Pure Functions

Pure Func

A pure func cannot perform I/O, call impure functions, mutate global state, or accept impure callbacks.

pure func add_one(n: Int) -> Int:
    n + 1

Local Mutation

Local var bindings are allowed when the mutation cannot be observed outside the function.

pure func total(xs: List[Int]) -> Int:
    var sum: Int = 0
    for x in xs:
        sum += x
    sum

Effect Boundaries

Keep reading, printing, files, and processes in impure functions such as main.

pure func label(n: Int) -> String:
    n.to_string()

func main(args: List[String]) -> Void:
    print(label(42))

Pure Callbacks

Collection and parallel APIs can accept pure callbacks when they need deterministic work.

items.parallel(
    pure func(chunk): chunk.map(pure func(x): x * 2)
)

Example

pure func average(scores: List[Int]) -> Int:
	var total: Int = 0
	for score in scores:
		total += score
	total / scores.length()


func main(args: List[String]) -> Void:
	scores: List[Int] = [82, 91, 77]
	print(average(scores).to_string())

Try It

blorp run average.brp