callbag-expr

callbag-expr

Create expressions based on callbags.

npm i callbag-expr

👉 Expressions with callbags:

import interval from 'callbag-interval'
import pipe from 'callbag-pipe'
import subscribe from 'callbag-subscribe'
import expr from 'callbag-expr'

const a = interval(1000)

pipe(
  expr($ => 'Hellow: ' + $(a, 0)),
  subscribe(console.log)
)

► TRY IT!

👉 Conditional expressions:

import interval from 'callbag-interval'
import pipe from 'callbag-pipe'
import subscribe from 'callbag-subscribe'
import expr from 'callbag-expr'

const a = interval(1000)
const b = interval(600)
const c = interval(3000)

pipe(
  expr($ => {
    if($(c, 0) % 2) return 'A = ' + $(a)
    else  return 'B = ' + $(b)
  }),
  subscribe(console.log)
)

► TRY IT!

👉 Passive tracking, i.e. using latest value from callbag without re-evaluating when it emits:

import interval from 'callbag-interval'
import pipe from 'callbag-pipe'
import subscribe from 'callbag-subscribe'
import expr from 'callbag-expr'

const a = interval(1000)
const b = interval(300)

pipe(
  expr(($, _) => $(a) + _(b)),
  subscribe(console.log)
)

► TRY IT!

Gotchas

⚠️⚠️ Don't create new callbags in the expression:

// WRONG:
expr($ => $(interval(1000)));

// CORRECT:
const i = interval(1000);
expr($ => $(i));

⚠️⚠️ Make sure your callbags have initial value, or provide initial value. If you don't, $(a) might default to undefined (so take that into account).

// WRONG:
const a = interval(1000);
expr($ => $(a));         // --> initially will be `undefined`

// CORRECT:
const a = interval(1000);
expr($ => $(a, 0));     // --> initially will be 0

// CORRECT:
const a = makeBehaviorSubject(42);
expr($ => $(a));        // --> initially will be 42

⚠️⚠️ Make sure some callbag is actively tracked:

// WRONG:
const a = interval(1000);
const b = interval(1000);

//
// --> a is not actively tracked, b is also not tracked initially,
// --> so the expression is never re-evaluated.
//
expr($ => _(a, 0) > 5 ? $(b) : 32);

Contribution

Play nice and respectful. Useful commands for development:

git clone https://github.com/loreanvictor/callbag-expr.git

npm i               # --> install dependencies

npm start           # --> run samples/index.ts

npm test            # --> run tests

npm run cov:view    # --> view coverage

Inspired by RxJS Autorun.