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regression: s.end(true) failing to end stream? #176

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leeoniya opened this issue Apr 27, 2018 · 3 comments

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@leeoniya
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commented Apr 27, 2018

hey @paldepind,

i'm having an issue with the latest flyd relative to a prior version. it seems like s.end(true) is failing to end the stream. in my case this is causing an infinite loop and stack explosion:

v0.2.6 works:
https://jsfiddle.net/tbtpoxwk/

master (Maximum call stack size exceeded):
https://jsfiddle.net/tbtpoxwk/1/

i haven't done a bisect to figure out which commit broke things :/

@nordfjord

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commented Apr 28, 2018

Your problem is here:

	var sub = flyd.on(val => {
		// this check ignores the initial call during subscription
		if (!sub) return;

		sub.end(true);
		then();
	}, s);

You are assuming that the method passed to flyd.on will run immediately. This is not the case if it happens inside a stream body. This is unfortunately flyd working as intended.

It seems the function you passed to flyd.on is trying to do two things:

  1. drop 1 value
  2. execute only once

We can implement this generically as drop and take

const drop = n => s => {
  return flyd.combine(function(s, self){
   if (--n < 0) self(s());
  }, [s]);
}

const take = n => s => {
  return flyd.combine(function(s, self) {
    if (--n < 0) {
      self.end(true);
      return;
    }
    self(s());
  }, [s])
}

then to achieve the desired result you can utilise these methods like so:

function on(s, then) {
  var sub = s
    .pipe(drop(1))
    .pipe(take(1))
    .map(_ => then());

  return s();
}
@nordfjord

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commented Apr 28, 2018

here is a fiddle demonstrating the resolution: https://jsfiddle.net/kfmtun8w/1/

For further context about this behaviour of flyd take a look at #160 and #163

@leeoniya

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commented Apr 30, 2018

awesome.

thanks @nordfjord!

@leeoniya leeoniya closed this Apr 30, 2018

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