A function as an argument in Julia (best practice / code design)

What is the best/most elegant code design in the Julia programming language for the following problem:

The modules mod1, mod2, mod3, ... implement a complicated function fun(a,b,c), which calculates and returns a matrix M.

A function foo of my program needs to calculate the matrix M repeatably via the function fun with varying input parameters a,b,c. However, the user of my program can specify which module is used for fun. Note, that the user is no programmer and only wants to type something like julia program.jl and specifies the used module with a string like "MODULE = mod2" in some input file.

The most naive and inelegant solution would probably be via if...elseif...

function foo(...
[...]
    if SpecifiedModule == "mod1"
        M = mod1.fun(a,b,c)
    elseif SpecifiedModule == "mod2"
        M = mod2.fun(a,b,c)
    SpecifiedModule == "mod3"
        M = mod3.fun(a,b,c)
    end
    # process M

An alternative way would be to use function-pointers. Or even functors as known from C++?

What is best practice in Julia for this case? Is there a way to get rid of this ugly if..elseif... code?

PS: there are similarities to the case of finding an optimized value of a function, like in the module Optim.jl (https://github.com/JuliaNLSolvers/Optim.jl/). If I am not wrong, a user specified function f is passed as an argument to the optimize(f, ...) function.