Can you give me an example of how to use Ramda lift?

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I am reading ramda documentation

const madd3 = R.lift((a, b, c) => a + b + c);

madd3([1,2,3], [1,2,3], [1]); //=> [3, 4, 5, 4, 5, 6, 5, 6, 7]

It looks like a really useful function. I can't see what would be a use case for it.

Thanks

This function can only accept numbers:

const add3 = (a, b, c) => a + b + c;
add3(1, 2, 3); //=> 6

However what if these numbers were each contained in a functor? (i.e. a thing that contains a value; an array in the example below)

add3([1], [2], [3]); //=> "123"

That's obviously not what we want. You can "lift" the function so that it can "extract" the value of each parameter/functor:

const add3Lifted = lift(add3);
add3Lifted([1], [2], [3]); //=> [6]

Arrays can obviously hold more than one value and combined with a lifted function that knows how to extract the values of each functor, you can now do this:

add3Lifted([1, 10], [2, 20], [3, 30]);
//=> [6, 33, 24, 51, 15, 42, 33, 60]

Which is basically what you'd have got if you had done this:

[
  add3(1, 2, 3),    // 6
  add3(1, 2, 30),   // 33
  add3(1, 20, 3),   // 24
  add3(1, 20, 30),  // 51
  add3(10, 2, 3),   // 15
  add3(10, 2, 30),  // 42
  add3(10, 20, 3),  // 33
  add3(10, 20, 30)  // 60
]

Note that each array doesn't have to be of the same length:

add3Lifted([1, 10], [2], [3]);
//=> [6, 15]

So to answer your question: if you intend to run a function with different sets of values, lifting that function may be a useful thing to consider:

const results = [add3(1, 2, 3), add3(10, 2, 3)];

is the same as:

const results = add3Lifted([1, 10], [2], [3]);

Ramda Documentation, This would turn, for instance, R.map function into one that more closely const nums = [1, 2, 3, -99, 42, 6, 7]; R.apply(Math.max, nums); //=> 42 Note: R.bind does not provide the additional argument-binding capabilities of "lifts" a function of arity > 1 so that it may "map over" a list, Function or other See also mean. Looking at the source for Ramda.js, specifically at the "lift" function. lift. liftN. Here's the given example: var madd3 = R.lift(R.curry((a, b, c) => a + b + c));madd3([1,2,3], [1,2,3], [1]); //=> [3, 4, 5, 4, 5, 6, 5, 6, 7] So the first number of the result is easy, a, b, and c, are all the first elements of each array.

R.lift, applying g to zero or more arguments will give true if applying the same It will throw an error if you concat an Array with a non-Array value. arity of the function returned is specified to allow using variadic constructor functions. For more details on this behavior, see: https://developer.mozilla.org/en-US/ For example: 5 Can you give me an example of how to use Ramda lift? View more network posts → Top tags (9) drums. Score 3. Posts 4. Posts % 100. technique. Score 2. Posts 2.

What hasn't been mentioned in the current answers is that functions like R.lift will not only work with arrays but any well behaved Apply1 data type.

For example, we can reuse the same function produced by R.lift:

const lifted = lift((a, b, c) => a + b - c)

With functions as the Apply type:

lifted(a => a * a,
       b => b + 5,
       c => c * 3)(4) //=> 13

Optional types (dispatching to .ap):

const Just = val => ({
  map: f    => Just(f(val)),
  ap: other => other.map(otherVal => val(otherVal)),
  getOr: _  => val
})

const Nothing = {
  map: f    => Nothing,
  ap: other => Nothing,
  getOr: x  => x
}

lifted(Just(4), Just(6), Just(8)).getOr(NaN) //=> 2

lifted(Just(4), Nothing, Just(8)).getOr(NaN) //=> NaN

Asynchronous types (dispatching to .ap):

const Asynchronous = fn => ({
  run: fn,
  map: f    => Asynchronous(g => fn(a => g(f(a)))),
  ap: other => Asynchronous(fb => fn(f => other.run(a => fb(f(a)))))
})

const delay = (n, x) => Asynchronous(then => void(setTimeout(then, n, x)))

lifted(delay(2000, 4), delay(1000, 6), delay(500, 8)).run(console.log)

... and many more. The point here is that anything that can uphold the interface and laws expected of any Apply type can make use of generic functions such as R.lift.

1. The argument order of ap as listed in the fantasy-land spec is reversed from the order supported by name dispatching in Ramda, though is still supported when using the fantasy-land/ap namespaced method.

What is R.lift? · Issue #1543 · ramda/ramda · GitHub, I just that its combinatoricly adding the numbers together, but I'm not sure why you'd use it -- yet I hear its a very important function. When would  The Sustain Pedal on the piano is played with your foot while your hands are playing on the keyboard. Some pianos actually have three pedals and if you happen to have one of those pianos, today we're just going to use the one on the far right - the Sustain Pedal. If you only have one pedal with your keyboard, that will be your Sustain Pedal.

Basically it is taking a cartesian product and applies a function to each array.

const
    cartesian = (a, b) => a.reduce((r, v) => r.concat(b.map(w => [].concat(v, w))), []),
    fn = ([a, b, c]) => a + b + c,
    result = [[1, 2, 3], [1, 2, 3], [1]]
        .reduce(cartesian)
        .map(fn);

console.log(result); // [3, 4, 5, 4, 5, 6, 5, 6, 7]

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Comments
  • Hmm, I thought we'd cleaned up that documentation. A clearer example would be madd3([100, 200], [30, 40], [5, 6, 7]) //=> [135, 136, 137, 145, 146, 147, 235, 236, 237, 245, 246, 247]. The answers given are great, but see also stackoverflow.com/q/36558598/1243641.
  • In general, lift, or more precisely liftA2 is equivalent to ap(map(x)), not ap(ap(x)). So this is Ramda specific and deviates from the standard.
  • @reify: Ramda does implement liftN (and therefore lift) with an initial map, reducing ap calls on the result.