Requirements

A requirement lists the effects a function may ask for.

let nextTick () : Int require { Clock } :=
  ask Clock.tick();

Write required effects in the signature

let nextTick () : Int require { Clock } := ask Clock.tick();

The signature tells callers that Clock must be available.

Requirements help testing

A test can provide a small fake answer instead of using the real clock, file system, or network.

Keep requirements narrow. Do not require a broad runtime when one small effect is enough.

Read requirement order

Read the signature left to right:

let nextTick () : Int require { Clock } := ask Clock.tick();

The empty parameter list says the function takes no arguments. : Int says it returns an integer. require { Clock } says the function needs a Clock effect provider.

Why not hide it

Without the requirement, the call site would not show that outside work may happen. A visible requirement keeps effectful code honest.