Julia create multiple slightly modified versions of a function - function

Question

I have a function that looks like
function eom!(du, u, p)
#views a, b = u[:,1], u[:,2];
#views da, db = du[:,1], du[:,2];
y = # some stuff involving p and a;
da .= f(a, b, y);
db .= g(b, a);
end
I now want to create a second a function that is the exact same, except the last line reads
db .= g(b, y);
How can I do this most cleanly? Of course, I could just copy and paste and give the functions slightly different names, but this seems unideal, especially if, as is plausible, I later want more functions where the second argument of g could be something else. Is there perhaps a way that I could pass into the function eom! an expression (via the argument p) that would specify the second argument of g? Or is there a way I could make some function eom_generator which can output all of the functions that I want? Perhaps macros are the central tool in doing this, but I am not sure.

Answer 1

You could produce a closure:
function eom_generator(g)
return function eom!(du, u, p)
#views a, b = u[:,1], u[:,2]
#views da, db = du[:,1], du[:,2]
y = nothing # some stuff involving p and a;
da .= f(a, b, y)
db .= g(a, b, y)
end
end
const eom1! = eom_generator((a, b, y) -> g(b, a))
const eom2! = eom_generator((a, b, y) -> g(b, y))
But since this is at the core of a differential equation, be sure to test whether you don't have any performance issues that way.
If you decide that you really need metaprogramming, you can use #eval in a loop:
for (i, expr) in enumerate((:(g(b, a)), :(g(b, y))))
#eval function $(Symbol("eom", i, "!"))(du, u, p)
#views a, b = u[:,1], u[:,2]
#views da, db = du[:,1], du[:,2]
y = nothing # some stuff involving p and a;
da .= f(a, b, y)
db .= $expr
end
end
end