Course of Raku / Advanced / More about built-in types / Enumerations 🆕 / Exercises / Traffic lights

Solution: Traffic lights

Here is a possible solution to the task.

Code

enum Light <red amber green>;

my Light $current = red;
say "$current is {$current.value}";

$current = amber;
say "$current is {$current.value}";

$current = green;
say "$current is {$current.value}";

🦋 You can find the source code in the file traffic-enum.raku.

Output

red is 0
amber is 1
green is 2

Comments

1. my Light $current is an ordinary, mutable variable — only constrained to the Light type. Assigning the next constant advances the light, and the number follows the name each time: 0, 1, 2.

2. $current++ looks like it should step to the next light on its own, but it fails with a type-check error. ++ treats the constant as its plain number and hands back an Int (red becomes 1), and an Int no longer fits the Light type. So you move the light by assigning the next constant, as above.

3. The type constraint applies to every assignment, not just the first. $current will take red, amber, or green, but assigning something that is not a Light — a bare number or a string — would be a type-check error.

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