Re: Type family constraints

On Apr 23, 2018, at 6:56 AM, Alan & Kim Zimmerman 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