First API

In this tutorial you will build three routes:

Route	What it shows
GET /hello	Plain text responses.
GET /users/:id	Path parameters and JSON responses.
POST /echo	JSON request bodies and custom status codes.

Start with:

flint new my-api
cd my-api
flint serve

The server starts with hot reload enabled — it watches for changes to .fl and .flint.ui files and automatically recompiles on save.

Open the routes/ directory in another terminal or editor.

How Route Files Work

The server loads every .fl file directly inside routes/:

routes/
├── hello.fl
├── users.fl
└── echo.fl

Each file may define functions and route declarations. A route declaration maps an HTTP method and path to a function:

section .route
    GET "/hello" -> say_hello

Route 1: Plain Text

Create or replace routes/hello.fl:

section .route
    GET "/hello" -> say_hello

section .text
say_hello:
    mov r0, "Hello from Flint!"
    ncall http.text, r0
    ret

Test it:

curl http://127.0.0.1:3000/hello

Expected response:

Hello from Flint!

The request flow:

1. The router matches GET /hello.
2. The runtime creates a fresh VM.
3. The VM calls say_hello.
4. http.text sets the response body.
5. ret returns to the HTTP runtime.

Route 2: Path Params and JSON

Create routes/users.fl:

section .route
    GET "/users/:id" -> show_user

section .text
show_user:
    mov r0, "id"
    ncallr r1, http.param, r0

    ncallr r2, json.object
    mov r3, "id"
    ncallr r2, json.set, r2, r3, r1

    mov r3, "source"
    mov r4, "Flint"
    ncallr r2, json.set, r2, r3, r4

    ncall http.json, r2
    ret

Test it:

curl http://127.0.0.1:3000/users/42

Expected response:

{"id":"42","source":"Flint"}

The key pattern is:

mov r0, "id"
ncallr r1, http.param, r0

http.param receives the parameter name and returns the value captured from the path.

Building JSON

There is no JSON literal syntax. Build JSON values with native functions:

Goal	Call
Empty object	ncallr r0, json.object
Empty array	ncallr r0, json.array
Set object field	ncallr r0, json.set, r0, key, value
Push array item	ncallr r0, json.push, r0, value

json.set and json.push return new JSON documents. Store the result:

ncallr r0, json.object
mov r1, "name"
mov r2, "Ada"
ncallr r0, json.set, r0, r1, r2

Route 3: Read a POST Body

Create routes/echo.fl:

section .route
    POST "/echo" -> echo_body

section .text
echo_body:
    ncallr r0, http.json_body

    ncallr r1, json.object
    mov r2, "received"
    ncallr r1, json.set, r1, r2, r0

    mov r2, 201
    ncall http.set_status, r2
    ncall http.json, r1
    ret

Test it:

curl -i -X POST http://127.0.0.1:3000/echo \
  -H 'Content-Type: application/json' \
  -d '{"name":"Ada"}'

Expected body:

{"received":{"name":"Ada"}}

http.json_body parses the request body as JSON. Invalid JSON becomes a runtime error and the HTTP runtime returns 500.

Change the Status Code

Every response starts as 200 OK. Set another status with:

mov r0, 201
ncall http.set_status, r0

The response is sent after the handler returns, so the status can be set before or after the body.

Try Everything

curl http://127.0.0.1:3000/hello
curl http://127.0.0.1:3000/users/42
curl -i -X POST http://127.0.0.1:3000/echo \
  -H 'Content-Type: application/json' \
  -d '{"x":1}'
curl -i http://127.0.0.1:3000/missing

Next: UI Pages.