Iterating over different functions with different number of parameters in Julia

Question

I'm trying to run a loop over different functions with different number of arguments. The variables are created at runtime inside the loop, and I want to use eval at each iteration to instantiate a Struct using the variable :symbol. However, I can't do this since eval only works in the global scope. This is the MWE for the case that works:

function f1(x); return x; end
function f2(x1,x2); return x1+x2; end

handles = [f1,f2]
args =[:(x1),:(x1,x2)]

x1 = 1; x2 = 1;
for (i,f) in enumerate(handles)
    params = eval(args[i])
    @show f(params...)
end

f(params...) = 1
f(params...) = 2

However, if I move the variable definitions inside the loop, which is what I actually want, it doesn't work after restarting Julia to clear the workspace.

function f1(x); return x; end
function f2(x1,x2); return x1+x2; end

handles = [f1,f2]
args =[:(x1),:(x1,x2)]

for (i,f) in enumerate(handles)
    x1 = 1; x2 = 1;
    params = eval(args[i])
    @show f(params...)
end

ERROR: UndefVarError: x1 not defined

I've tried several of the answers, such as this one, but I can't seem to make it work. I could write a custom dispatch function that takes[x1,x2] and calls f1 or f2 with the correct arguments. But still, is there any way to do this with eval or with an alternative elegant solution?

EDIT: here are more details as to what I'm trying to do in my code. I have a config struct for each algorithm, and in this I want to define beforehand the arguments it takes

KMF_config = AlgConfig( 
    name = "KMF",
    constructor = KMC.KMF,
    parameters = :(mu,N,L,p),
    fit = KMC.fit!)
MF_config = AlgConfig( 
    name = "MF",
    constructor = KMC.MF,
    parameters = :(mu,N,L),
    fit = KMC.fit!)

alg_config_list = [KMF_config, MF_config]
for (i,alg_config) in enumerate(alg_config_list)
    mu,N,L,p,A,B,C,D,data = gen_vars() #this returns a bunch of variables that are used in different algorithms
    method = alg_config.constructor(eval(method.parameters)...)
    method.fit(data)
end

One possible solution is to have a function take all the variables and method, and return a tuple with a subset of variables according to method.name. But I'm not sure if it's the best way to do it.

can you show an example of the struct generation in the case that works? maybe an alternative solution can be done without the use of `eval` — longemen3000, Dec 24 '19 at 22:30
It doesn't really work since my variables are not global, but I've added an example with what I'm actually trying to do. — Pedro G., Dec 24 '19 at 23:06

Carl Morris · Accepted Answer · 2019-12-25T05:39:17.443

Here's an approach using multiple dispatch rather than eval:

run_a(x, y) = x + 10*y
run_b(x, y, z) = x + 10*y + 100*z

extract(p, ::typeof(run_a)) = (p.x, p.y)
extract(p, ::typeof(run_b)) = (p.x, p.y, p.z)
genvars() = (x=1, y=2, z=3)

function doall()
    todo = [
        run_a,
        run_b,
    ]
    for runalg in todo
        v = genvars()
        p = extract(v, runalg)
        @show runalg(p...)
    end
end

In your example you would replace run_a and run_b with KMC.KMF and KMC.MF.

Edit: Cleaned up example to avoid structs that don't exist in your example.

Iterating over different functions with different number of parameters in Julia

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