Nice!

Does this look ok to merge?

I think so... I didn't see anything wrong in the code, but my review was pretty superficial. The new errors look awesome! Does the package set / Halogen, etc. still compile?

I tested Halogen but not the whole package set. Let me see ...

If Halogen passes it's probably fine, it's probably one of the harder tests due to the great deal of existentials 😄. I guess maybe lensy things would be the only other thing potentially tripped up by it?

Well Thermite is okay too, which uses lenses quite a bit. But I'll test the package set anyway. All 148 packages is probably a pretty thorough test :)

@garyb Everything builds, except for Data.StrMap, but that's because it actually contains some escaped skolems 😄

The error is this:

Error found:
in module Data.StrMap
at /private/tmp/x/.psc-package/psc-0.10.6/maps/v2.1.1/src/Data/StrMap.purs line 169, column 14 - line 169, column 42

  The type variable h, bound at

    /private/tmp/x/.psc-package/psc-0.10.6/maps/v2.1.1/src/Data/StrMap.purs line 169, column 21 - line 169, column 42

  has escaped its scope, appearing in the type
                             
    forall h e.              
      STStrMap h a24         
      -> Eff                 
           ( st :: ST h      
           | e               
           )                 
           (STStrMap h26 a24)
                             

in the expression \s ->           
                    ((poke s) k) v
in value declaration insert

and you can fix it by inserting a few voids in insert and similar functions. Since this won't go out until 0.11, I think that's probably okay since we can make a new release of maps. Right?

Yeah, I think we have a bunch of breaking changes in the core libs queued up, so makes sense to do that with 0.11 :)

Oh, if it doesn't need a breaking change, but just a fix before this goes out - even better.