Ramda chain usage

Question

From the documentation:

var duplicate = n => [n, n];
R.chain(duplicate, [1, 2, 3]); //=> [1, 1, 2, 2, 3, 3]
R.chain(R.append, R.head)([1, 2, 3]); //=&g         


        

Answer 1

chain is (approximately) defined as (for functions): (fn, monad) => x => fn(monad(x))(x)

Therefore, we can transform R.chain(R.append, R.head)([1, 2, 3]); like so:

R.chain(R.append, R.head)([1, 2, 3]);

R.append(R.head([1, 2, 3]), [1, 2, 3]); // This is the important step

R.append(1, [1, 2, 3]);

[1, 2, 3, 1];

The actual source:

var chain = _curry2(_dispatchable(['fantasy-land/chain', 'chain'], _xchain, function chain(fn, monad) {
  if (typeof monad === 'function') {
    return function(x) { return fn(monad(x))(x); };
  }
  return _makeFlat(false)(map(fn, monad));
}));

The important part of that is this:

function chain(fn, monad) {
  if (typeof monad === 'function') {
    return function(x) { return fn(monad(x))(x); };
  }
  return _makeFlat(false)(map(fn, monad));
}

According to the comment in the code, "_makeFlat is a helper function that returns a one-level or fully recursive function based on the flag passed in."

_makeFlat(false) seems to be equivalent to unnest and _makeFlat(true) seems to be equivalent to flatten.