Confusion with the signature of the function used in chain

[paulvictor]

Am here refering to the chain method in

fantasy-writers/src/writer.js

Lines 17 to 23 in da0e141

	Writer.prototype.chain = function(f) {
	return Writer(() => {
	const result = this.run();
	const t = f(result._1).run();
	return Tuple2(t._1, result._2.concat(t._2));
	});
	};

.

Writer.prototype.chain = function(f) {
    return Writer(() => {
        const result = this.run(); //1
        const t = f(result._1).run(); //2
        return Tuple2(t._1, result._2.concat(t._2)); //3
    });
};

Am using T, S, R as arbitrary type variables.
Here let the type of result from line 1 is Tuple2(T, Monoid), so result._2 is a Monoid, t is of type Tuple2(S, Monoid), so t._2 is a Monoid, both got by run()ning the monad constructor. In line 3 the 2nd part of the tuple is result._2.concat(t._2), which translates to type Monoid.concat(Monoid). I feel something's not right here, right now am using js Array which kind of is easy in that it doesn't care if I concat an array of an element or the element itself, the result is the same. Is there something amiss here?