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Created Jun 11, 2019
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Prisma Client Promise Weridness

The prisma client has what it calls a "fluent" API, which allows you to chain requests for sub-types but work with them all as if they were a promise (docs). Example:

const organization = await prisma.schedule({id: "foo"{}).organization();

All of the examples in the documentation demonstrate using async/await syntax when working with the client. Inspecting what gets returned from a call to the client shows then and catch properties, like a normal promise, as well as the other fields exposed as functions (which is what allows the fluent API).

The Problem

The ReasonML bindings I've written assume normal JavaScript promise behavior - async/await isn't supported in Reason. When encountering a Prisma error, however, my code wasn't actually handling the error like I'd expect - in fact, the catch function wasn't called.

Here's the generated output from the Reason Compiler (Repromise is a 3rd party lib that makes promises easier to work with in Reason. In this case match[0] is a promise getting returned from the function, and match[1] is the resolve function to resolve that promise. Essentially we're creating a new promise and returning it, and resolving it inside the handlers for Prisma's promise.)

function create$2(input) {
  var match = Repromise.make(/* () */ 0);
  var resolve = match[1];
    .then(function(res) {
      return Promise.resolve(
        Curry._1(resolve, /* Ok */ Block.variant("Ok", 0, [res])),
    .catch(function(err) {
      return Promise.resolve(
        Curry._1(resolve, /* Error */ Block.variant("Error", 1, [err])),
  return match[0];

This code has an intentional error in it (the input being passed is for creating a schedule, not querying for one) and so Prisma returns an error... but the catch block is never executed, and the debugger statement is never hit.

If I manually edit the compiled output to use async/await, it works as expected:

async function create$2(input) {
  var match = Repromise.make(
  /* () */
  var resolve = match[1];

  try {
    const res = await PrismaClient.prisma.schedule(input);
    /* Ok */
    Block.variant("Ok", 0, [res]));
  } catch (err) {

    /* Error */
    Block.variant("Error", 1, [err]));

  return match[0]; 

In this case, the debugger is triggered and the error is reported correctly.

I'm thinking that something about the way Prisma's client is modifying a base Promise to extend it with the additional fields must somehow be affecting normal Promise behavior?

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