mymap.jl

macro fcall(Xs, nargs)
    res = Expr(:call, :f)
    for i in 1:nargs
        push!(res.args, :($(Xs)[$i][i]))
    end
    return res
end

mymap(f, Xs...) = _map(f, Xs...)

@generated function _map{N}(f, Xs::Vararg{Array, N})
    return quote
        shp = size(zip(Xs...))
        len = prod(shp)
        i = 1
        i <= len || return similar(Xs[1], Base.return_types(f, tuple([ eltype(X) for X in Xs ])))
        v = @fcall Xs $N
        dest = similar(Xs[1], typeof(v))
        dest[i] = v
        map_to!(f, dest, Xs, i+1, len)
    end
end

@generated function map_to!{T, N}(f, dest::Array{T}, Xs::Tuple{Vararg{Array, N}}, offs, len)
    return quote
        i = offs
        while i <= len
            @inbounds el = @fcall Xs $N
            S = typeof(el)
            if S === T || S <: T
                @inbounds dest[i] = el::T
                i += 1
            else
                R = typejoin(T, S)
                new = similar(dest, R)
                copy!(new, 1, dest, 1, i-1)
                @inbounds new[i] = el
                return map_to!(f, new, Xs, i+1, len)
            end
        end
        return dest
    end
end