Writer monad

[ulfryk]

As a scratch to start with:

interface Semigroup<T> {
    concat(a: T): T
}

class Writer<V, L extends Semigroup<L>> {

    public static of<X, Y extends Semigroup<Y>>(empty: Y) {
        return (value: X) => new Writer<X, Y>(value, empty)
    }

    constructor(
        private readonly value: V,
        private readonly log: L,
    ) { }

    public map<Z>(fn: (v: V) => Z): Writer<Z, L> {
        return new Writer(fn(this.value), this.log)
    }

    public flatMap<Z>(fn: (v: V) => Writer<Z, L>): Writer<Z, L> {
        const { value, log } = fn(this.value)

        return new Writer(value, this.log.concat(log))
    }

    public toString() {
        return `(${this.value}, "${this.log}")`
    }

    public equals(other: Writer<V, L>) {
        return String(this) === String(other)
    }

}

const unit = Writer.of<number, string>('')
const add5 = (a: number) => Writer.of<number, string>('added 5, ')(a + 5)
const add8 = (a: number) => Writer.of<number, string>('added 8, ')(a + 8)
const theOne = new Writer(1, 'Initial One, ')

console.assert(unit(3).flatMap(add5).equals(add5(3)), 'Left identity')
console.assert(theOne.flatMap(unit).equals(theOne), 'Right identity')
console.assert(theOne.flatMap(add5).flatMap(add8).equals(theOne.flatMap(one => add5(one).flatMap(add8))) , 'Associativity')