jsaddle-dom import error suggests adding dependency I've already added

Question

When trying to compile my ghcjs/jsaddle based Haskell website, I get

Main.hs:20:1: error:
    Failed to load interface for ‘GHCJS.DOM.Element’
    It is a member of the hidden package ‘jsaddle-dom-0.9.2.0’.
    Perhaps you need to add ‘jsaddle-dom’ to the build-depends in your .cabal file.
    Use -v to see a list of the files searched for.

even though jsaddle-dom is already in the build-depends of my cabal file.

So I can't do what the error message suggests.

What's going on?

Answer 1

You should put build-depends: ghcjs-dom instead of build-depends: jsaddle-dom in your cabal file.

This is because ghcjs-dom is the multiplexer between the real ghcjs-dom-jsffi and jsaddle-dom.

Check out the ghcjs-dom cabal file to see that.

Why is ghc's error message so misleading? Thanks to jonored from the #reflex-frp freenode IRC channel for this explanation:

ghcjs-dom is just a reference to the module in jsaddle on that platform. so it's not where it actually canonically is, and ghc tells you about where it is. But you want ghcjs-dom, so that it tells you the ghcjs-based implementation on ghcjs.