If you wanted to do as you started with, then you need to tell Mathematica to evaluate `expr` in the RHS, like thisAnother option is to use transformation Rules, like thisand if you really still want to use a function, you can use transformation rules with that as well. The good thing about using rules is that you do not have to worry about mixing the order of parameters, and it is more clear when reading the code which is a,b and y

One can eliminate many "issues" in programming with Mathematica by using a simple rule: Avoid global variables, and build your program from smaller functions up.

In your example, "func[]" was not really function. The example I gave for "func[]" was not a real function either. A real function, is like a mathematical function, it will accept all its input via arguments, and return its output by its final result (what it evaluates to).  If you look at how "func[]" is used above, you will see it updates a global variable called "expr".  So half its input comes via arguments, and then it uses these inputs and applies them on a global variable called "expr". A real function will take all its input from the argument list and would have no side effects other than its result (what it evaluates to).

In your simple example, there was actually no need for a function at all, since the whole operation just needs one computational step and using a function seems a little awkward here. But if you need to do many more computations, then you can start thinking about making a function then. This is really no different than when programming in Fortran or many other languages. (except in Mathematica it takes much less code to do the same thing