Monoid.reduce has different signatures for objects and arrays

[taras]

Same monoid reduce will produce a different result for [1, 2, 3, 4] than { "1": 1, "2": 2, "3": 3, "4": 4 }. This makes it inconvenient when trying to use the same monoid for both arrays and objects. I would expect to be able to use the same monoid for both without having to make special accommodation for both types.

When reducing arrays, the append function for the array receives a value for a and b. Reducing objects, sends memo for a and b. This is caused by the difference in implementation of foldl for Arrays and Objects.

I believe that both foldl for Arrays and Objects should send data of same shape to the folding function. It should be other value or memo, not either.

Failing test: https://github.com/taras/funcadelic.js/blob/monoid-signature-failing-test/tests/funcadelic-test.js#L67

[cowboyd]

The "signature" for reduce is reduce(Monoid, list) where list is a list of monoids. Or, alternatively, Monoid.reduce(list). In both cases, what you have to reduce is a list. That's just how Monoids work.

So testcase Sum.reduce({"a": 1, "b": 2, "c": 3, "d": 4})) this is an invalid usage of the api. My intuition is to keep the moving parts as simple as possible because being hard-nosed about it thus far has yielded such amazing dividends.

To get the behavior of Sum.reduce(values({"a": 1, "b": 2, "c": 3, "d": 4})), perhaps the introduction of another typeclass in order to maximize composability without overloading existing typeclasses?

const Values = type(class Values {
  values(holder) {
    return this(holder).values(holder);
  }
);

Values.instance(Array, {
  values(array) { return array; }
});

Values.instance(Object, {
  values(object) { return Object.values(object); }
});

[taras]

Yes, this would work nicely! I'll make a PR.

[taras]

We can close this issue and PR. I'll make another and reference this one.

[cowboyd]

Maybe we should try it out in libraries consuming it first, a-la Monad.

Also, I think that this highlights a weakness with error handling and messaging.