On Apr 6, 2021, at 7:44 AM, Anthony Clayden wrote:

...

But why does this matter?

Because I want the semantics of that equality constraint, without switching on any of these, which I don't otherwise use:

GADTs TypeFamilies TypeOperators

And if that means I can't use infix `~` in my constraints, I'll put up with that. (I'd user-define a conventional class, say `TypeCast`. Could be that has to be defined in terms of `~`, in which case I'd put that in a shim module as the only place with those extensions.)

A shim module would work for you here, indeed. Individuals' opinions of extensions will differ, but TypeOperators is much less powerful than either of the others: it affects only parsing, and in a fully backward-compatible way. Richard