Availability of Coercible Instances at TH Runtime

Richard Eisenberg rae at cs.brynmawr.edu
Fri May 4 18:01:52 UTC 2018

I don't think there's an easy way to do this. We could imagine extending Quasi to have a method to check for coercibility, but I don't think there's a way to do this in the current TH API. Sorry!


> On May 4, 2018, at 3:44 AM, Travis Whitaker <pi.boy.travis at gmail.com> wrote:
> Given that Coercible instances are Deeply Magical, perhaps I'm being a bit naive here, but I recently tried to write a TH function that can check if one type is a newtype of another (or a newtype of a newtype of another, etc).
> coercibleToFrom :: Type -> Type -> Q Bool
> coercibleToFrom tx ty = (&&) <$> isInstance ''Coercible [tx, ty]
>                              <*> isInstance ''Coercible [ty, tx]
> If this worked as I'd hoped, I'm almost certain checking reflexively is redundant. However, I can't seem to get reifyInstances to ever return an InstanceDec for a Coercible instance. Given that these instances are generated on the fly by the typechecker, there's no way to make them available at TH runtime, correct? And, given that, is there an easy way to find out with TH whether not I'll be able to use coerce without taking all the Decs apart to hunt for NewtypeD?
> Travis
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