Pipe

ReScript provides a tiny but surprisingly useful operator ->, called the "pipe", that allows you to "flip" your code inside-out. a(b) becomes b->a. It's a simple piece of syntax that doesn't have any runtime cost.

Why would you use it? Imagine you have the following:

validateAge(getAge(parseData(person)))

This is slightly hard to read, since you need to read the code from the innermost part, to the outer parts. Use pipe to streamline it:

person
  ->parseData
  ->getAge
  ->validateAge

Basically, parseData(person) is transformed into person->parseData, and getAge(person->parseData) is transformed into person->parseData->getAge, etc.

This works when the function takes more than one argument too.

a(one, two, three)

is the same as

one->a(two, three)

This also works with labeled arguments.

Pipes are used to emulate object-oriented programming, e.g. what's myStudent.getName in other languages like Java would be myStudent->getName in ReScript (aka getName(myStudent)). This allows us to have the readability of the good parts of OOP without its downside of dragging in a huge class system just to call a function on a piece of data.

Tips & Tricks

Do not to abuse pipes; they're a means to an end. Inexperienced engineers sometimes shape a library's API to take advantage of the pipe. This is backward.

JS Method Chaining

This section requires understanding of our binding API.

JavaScript's APIs are often attached to objects, and often chainable, like so:

JS
const result = [1, 2, 3].map(a => a + 1).filter(a => a % 2 === 0);

asyncRequest()
  .setWaitDuration(4000)
  .send();

Assuming we don't need the chaining behavior above, we'd bind to each case this using bs.send from the aforementioned binding API page:

@bs.send external map: (array<'a>, 'a => 'b) => array<'b> = "map"
@bs.send external filter: (array<'a>, 'a => bool) => array<'a> = "filter"

type request
@bs.val external asyncRequest: unit => request = "asyncRequest"
@bs.send external setWaitDuration: (request, int) => request = "setWaitDuration"
@bs.send external send: request => unit = "send"

You'd use them like this:

let result = Js.Array2.filter(
  Js.Array2.map([1, 2, 3], a => a + 1),
  a => mod(a, 2) == 0
)

send(setWaitDuration(asyncRequest(), 4000))

This looks much worse than the JS counterpart! Clean it up visually with pipe:

let result = [1, 2, 3]
  ->map(a => a + 1)
  ->filter(a => mod(a, 2) == 0)

asyncRequest()->setWaitDuration(4000)->send

Pipe Into Variants

You can pipe into a variant's constructor as if it was a function:

let result = name->preprocess->Some

We turn this into:

let result = Some(preprocess(name))

Note that using a variant constructor as a function wouldn't work anywhere else beside here.

Pipe Placeholders

A placeholder is written as an underscore and it tells ReScript that you want to fill in an argument of a function later. These two have equivalent meaning:

RES
let addTo7 = (x) => add3(3, x, 4)
let addTo7 = add3(3, _, 4)

Sometimes you don't want to pipe the value you have into the first position. In these cases you can mark a placeholder value to show which argument you would like to pipe into.

Let's say you have a function namePerson, which takes a person then a name argument. If you are transforming a person then pipe will work as-is:

makePerson(~age=47, ())
  ->namePerson("Jane")

If you have a name that you want to apply to a person object, you can use a placeholder:

getName(input)
  ->namePerson(personDetails, _)

This allows you to pipe into any positional argument. It also works for named arguments:

getName(input)
  ->namePerson(~person=personDetails, ~name=_)

Triangle Pipe (Deprecated)

You might see usages of another pipe, |>, in some codebases. These are deprecated.

Unlike -> pipe, the |> pipe puts the subject as the last (not first) argument of the function. a |> f(b) turns into f(b, a).

For a more thorough discussion on the rationale and differences between the two operators, please refer to the Data-first and Data-last comparison by Javier Chávarri