How to user-define a type equality constraint?

[Richard Eisenberg]

> On Apr 6, 2021, at 7:44 AM, Anthony Clayden <anthony_clayden at clear.net.nz> 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
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