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📝 Making an HTTP server in ReasonML egghead Collection Notes

Async / Await in ReasonML

  • npm install bs-let
  • add 'ppx-flags': ['bs-let/ppx'] to your bsconfig.json to extend the bucklescript compiler
  • this lets us go let%Async person = Swapi.getPerson()
  • let%Async is an extension of let that we have to define ourselves.
module Async = {
let let* = (prom, cb) => Js.Promise.then*(cb, prom)
  • let_ is what the bs-let extension is expecting in the Async module to make this work.

Automatically decode JSON into records in ReasonML using Decco

  • We need a type to turn json into
type person = {
name: string
  • npm install decco
  • add decco to bs-dependencies array in bsconfig.json
  • add decco/ppx to ppx-flags array in bsconfig.json
  • Now we add a decorator to our type:
type person {
name: string
  • >showppx in VS Code to view the compiled version of the decorator
  • This shows how decco is giving us a decode and encode function to turn our json string into a record.
  • decode will return an Ok or an Error
open Async;
type person = {firstName: string};
let go = () => {
Js.log("Here's some JSON of a person");
let%Async json = Swapi.getPerson();
let result = person_decode(json);
switch (result) {
| Ok(luke) => Js.log2("Here is the name", luke.firstName)
| Error(msg) => Js.log2("Our decoding failed", msg)

Write a simple custom JSON decoder for Decco in ReasonML

  • Decco doesnt have support for js dates so we have to write our own "codec"
  • decco expects us to define a type t, t_encode and t_decode
module DateAsFloat = {
let t_decode = json => {
let t_encode = date => Js.Date.getTime(date)->Decco.floatToJson;
type t = Js.Date.t;
  • Now we can use this special type:
type data = {date: DateAsFloat.t};
  • example:
let go = () => {
let json = Data.data;
Js.log2("Json", json);
switch (data_decode(json)) {
| Ok(data) => Js.log2("data", data)
| Error(msg) => Js.log2("failed", msg)
Js.log2("Data", data_decode(json));
Js.log2("Json again", data_encode({date: Js.Date.make()}));

Working with cryptic error messages from ReasonML when writing custom Decco decoders.

  • When you get weird compiler errors, start adding type signatures to your code to see specifically whats going on.`
  • Type Level Debugging

Make a basic server with ReasonML and Node.js

  • serbet wraps express in reasonML
  • install decco and bs-let with serbet
  • open Serbet.Endpoint to get access to Serbet functionality.
  • We create a module HelloEndpoint to create an endpoint on our server
  • call Serbet.endpoint and pass it a record with {verb, path, handler}
  • handler must return a promise, so Serbet has an Async module we use to lift our string into a promise.
  • Now you call Serbet.application with a list of endpoints to create the server.
open Async;
open Serbet.Endpoint;
module HelloEndpoint = {
type params = {name: string};
let endpoint =
verb: GET,
path: "/hello/:name",
handler: req => {
let%Async params = req.requireParams(params_decode);
OkString("Hello, " ++ params.name ++ "!!!!!")->async;
let app = Serbet.application([HelloEndpoint.endpoint]);

Handle JSON request bodies and send JSON responses from a Node.js server using ReasonML

  • Field Punning -> makes it so you don't have to go {body_in_encode: body_in_encode} and can just go {body_in_encode}
  • a POST endpoint expects a body_in_decode and body_out_encode functions to be passed to it so it knows how to handle the json coming in and out.
module HelloJsonEndpoint = {
type body_in = {name: string};
type body_out = {greeting: string};
let endpoint =
verb: GET,
path: "/hello/:name",
handler: (body, \_req) => {
{greeting: "Hey there, " ++ body.name}->async;