As mentioned <a href="https://github.com/fantasyland/fantasy-land/issues/7#issuecommen

There's an Unfoldable package that gives some guidance here: <a href="http://hacka

Just an idea: chainRec of Array could be used as <cod

Cofoldable about fantasy-land HOT 3 OPEN

fantasyland commented on July 18, 2024

Cofoldable

from fantasy-land.

Comments (3)

DrBoolean commented on July 18, 2024

There's an Unfoldable package that gives some guidance here:
http://hackage.haskell.org/package/unfoldable-0.8.1/docs/Data-Unfoldable.html
It even provides fromList

Regarding #7, it seems like Cofoldable makes sense over Unfoldable if the whole purpose is to provide some laws. Although in that case, my opinion is that we're being really specific about a natural transformation or isomorphism.

Anyways, for unfold:

Since this is fantasy land, I think we should use Option to flag the end of the unfold rather than a guard fn or null.

Array's instance could be:

//+ unfold :: (b -> Option([a, b])) -> b -> [a]
Array.prototype.unfold = function(step, seed) {
  var output = this;
  return step(seed).cata({
    Some: function(result) {
      return output.concat(result[0]).unfold(step, result[1]);
    },
    None: function() {
      return output;
    }
  });
}

var result = [].unfold(function(x){ return (x > 10) ? None : Some([x, x+1]); }, 0)
console.log(result);

from fantasy-land.

SimonRichardson commented on July 18, 2024

👍 on using Option instead of a guard func & null.
Looks nice and clean to me.

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safareli commented on July 18, 2024

Just an idea: chainRec of Array could be used as unfold

Array.chainRec(function(next, done, x) {
  return (x == 10) ? [done(x)] : [done(x), next(x+1)]
}, 0) // [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ]

// implementation of Array.chainRec
function stepNext(x) { return {value: x, done: false }; }
function stepDone(x) { return {value: x, done: true }; }

Array.chainRec = function _chainRec(f, i) {
  var todo = [i];
  var res = [];
  var buffer, xs, idx;
  while (todo.length > 0) {
    xs = f(stepNext, stepDone, todo.shift());
    buffer = [];
    for (idx = 0; idx < xs.length; idx += 1) {
      (xs[idx].done ? res : buffer).push(xs[idx].value);
    }
    Array.prototype.unshift.apply(todo, buffer);
  }
  return res;
};