Can't wrap my head around “lift” in Ramda.js

Question

Looking at the source for Ramda.js, specifically at the \"lift\" function.

lift

liftN

Here\'s the given example:

var madd3 = R.lift(R         


        

Answer 1

lift/liftN "lifts" an ordinary function into an Applicative context.

// lift1 :: (a -> b) -> f a -> f b
// lift1 :: (a -> b) -> [a] -> [b]
function lift1(fn) {
    return function(a_x) {
        return R.ap([fn], a_x);
    }
}

Now the type of ap (f (a->b) -> f a -> f b) isn't easy to understand either, but the list example should be understandable.

The interesting thing here is that you pass in a list and get back a list, so you can repeatedly apply this as long as the function(s) in the first list have the correct type:

// lift2 :: (a -> b -> c) -> f a -> f b -> f c
// lift2 :: (a -> b -> c) -> [a] -> [b] -> [c]
function lift2(fn) {
    return function(a_x, a_y) {
        return R.ap(R.ap([fn], a_x), a_y);
    }
}

And lift3, which you implicitly used in your example, works the same - now with ap(ap(ap([fn], a_x), a_y), a_z).