eslisp-chain

An eslisp macro for chaining method calls on a single object.

Examples

The macro is best suited for APIs whose methods return the same object to make it possible to chain sebsequent method calls. eslisp's functional syntax makes such API rather tedious to use.

Consider the following example in JavaScript:

service.method('greet', function (name) {
    return 'Hello, ' + (name + '!');
}).listen();

This can be expressed in eslisp with the following code:

((. 
  ((. service method) "greet" (lambda (name)
    (return (+ "Hello, " name "!"))))
  listen))

As the number of method calls increases, this inside-out functional syntax can get unwieldy, with multiple nested ((. invocations.

The eslisp-chain macro makes it possible to write the same code in a sequential manner:

(macro -> (require "eslisp-chain"))
(-> service
  (method "greet" (lambda (name)
    (return (+ "Hello, " name "!"))))
  (listen))

This syntax is convenient for chaining promises:

(macro -> (require "eslisp-chain"))
(-> (read "data.json")
  (then (. JSON parse))
  (then (lambda (obj)
    (return ((. Object keys) obj))))
  (catch (. console error)))

↓

read('data.json').then(JSON.parse).then(function (obj) {
    return Object.keys(obj);
}).catch(console.error);

It can be used to compose array methods:

(macro -> (require "eslisp-chain"))
(-> Object
  (keys data)
  (filter (lambda (key)
    (return (!== (get key 0) "_"))))
  (forEach processPublicMembers))

↓

Object.keys(data).filter(function (key) {
    return key[0] !== '_';
}).forEach(processPublicMembers);

The macro offers an alternative even for single method calls when there is no chaining:

(macro -> (require "eslisp-chain"))
(-> app
  (get "/" (lambda (req, res)
    ((. res send) "Hello, world!"))))

(-> app
  (listen 3000))

↓

app.get('/', function (req, unquote(res)) {
    res.send('Hello, world!');
});
app.listen(3000);