Values and functions

An ordinary function can calculate a value without an interface or injected dependency. This example calculates an order total.

Create a new folder with these two files. Continue using the published CLI from the first chapter.

main.aug

import total from prices

int price = 7
int quantity = 3
print(value=total(price, quantity))
print(value=total(quantity=0, price))

prices.aug

/** Calculate the price for a positive quantity; otherwise return zero. */
total(int price, int quantity):
    if quantity > 0:
        return price * quantity
    return 0

Run aug check ., then aug run .. The output is:

21
0

Read the call

int price = 7 gives the value a type and a name. int price to 7 means the same thing. You can omit the type when it can be inferred, as in price = 7.

Every call input has a label. total(price, quantity) is shorthand for total(price=price, quantity=quantity): the local names match the labels. The second call supplies a different quantity. Labels let you reorder inputs without making the reader guess which argument is which.

A function declaration starts with its name. The compiler infers the integer result from the returns. VS Code shows returns int as a hint beside the header. Write the return type yourself when you want the compiler to enforce it.

Read the decision

if quantity > 0 chooses the first return when the quantity is positive. That return ends the call. Otherwise execution reaches return 0. Every path in a function with a non-void result must return or throw.

Boolean conditions use and, or, and not. For example, price > 0 and quantity > 0 requires both comparisons to hold. and and or short-circuit: the second operand runs only when the first operand makes it necessary.

Try a change

Change the first quantity to 4. Run the program and check that the first result is 28. Then change the second call's quantity label to amount and run aug check .. That call should fail checking because total has no input named amount. Restore the label before continuing.

Run aug spec . and read prices.aug.md. It should explain the two return paths. The spec explains both return paths and includes the comment about nonpositive quantities. The next chapter makes an invalid input an explicit failure instead.