Optional Attribute Access in Reason


One thing that you'll run into when interfacing with complex javascript objects in Reason, is that dealing with optional objects can be a huge pain. The most common place to run into this is when using the excellent graphql-ppx syntax plugin (often together with reason-apollo), because the result of a graphql query is a large complex javascript object, with many parts of it that are optional.

The problem

For example, say you have the following query:

query MyAwesomeQuery {
  user {
    bestFriend {
      profilePicture {

Where everything except the final url is optional.

The response type then looks like:

type queryResponse = {.
  "user": option({.
    "bestFriend": option({.
      "profilePicture": option({.
        "url": string

If you want to get the best friend's profile URL, it's quite a bit of code:

let url = switch (response##user) {
| None => None
| Some(user) => switch (user##bestFriend) {
  | None => None
  | Some(bestFriend) => switch (bestFriend##profilePicture) {
    | None => None
    | Some(profilePicture) => Some(profilePicture##url)

And this might be OK to write once, but it really adds up if you're doing lots of graphql queries in your app.

A solution!

Gregiorevda, an active community member and core reason constributor, proposed a modification to the reason parser that would add "safe call operators" to address exactly this use case. It's had a fair amount of discussion back and forth, and I personally was hesitant about making a syntax change like this without first thoroughly testing it out.

And so I made a PPX to test it out! You can install https://github.com/jaredly/get_in_ppx and experience the magic today.

This ppx includes two operators, that are only valid within the [%get_in ] form.

  • #?? is to be used when both sides are optional. E.g. the object on the left is optional, and the attribute you're getting out is also optional. option({. "attr": option(int)})
  • #? is to be used when only the object is optional, but the attribute you're getting out is not. e.g. option({. "attr": int})

For the example query above, you could do:

let url = [%get_in response##user#??bestFriend#??profilePicture#?url]


  • npm install get_in_ppx
  • add this line to your bsconfig.json: "ppx-flags": ["get_in_ppx/ppx"]

And you should be good to go! If you run into any issues, leave me some feedback on the github repo.