Functional side effects is when a function changes a two-way variable or nonlocal variable.

Problem:
When a function referenced in an expression alters another operand of the expression.

x = 10; y = x + function(&x);

Two possible solutions:
Solution 1

Write the language definition to disallow functional side effects. No two-way parameters and nonlocal references in functions. Disadvantage: Programmer want the flexibility of two-way parameter and nonlocal references.

Solution 2

Write the language definition such that the operand evaluation order is fixed. Disadvantages: Certain compiler optimization may be limited.