Add S.length function

[svozza]

I went with the Countable suggestion as mentioned in #145.

[davidchambers]

What's the appeal of this function over R.prop('length')? I'm curious. :)

[svozza]

Hmmm, good question. Not a lot, aside from a more descriptive error message than:

TypeError: Cannot read property 'length' of undefined

[davidchambers]

I think we should start by defining a function for accessing record fields. It would be similar to R.prop but would throw if the requested property is not present.

[svozza]

How would that work? We couldn't do something like:

def('prop', {}, [$.RecordType, $.Any]...)

[davidchambers]

How about something like this?

var prop =
def('prop',
    {a: [Accessible]},
    [$.String, a, b],
    function(name, obj) {
      if (name in obj) {
        return obj[name];
      } else {
        throw new TypeError('‘prop’ expected object to have a property named ' +
                            '‘' + name + '’; ' + show(obj) + ' does not');
      }
    });

[svozza]

Do we want to traverse the prototype chain to find properties? I know not doing so would make this an O(n) function but I think it might be less surprising to users.

[davidchambers]

How do you feel about length now that we have prop, Stefano?

[svozza]

Only seeing your comment now, David. I think we can close this one. I really don't like this Countable type thing.

[davidchambers]

Okay. Closing without prejudice.