Slide 13 of 37

Pattern matching with is

Variant patterns

type Shape {
	Circle    { radius: Int }
	Rectangle { width: Int, height: Int }
}

proc main(): void {
	shape := Shape.Circle(4)
	if shape is Shape.Circle(r)            { echo "circle, r={r}" }
	else if shape is Shape.Rectangle(w, _) { echo "rect, w={w}" }
}

Vector patterns

proc main(): void {
	command := ["move", "north", "2"]
	if command is ["move", dir, ...steps] { echo "{dir}, {len(steps)} more" }
	else if command is ["stop"]           { echo "halt" }
}

String patterns

proc main(): void {
	path := "/users/42/posts/7"
	if path is "/users/{id}/posts/{postId}" { echo "{id} / {postId}" }
}

`is` checks a value against a pattern and yields a Bool, narrowing it and binding names usable right away — in the rest of the condition after &&, and in the branch body. There is no match statement; every match is an ordinary if/else if chain. Four kinds of pattern exist: a variant, a Result, a vector matched positionally with an optional ...rest, and a string template whose {holes} bind the text between literal pieces.

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