pairwise operator

[sxd1140]

Is have pairwise operator in wonka?

[kitten]

Hiya, for the sake of not having to cross-check this operator:

• what's pairwise and what does it do?
• what are you trying to achieve?

That may give me more information on what you're trying to get out of wonka 👍 Are you referring to it as an Rx operator? Would love to make sure that wonka supports every necessary use-case, but I'd love to keep the operators to a minimum, since I think it already has all the operators needed to fulfill lots of usecases

[sxd1140]

I need handle scroller change event, so need current scroller position and last scroller position to know scrolling direction.

[kitten]

Sounds like you were thinking of the combine operator, but if you have a single stream of position events you could also use the scan operator to transform a single value into a stream of { last, current } or sth similar

[sxd1140]

Thanks, I look scan operator.

scan((acc, val) => acc + val) in Example. But in d.ts file https://github.com/kitten/wonka/blob/acc4e71e338ca178ad6da5748d7ffa6c3a3446c8/src/operators/wonka_operator_scan.d.ts#L3

What the second param I should pass?

[kitten]

Hm, seems like that example has a typo! As can be seen in the Reason example for it (and in the typings as you’ve already pointed out) there is a second argument indeed.

Like for reduce with Arrays that second argument is for the initial value of the accumulation, but it’s not optional for scan

[sxd1140]

Ok. But seem like scan just pass one value.

pipe(
   fromArray([1, 2, 3, 4, 5, 6]),
   scan((last, current) => current, 0),
   subscribe( [`I need two value here, last and current`] => {}),
);

[kitten]

You can just store the previous value using scan temporarily, then calculate your scroll trajectory separately:

  scan(
    ({ current: prev }, current) => ({ prev, current }),
    { current: null }
  ),
  map(({ prev, current }) => prev !== null ? current - prev : 0)

[sxd1140]

I got it. Thanks a lot.