causing side effects in user-defined functions

[Brian Lewis]

I'd like a user-defined function to say x="M"$(N), increment N, and
return x. I know how to write user-defined functions, but I don't
understand how they can cause side effects like the incrementing of the
variable N. In case it isn't clear, this function would return M0 the
first time, M1 the second time, etc. User-defined functions in make seem
functional. Maybe I can use semicolons? Is the value of a function the
value of its last expression? Thanks.