[julia-users] Pasting an expression inside a macro

[Tim Besard]

I'm trying something with macro's, and I can't understand the following 
behavior:
julia> data = [1 2 3]
1x3 Array{Int64,2}:
 1  2  3

julia> eval(:(println($(length(data)))))
3

julia> eval(parse("println(\$(length(data)))"))
ERROR: unsupported or misplaced expression $
Why do these behave differently?

Placed in context, I'm trying to generate a function from within macro, 
which on its turn generates an expression containing a the result of a 
subexpression evaluated when the function was called. Or, in code:
macro outer(ex)
    ex = Expr(:quote, :(println($ex)))
    fdef = quote
        function inner(data)
            $ex
        end
    end
    eval(fdef)
end

function inner_wanted(data)
    :(println($(length(data))))
end

function main()
    @outer(length(data))

    data = [1 2 3]

    println("What I want:")
    ex = inner_wanted(data)
    println(ex)
    eval(ex)
    
    println("\n\nWhat I have:")
    ex = inner(data)
    println(ex)
    eval(ex)
end

main()

The problem here is that I cannot seem to generate ":($(something))" within 
a quote block... I even tried parse()ing that very construct, as seen in 
the beginning of this mail, but even that fails. Can anybody help me out?

Thanks