ATL-S syntax (5 minutes)

Write HTTP apps in short .atl. Sugar expands to the same runtime as classic forms; older verbose styles still work.

Hello route

module app

get "/hello" {
  def hello(req) {
    return json({ message: "hi" })
  }
}

fn main() -> Int {
  return 0
}

atlantus check --path .

Route blocks

get    "/users"      { def index(req)   { return json(list()) } }
get    "/users/{id}" { def show(req)    { return json(get_one(req.param("id"))) } }
post   "/users"      { def create(req)  { return json(create(req.json()), 201) } }
put    "/users/{id}" { def update(req)  { return json(update(req.param("id"), req.json())) } }
delete "/users/{id}" { def destroy(req) {
  if delete_one(req.param("id")) {
    return status(204)
  }
  return json({ error: "not_found" }, 404)
}}

One def / fn per block
Method + path become HTTP routes
Untyped req is the request; omitted return is a response

`def` and `fn`

def add(a: Int, b: Int) -> Int {
  return a + b
}

fn add(a: Int, b: Int) -> Int {
  return a + b
}

JSON responses

Write this	Means this
`json(body)` / `ok(body)`	HTTP 200 + JSON body
`json(body, 201)` / `created(body)`	HTTP 201 + JSON body
`status(204)` / `no_content()`	Status only

Request helpers

Write this	Means this
`req.param("id")`	Path param `{id}`
`req.json()`	Parsed JSON body
`req.header("authorization")`	Header value
`req.method()` / `req.path()`	Method / path

Elegant handlers

Short status + early-exit sugar (desugars to the same natives as above):

Write this	Means this
`not_found(msg?)`	404 error envelope
`unauthorized` / `forbidden` / `bad_request`	401 / 403 / 400
`unprocessable(msg_or_fields?)`	422 validation_failed
`let x = require_found(e, msg?)`	value or 404
`let x = require_json(req)`	body or 422
`let x = require_auth(req)`	user id or 401
`require_fields(map, ["a","b"])`	422 if missing field

get "/users/{id}" {
  def show(req) {
    let user = require_found(db_get("users", req.param("id")), "not found")
    return ok(user)
  }
}

post "/users" {
  def create(req) {
    let body = require_json(req)
    require_fields(body, ["name"])
    return created(body)
  }
}

Null checks with `none`

let user = find(id)
if user is none {
  return not_found()
}
return ok(user)

Control flow

if n > 0 {
  return ok({ ok: true })
} else if n == 0 {
  return ok({ ok: "zero" })
} else {
  return bad_request("n must be >= 0")
}

Maps and arrays

let user = { "name": "Ada", "id": 1 }
let tags = ["atl", "web"]

Next: Language overview · HTTP & JSON.

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