I am attempting to write my first macro. Cairo.jl, following its C roots,
requires a context argument to be added at the start of many of its drawing
functions. This results in lots of typing, which I think could be cleaned
up with a macro.
Here's a snippet from one of the examples[1].
using Cairo
c = CairoRGBSurface(256,256);
cr = CairoContext(c);
...
new_path(cr); # current path is not consumed by cairo_clip()
rectangle(cr, 0, 0, 256, 256);
fill(cr);
set_source_rgb(cr, 0, 1, 0);
move_to(cr, 0, 0);
line_to(cr, 256, 256);
move_to(cr, 256, 0);
line_to(cr, 0, 256);
set_line_width(cr, 10.0);
stroke(cr);
What I would like to be able to do is use a macro to insert a common
argument into each expression. The result might look something like...
using Cairo
c = CairoRGBSurface(256,256)
cr = CairoContext(c)
@withcontext cr, begin
...
new_path()
rectangle(0, 0, 256, 256)
fill()
set_source_rgb(0, 1, 0)
move_to(0, 0)
line_to(256, 256)
move_to(256, 0)
line_to(0, 256)
set_line_width(10.0)
stroke()
end
I have clumsily got this far..
macro withcontext(exprs...)
common_arg = exprs.args[1]
for i in 2:length(exprs)
expr = exprs[i]
insert!(expr, 2, common_arg)
end
return exprs[2:end]
end
..but haven't seen much success. Both lines of this tiny experiment are
being wrapped up as a single expression.
julia> a = [1,2,3,4,5]
julia> @withcontext begin
a
methods()
end
ERROR: type: getfield: expected DataType, got (Expr,)
I'm quite new to Julia and newer to macros - is this kind of thing even
possible? Any suggestions?
With thanks,
Tim McNamara
http://timmcnamara.co.nz
[1] https://github.com/JuliaLang/Cairo.jl/blob/master/samples/sample_clip.jl
