[julia-users] Metaprogramming question

[Christoph Ortner]

This is my first attempt at metaprogramming, so apologies if this should be 
obvious from the documentation (or just obvious):

For defining an interface I am implementing "dummy functions" which just 
return a tailored error message if an interface function has not been 
implemented. At the moment this largely duplicates the standard Julia 
behaviour, but it just gives a little more information (e.g. the 
information that a certain function ought to be part of an interface), but 
long-term I want to give it additional functionality, e.g. more information.

Because I am implementing many of these interface functions, I tried to 
write a macro:

module Test
using Docile, Lexicon, Compat
export ffun

macro protofun(fname, argtypes...)
    # preprocess the arguments to generate an error message
    errmsg = "AbstractAtomsInterface: `$(string(fname))("
    for n = 1:length(argtypes)-1
        errmsg *= string(argtypes[n]) * ", "
    end
    errmsg = errmsg[1:end-2] * ")' has no implementation."
    
    eval(quote
         @doc $(argtypes[end])->
        function $(fname)($(argtypes[1:end-1]...))
            error($errmsg)
        end
    end)
end

@protofun(ffun, ::Number, ::String, doc"yads")
end


When I run this:


julia> using Test

julia> ffun(1.0, "abc")
ERROR: AtomsInterface: `ffun(::Number, ::String)' has no implementation.
 in ffun at /Users/ortner/gits/Atoms.jl/Test.jl:20


What I would like to see is 

ERROR: AtomsInterface: `ffun(::Float64, ::ASCIIString)' has no 
implementation.
 in ffun at /Users/ortner/gits/Atoms.jl/Test.jl:20

It is clear to me why my implementation does not work as I hope, but I 
don't see how to make this work. I would be grateful about any suggestions.

Many thanks,
Christoph