MonadWriter

monadwriter.ts

class MonadWriter<T, V> {
    private v:T;
    private log:V[];

    constructor(v:T, log:V[] | V) {
        this.v = v;     
        if(log instanceof Array)
            this.log = log;
        else
            this.log = [log];
    };
    
    public Map = <U>(f:(a:T) => U) : MonadWriter<U, V> => new MonadWriter(f(this.v), this.log);

    public Run = ():[T, V[]] => [this.v, this.log];

    public FlatMap = <U>(f:(b:T) => MonadWriter<U, V>) : MonadWriter<U, V> => {
        const a = f(this.v)

        return new MonadWriter(a.v,[ ...this.log, ...a.log])
    }
}

const sum = x => y => new MonadWriter(x + y, "sum")
const mul = x => y => new MonadWriter(x * y, "mul")
const tostr = x => new MonadWriter(x.toString(), "tostr")
const a = sum(5)(5)
const b = a.FlatMap(mul(2))
const c = b.FlatMap(mul(8))

const d = c.Map(f => f.toString())

const e = c.FlatMap(tostr)

console.log(c.Run()) // [160, ['sum', 'mul', 'mul']]
console.log(d.Run()) // ['160', ['sum', 'mul', 'mul']]
console.log(e.Run()) // ['160', ['sum', 'mul', 'mul', 'tostr']]