Re: Expanding a particular type family in type errors

On May 5, 2020, at 04:00, Richard Eisenberg wrote:

reportWanteds reports errors, yes, but it also fills in deferred type errors. [snip] It's a bit awkward that a function called reportWanteds does this, but it actually makes good sense, because reportWanteds knows the text that should be in the runtime error call.

Yes, that does make sense; thank you for the explanation.

You say that they're not, in general, fully known. But are they often known in practice? My suggestion here isn't to remove the type families entirely, but maybe to short-circuit around them. If that's often possible, it takes some pressure off fixing the pretty-printing.

You’re right that this might be feasible, but I think it would be awkward, and it probably would break down just often enough to be an unsatisfying solution.

I wonder if we should have a preprocessTypeForPrinting :: DynFlags -> Type -> Type function. This could be run *before* the conversion to IfaceType. It could handle both your new idea and the existing idea for -f(no-)print-explicit-runtime-reps. The supplied DynFlags could be gotten from sdocWithDynFlags, I think. I bet that once we have this facility working, we'll find more uses for it.

If you think this is a reasonable idea, I’m happy to give it a shot—it definitely seems like the nicest way to do it to me. I’ll try it out and report back if I run into any trouble. Thanks again! Alexis