Recursion

Mutual Recursion

To compute the value of an expression, we implement three functions expression_value, term_value, and factor_value.
The expression_value function calls term_value, checks to see if the next input is + or -, and if so calls term_value again to add or subtract the next term.
The term_value function calls factor_value in the same way, multiplying or dividing the factor values.
The factor_value function checks whether the next input is '(' or a digit, calling either expression_value recursively or returning the value of the digit.
The termination of the recursion is ensured because the expression_value function consumes some of the input characters, ensuring that the next time it is called on a shorter expression.