Storing evaled function inside a struct

General Usage
7

You are right, I had hoped it would be easier without this but I guess I’ll start from the beginning:

I have a bunch of functions and a cost function

f1(a, b) = ...
f2(a, d) = ...
f3(b, c) = ...
cost(a, b, c, d) = f1(a, b) + f2(a, d) + f3(b, c)

with some other equations to calculate the values a, b, c, d:

a = 2d
c = a * b
...

Now the user has the choice of setting some of those values as fixed, some to be in some range, some as free (do with these whatever). Then I want to find the best values for those parameters I can change via optimization.
If I set a as fixed and b to be in some range (to be optimized), I can get both c and d from the equations above, so my function would look like

function (x)
    a = 2  # the const value I want
    d = 1  # == a / 2;  I get this value by evaluating this equation while generating the expressions
    b = x[1]
    c = a * b
    cost(a, b, c, d)
end

If I only set b to be fixed, d can be optimized and the function will look like

function (x)
    b = 5  # the const value I want
    d = x[1]
    a = 2d
    c = a * b
    cost(a, b, c, d)
end

The resolving of these dependencies takes some time, which is why I wish to do this once instead on doing this in the cost function.

I don’t know how to do this just with functions without a big performance penalty to the cost function, that’s why I create the function from expressions and need the eval.