npm i @susisu/effectful
# or
yarn add @susisu/effectfulimport { Effectful, makeEffect, run } from "@susisu/effectful";
// 1. Augment `EffectDef<A>` to define new kinds of effects
declare module "@susisu/effectful" {
interface EffectDef<A> {
"state/get": {
k: (x: number) => A; // constrains A = number
};
"state/put": {
value: number;
k: (x: undefined) => A; // constrains A = undefined
};
}
}
// 2. Define effect constructors using `makeEffect`
const getState = makeEffect<"state/get", number>("state/get", {
k: x => x,
});
const putState = (value: number) =>
makeEffect<"state/put", undefined>("state/put", {
value,
k: x => x,
});
// 3. Write effect handlers
function runState<T>(
comp: Effectful<"state/get" | "state/put", T>,
state: { current: number }
): T {
return run(comp, x => x, {
"state/get": (eff, resume) => {
return resume(eff.value.k(state.current));
},
"state/put": (eff, resume) => {
state.current = eff.value.value;
return resume(eff.value.k(undefined));
},
});
}
// 4. Write computations using generators
function* getAndMultiplyState(
multiplier: number
): Effectful<"state/get", number> {
// use `yield*` to perform an effect
const value = yield* getState;
return multiplier * value;
}
function* main(): Effectful<"state/get" | "state/put", string> {
// computations can be nested
const newValue = yield* getAndMultiplyState(3);
yield* putState(newValue);
return `current state is ${newValue}`;
}
// 5. Run the computation
const state = { current: 2 };
const result = runState(main(), state);
console.log(result);
// => "current state is 6"
console.log(state);
// => { current: 6 }