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ECMAScript proposal: chainable do sugar

For purposes of this document, a “chainable” is any value that:

  1. Is a wrapper around another value (which can be anything).
  2. Has a .map(value => value + 1) method which takes a function that modifies the wrapped value.
  3. Has a .chain(value => Wrapper(value + 1)) method which takes a function that returns another wrapper (must be the same wrapper type), and “unwraps” the nested wrapper.

Chainables are a powerful programming pattern that can be used to express a large number of pure solutions to programming problems including but not limited to: asynchronous code, safe null checks, dependency injection, state changes, observables, error handling, and lazy evaluation.


const foo = {bar: null}
const baz = 
    .chain(foo =>
      option( => 
        option(bar.baz).chain(baz =>
          option( =>

Nested chainable calls can quickly become verbose and clunky to deal with. Async/await helps with this, but is specialized to Promises.

The intent behind this nested sequence of code can be expressed clearer with just a small amount of syntax sugar:

const fooOption = option({bar: null})
const baz = 
  do {
    foo <- fooOption
    bar <- option(
    baz <- option(bar.baz)
    biz <- option(



The rules for the syntax are rather simple:

Anytime a <- expression is inside a do block, it’s split into two sides. The RHS value has a chain method invoked, and the LHS is presented as the sole parameter to the chain method. For the last <- expression, chain is replaced with map, and the very last expression of the do block is returned from this method

do {
  {paramA} <- {exprA}
  {paramB} <- {exprB}


Is de-sugared to:

{exprA}.chain({paramA} => {
  return {exprB}.map({paramB} => {
    return {exprC}


Since the only requirements for this are that a value has a .chain and .map method, it can work for all kinds of chainables included but not limitied to:

Asynchronous chainables:

do {
  user   <- fetchUser('paul')
  img    <- fetchImage(user.profileImage)
  friend <- fetchUser(user.friend_id)

  {...user, img, friend}

Option (Maybe) chainables:

const baz = 
  do {
    posts     <- option(user.posts)
    firstPost <- option(posts[0])

    firstPost.likeCount / posts.length

Error handling chainables:

const baz = 
  do {
    count1 <- Try(() => parseInt('15'))
    count2 <- Try(() => parseInt('30'))

    count1 * count2

These are just some examples. There are many more!

Example Implementation

This babel plugin implements this proposal with a few caveats:

  1. << is used instead of <-.
  2. A destructuring expression doesn’t work as the LHS of a << expression.

These caveats are simply because this is implemented as a babel plugin. An update to babel would be needed to add the <- operator and support destructuring on the LHS.

Prior Art

Scala has for comprehensions

for {
   n1 <- fetchNumber(...)
   n2 <- fetchNumber(...)
   n3 <- fetchNumber(...)
} yield (n1 + n2 + n3)

Haskell has do notation

do { x1 <- action1
   ; x2 <- action2
   ; mk_action3 x1 x2 }


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