Sep 22, 2021 • Iván Stepaniuk

TL;DR:

Sentinel values and other kinds of “flag” values have no place in modern languages, use types!

A bit longer:

In languages like C, it is common to use sentinel values, sometimes also called flag value, signal value, etc., as a way to interrupt a loop. It is also common to return negative values as a way to indicate an error condition where positive values indicate that things went well.

int find(int buffer[], size_t len, int value)
{
    for (int i = 0; i < len; i++) {
        if (buffer[i] == value) {
            return i;
        }
    }
            
    return -1; // not found!
}

This makes a lot of sense in C or even straight assembler; the CPU can very quickly determine whether a value is positive and branch conditionally.

You will find similar APIs in modern languages as well, such as Python’s find(), that keep the tradition alive:

"an apple, a pear and a banana".find("orange")  // -1

However, when writing your own code in a typed language, this is just an anti-pattern, and a clear indication that you are using primitives where you should not.

So for example in Java, instead of doing:

public int findCustomerIdByUsername(Username username) {
    for (Customer customer : this.customers) {
        if (customer.getUsername().equals(username)) {
            return customer.getId();
        }
    }
    
    return -1;
}

Consider the much safer:

public CustomerId findCustomerIdByUsername(Username username) {
    for (Customer customer : this.customers) {
        if (customer.getUsername().equals(username)) {
            return customer.getId();
        }
    }
    
    return new NullCustomerId();
}

In this case, using a null object pattern (but a throw new CustomerNotFoundException() might be the best choice in some circumstances).