Ebuall/reactive-polyglot

FRP libraries / langs compared

Reactive polyglot

R. (Ramda) stands for standard functional toolkit.

Operators

Current ordering is subjective: from most common operators to less common.
Operator counterparts are not expected to be fully equivalent.
"Kinda the same" is the current definition.

Presence of operator is not "good" as well as abscence is not "bad".
Every decision has it's own benefits and drawbacks. Remember, that we're trying to make analogies over systems which were be designed with different goals / philosophies in mind.

Create

Create stream from non-stream values.

KefirJS	MostJS	RxJS	XStream
constant	of / just	just	of
– (sequentially)	– (from)	of	of
– (sequentially)	from	from	?
fromEvents	fromEvent	fromEvent	fromEvent
fromPromise	fromPromise	fromPromise	fromPromise
interval	periodic	interval + map	periodic
repeat	of + R.range	repeat	of + R.range
?	iterate	generate	?
?	generate	generate	?

Mapper

Modify events one to one.

KefirJS	MostJS	RxJS	XStream
map	map	map	map
– (map)	constant	mapTo	mapTo
delay	delay	delay	combine(delay(500))
– (map)	timestamp	timestamp	– (map)

Transforms

Modify events * to *.

MostJS	RxJS	XStream
scan	scan	fold
chain / flatMap	flatMap	map + flattenConcurrently
map + switch	switchMap / flatMapLatest	map + flatten
concatMap	concatMap	map + flattenSequentially
join	mergeAll	flatten
loop	scan + map	fold + map
– (custom)	bufferWithCount	?

Filters

Skip events by predicate or signal.

MostJS	RxJS	XStream
filter	filter	filter
skipRepeats	distinctUntilChanged	dropRepeats
skipRepeatsWith	– (scan)	dropRepeats
skip	skip	drop
take	take	take
slice	skip + take	drop + take
skipWhile	skipWhile	fold + filter + map
takeWhile	takeWhile	filter + endWhen
since / skipUntil	skipUntil	fold + filter + map
until / takeUntil	takeUntil	filter + endWhen
during	window + take(1)	?

Combinators

Combine multiple streams into single.

MostJS	RxJS	XStream
merge	merge	merge
combine	combineLatest	combine
sample	withLatestFrom	sampleCombine
sampleWith	sample	?
zip	zip	?
concat	concat	concat
ap	combineLatest	?

Side effects

Produce side effect for every event.

MostJS	RxJS	XStream
tap	do / tap	debug

Ending

Operators which target end event somehow.

MostJS	RxJS	XStream
empty	empty	empty
never	never	never
continueWith	?	concat

Concurrency

MostJS	RxJS
– (custom)	amb / race

History

MostJS	RxJS
– (custom)

Design diffs

RxJS

1. Three primitives: Observer, Observable, Subject.
2. Observables end on error.
3. Provides API to handle errors.
4. Does not provide API to handle ending.

KefirJS

1. Two primitives: Stream and Property. Properties have "current value" notion.
2. Observables does not end on error (by default).
3. Provides API to handle errors.
4. Provides API to handle ending.

MostJS

1. One primitive: Stream.
2. Separate packages for subject-like and property-like primitives.
3. Provides API to handle errors.
4. Provides API to handle ending.

XStream

1. One primitive: Stream.
2. Always multicast. A Stream is like an RxJS Subject.
3. Streams end on error.

Common

RxJS does not emit initial scan value as event (use startWith for that).

Rx.Observable.interval(100).map(x => 1)
  .scan(add, 0);
  .subscribe(console.log); // 1--2--3--...

Most.periodic(100, 1)
  .scan(add, 0);
  .observe(console.log); // 0--1--2--3--...

Found docs / API quirks

MostJS

startWith vs sampleWith vs continueWith + recoverWith vs skipRepeatsWith
(val vs none vs func vs stream)

tap is listed in "Transform" section.

RxJS

startWith is listed in "Combine" section.

mergeAll is listed in "Combine" section.

distinct is not listed in "Filtering" section.

takeUntil is not listed in "Filtering" section.

just / return should be deprecated in favor of of.

fromArray should be deprecated in favor of from.

Links

bacon-vs-kefir – BaconJS vs KefirJS API comparison

dataflows – web arch. dataflow comparison

stream-conversions – tool for cross-library stream conversions

Additional Read

cujojs/most#171

https://twitter.com/rpominov/status/689566111734599683

tc39/proposal-observable#66