Version 1 (modified by goldfire, 3 years ago) (diff) |
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This page discusses problems and solutions that come up when thinking about type family instances with repeated variables on the left-hand side.

## The Problem

Consider the following:

type family F a b type instance F x x = Int type instance F [x] x = Bool type family G type instance G = [G]

Here, `G` is a nullary type family, but its nullariness is just for convenience -- no peculiarity of nullary type families is involved.

These declarations compile just fine in GHC 7.6.3 (with `-XUndecidableInstances`), and on the surface, this seems OK. After all, the two instances of `F` cannot unify. Thus, no usage site of `F` can be ambiguous, right? Wrong. Consider `F G G`. We might simplify this to `Int`, using the first instance, or we might first simplify to `F [G] G` and then to `Bool`. Yuck!

I (Richard/goldfire) have tried to use this inconsistency to cause a seg fault, but after a few hours, I was unable to do so. However, my inability to do so seems more closely related to the fact the type families are strict than anything more fundamental.

It's worth noting that `-XUndecidableInstances` is necessary to exploit this problem. However, we still want `-XUndecidableInstances` programs to be type-safe (as long as GHC terminates).

## The Solution

We need to consider the two instances of `F` to be overlapping and inadmissible. There are a handful of ways to do this, but the best seems to be this: when performing the overlap check, check a version of the instance where all variables are distinct. Using such a check, the two instances for `F` above indeed conflict.

This can break existing code. But, a medium-intensity search did not find *any* uses of non-linear (i.e. with a repeated variable) family instances in existing code, so I think we should be OK. However, a change needs to be made -- the current system is subtly broken and has been so for years.

(Interestingly, proofs of the soundness of the existing system have been published. For example, see here and here.