equations) are added, so for these reasons it fails to solve the
> implication constraint above.
>
> See also GHC issues #10651 [2], #14921 [3], and #15649 [4], which all
> involve similar issues.
>
> -----
>
> I'm far from an expert on type inference, so I can't really offer a
> better inference algorithm that would avoid this problem. The best you
> can do, as far as I can tell, is to enable AllowAmbiguousTypes and
> hope for the best. Even then, you'll still run into situations where
> untouchability rears its ugly head, as in your `q3` definition. When
> that happens, your only available option (again, as far as I can tell)
> is to make use of TypeApplications. For example, the following /does/
> typecheck:
>
>     q3 :: Q3
>     q3 = Q3 @Bool t
>
> Hope that helps,
>
> Ryan S.
> -----
> [1] https://www.microsoft.com/en-us/research/wp-content/uploads/2016/02/jfp-outsidein.pdf
> [2] https://gitlab.haskell.org/ghc/ghc/issues/10651
> [3] https://gitlab.haskell.org/ghc/ghc/issues/14921
> [4] https://gitlab.haskell.org/ghc/ghc/issues/15649
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