Looks good. If you can please test it, I can merge.

Ok, I've tested it, and it worked for me in FF 33.0. I can do other browsers if you'd like.

Also how do you feel about getCanvasElementByIdImpl being exported?

Looks solid to me, but yeah ideally getCanvasElementByIdImpl would be hidden.

In that case, could you merge this and my other two PRs, and then I'll hide all the impl functions in one PR afterwards? Just so that I can do it all in one go.

Oops, just noticed, the type of getCanvasElementByIdImpl is a bit wrong. The way I usually type these things (not the only way by any means) is to say something like:

forall r eff.
  Fn3
    String
    (CanvasElement -> r)
    r
    (Eff (canvas :: Canvas | eff) r)

In the current type, the a is underconstrained.