Adds lensIndex, lensProp, composeL & pipeL

[scott-christopher]

This change introduces 2 new helpers for generating lenses,
lensIndex and lensProp, where lensIndex is used to
create a new lens with a focus on an array index, while
lensProp is used to create a new lens with a focus on an
object property.

var phraseLens = R.lensProp('phrase');
var obj1 = { phrase: 'Absolute filth . . . and I LOVED it!'};
phraseLens(obj1); // => 'Absolute filth . . . and I LOVED it!'
phraseLens.set('Ooh Betty', obj1); //=> { phrase: 'Ooh Betty'}

var headLens = R.lensIndex(0);
headLens([10, 20, 30, 40]); //=> 10
headLens.set('mu', [10, 20, 30, 40]); //=> ['mu', 20, 30, 40]

This change also introduces composeL and pipeL which
are used to compose lenses together, resulting in a new lens
which can be used to access and update nested properties
of a target object.

var headLens = R.lensIndex(0);
var secondLens = R.lensIndex(1);
var xLens = R.lensProp('x');
var secondOfXOfHeadLens = R.composeL(secondLens, xLens, headLens);

var source = [{x: [0, 1], y: [2, 3]}, {x: [4, 5], y: [6, 7]}];
secondOfXOfHeadLens(source); //=> 1
secondOfXOfHeadLens.set(123, source); //=> [{x: [0, 123], y: [2, 3]}, {x: [4, 5], y: [6, 7]}]

[megawac]

probably faster to do slice(xs, 0, n].concat([x], _slice(xs, n + 1))

[scott-christopher]

As suggested, I've updated this to use _slice(xs, 0, n).concat([x], _slice(xs, n + 1)) instead.

[buzzdecafe]

no doubt, this is a super useful pattern

[CrossEye]

🌿

Very nice stuff! I really like composeL/pipeL.

I wonder if we also want lensPath similar to lensProp using path and assocPath

[scott-christopher]

Is it worth pulling this out to a separate function, equivalent to assoc for arrays?

[buzzdecafe]

discussion about whether assoc should work on arrays as well: #773

[buzzdecafe]

🐄

this looks good. any input @davidchambers ?

[davidchambers]

👾

[davidchambers]

I haven't yet used lenses, but these functions make sense to me.

[kevinbeaty]

Love this. Was actually just looking for something like R.lensIndex and the other functions look just as useful. Thanks @scott-christopher !

[niant]

Looking forward to this! Hopefully next release is soon 👍

[jethrolarson]

Should lensIndex this use R.update for the setter?

Would this be too much dog food:
lensProp = converge(lens, prop, assoc);

[CrossEye]

It would not necessarily be too much dogfood. But I would want to compare that to the existing implementation, especially as converge is one of our fill-in-the-gaps functions. (useWith is the other main one.) These help us do things that we would really like to be able just to do with compose but cannot for various reasons. So as useful as converge is, I would rather not promote it overmuch. And the other important point is that the current implementation is already very clean. Just making it points-free because we can doesn't add much.

[scott-christopher]

The reason why update wasn't initially used to implement the setter in lensIndex was because update didn't exist at the time (well, for 4 days) :)

Using update would provide a nice symmetry with lensProp.

function lensProp(k) {
  return lens(prop(k), assoc(k));
}

function lensIndex(n) {
  return lens(nth(n), update(n));
}

[CrossEye]

I would definitely be in favor of that change.