How to use metaprogramming with function args? - scope

Question

its my second Day learning and experiment with Julia. Although I read the Documantation concerning Metaprogramming carefully (but maybe not carefully enough) and several simular threads. I still can't figure out how I can use it inside a function.
I tryed to make following function for simulation of some data more flexible:
using Distributions
function gendata(N,NLATENT,NITEMS)
latent = repeat(rand(Normal(6,2),N,NLATENT), inner=(1,NITEMS))
errors = rand(Normal(0,1),N,NLATENT*NITEMS)
x = latent+errors
end
By doing this:
using Distributions
function gendata(N,NLATENT,NITEMS,LATENT_DIST="Normal(0,1)",ERRORS_DIST="Normal(0,1)")
to_eval_latent = parse("latent = repeat(rand($LATENT_DIST,N,NLATENT), inner=(1,NITEMS))")
eval(to_eval_latent)
to_eval_errors = parse("error = rand($ERRORS_DIST,N,NLATENT*NITEMS)")
eval(to_eval_errors)
x = latent+errors
end
But since eval don't work on the local scope it dont work. What can I do to work arround this?
Also the originally function, don't seem to be that fast, did I make any major mistakes concerning perfomance?
I really appriciate any recommandation.
Thanks in advance.

Answer 1

There is no need to use eval there, you can retain the same flexibility by passing the distribution types as keyword args (or named args with default values). Parsing and eval'ing "stringly-typed" arguments will often defeat optimizations and should be avoided.
function gendata(N,NLATENT,NITEMS; LATENT_DIST=Normal(0,1),ERRORS_DIST=Normal(0,1))
latent = repeat(rand(LATENT_DIST,N,NLATENT), inner=(1,NITEMS))
errors = rand(ERRORS_DIST,N,NLATENT*NITEMS)
x = latent+errors
end
julia> gendata(10,2,3, LATENT_DIST=Pareto(.3))
...
julia> gendata(10,2,3, ERRORS_DIST=Gamma(.6))
...
etc.

Answer 2

You're not really supposed to use eval here (slower, won't produce type information, will interfere with compilation, etc) but in case you're trying to understand what went wrong, here's how you would do it:
Either separate it from the rest of the code:
function gendata(N,NLATENT,NITEMS,LDIST_EX="Normal(0,1)",EDIST_EX="Normal(0,1)")
# Eval your expressions separately
LATENT_DIST = eval(parse(LDIST_EX))
ERRORS_DIST = eval(parse(EDIST_EX))
# Do your thing
latent = repeat(rand(LATENT_DIST,N,NLATENT), inner=(1,NITEMS))
errors = rand(ERROR_DIST,N,NLATENT*NITEMS)
x = latent+errors
end
Or use interpolation with quoted expressions:
function gendata(N,NLATENT,NITEMS,LDIST_EX="Normal(0,1)",EDIST_EX="Normal(0,1)")
# Obtain expression objects
LATENT_DIST = parse(LDIST_EX)
ERRORS_DIST = parse(EDIST_EX)
# Eval but interpolate in everything that's local to the function
# And you can't introduce local variables with eval so keep them
# out of it.
latent = eval( :(repeat(rand($LATENT_DIST,$N,$NLATENT), inner=(1,$NITEMS))) )
errors = eval( :(rand($ERRORS_DIST, $N, $NLATENT*$NITEMS)) )
x = latent+errors
end
You can also use a single eval with a let block to introduce a self-contained scope:
function gendata(N,NLATENT,NITEMS,LDIST_EX="Normal(0,1)",EDIST_EX="Normal(0,1)")
LATENT_DIST = parse(LDIST_EX)
ERRORS_DIST = parse(EDIST_EX)
x =
#eval let
latent = repeat(rand($LATENT_DIST,$N,$NLATENT), inner=(1,$NITEMS))
errors = (rand($ERRORS_DIST, $N, $NLATENT*$NITEMS))
latent+errors
end
end
((#eval x) == eval(:(x)))
Well, hope you understand the eval thing a little better. Day two I mean, you should be experimenting ;)