The way the compiler is implemented right now, we can not use foreign functions as parameters (lol). They will then appear as identifiers in the AST, and we currently only look for them in App, in Desugar.hs. A FAIL constructor is emitted. It seems like a big limitation if we can only apply foreign functions, and not use them as parameters to e.g map.

The file good35.cam contains a super small example that duplicates the error.