No, that's not possible—expr( can only handle functions that return a value (no storing or executing commands that start with a capital letter).

I don't know what you need to do, but would using a list work? With lists you can calculate the offset to store to.

Hmm, I guess I'll either have to think of some tricky find-and-replace way of doing this or suck it up and use the 27-branch conditional. The reason why I wanted this was to let the user input a function of arbitrary variables, which is then searched for any variables and the user is prompted for their values. To then evaluate the function (or a transformation of the function), I need some way of actually assigning those values to the variables.

Why not just use Request?
Request “variable”, variable
That way variable is defined as the value. I can’t understand what you want to do.

We are talking about z80 Basic, not Nspire Basic.
And the equivalent to Request is Input, but that is not what he wants. He wants to put code that would store a value to a variable in a string and "execute the string" (if I understood correctly)

Why not just use Request?
Request “variable”, variable
That way variable is defined as the value. I can’t understand what you want to do.

We are talking about z80 Basic, not Nspire Basic.
And the equivalent to Request is Input, but that is not what he wants. He wants to put code that would store a value to a variable in a string and "execute the string" (if I understood correctly)

Oh, wow
I don't know why I automatically assumed it was Nspire

Hmm, I guess I'll either have to think of some tricky find-and-replace way of doing this or suck it up and use the 27-branch conditional. The reason why I wanted this was to let the user input a function of arbitrary variables, which is then searched for any variables and the user is prompted for their values. To then evaluate the function (or a transformation of the function), I need some way of actually assigning those values to the variables.

You can do something like Input "F=?",L1(6) to make it look like you're prompting for the value of F.

I had a program once that accepted an expression of any letter variables. The program scanned through the expression, replacing every instance of the letter with an appropriate list element and prompting for its value in that way, and finally called expr( on the tweaked expression.