Type family constraints

[Richard Eisenberg]

> On Apr 23, 2018, at 6:56 AM, Alan & Kim Zimmerman <alan.zimm at gmail.com> wrote:
> 
> Is there any way to express that `pass` must be valid for each value of `Pass` in the following instance head?
> 

No, there isn't. And for good reason: whatever you're trying to do likely requires some runtime decision-making, and that's impossible with just type-level information. (Even without that motivation, there's this: any way of doing this would likely break type soundness. See #7259 and #14420.)

My recommendation is to have

> class ValidPass p
> instance ValidPass Parsed
> instance ValidPass Renamed
> instance ValidPass Typechecked

My guess is that you'll need to add a method to ValidPass, anyway.

Richard
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