A quick note to help others using Juniper, a Rust GraphQL library

Published 19 February, 2020.

TL;DR: panicked at 'resolve() must be implemented by non-object output types' might just mean that your GraphQL query is badly formatted.

I’ve been spending some time with Rust recently, using it to build a GraphQL API.

It’s been a steep learning curve, but one I was enjoying until I started getting this error:

thread '…' panicked at 'resolve() must be implemented by non-object output types', …/juniper/src/types/async_await.rs:44:13

Googling found barely any mention of this error message outside of Juniper’s source code.

It took quite a bit of pairing and rubber-duck debugging to figure out what this message meant, but it turns out that it was because my GraphQL query was badly formatted; instead of this:

{
  products(searchTerm: "cool products") {
    pageInfo {
      hasPreviousPage
      hasNextPage
    }
    edges {
      node {
        id
      }
    } 
  }
}

…I was just asking for pageInfo.

{
  products(searchTerm: "cool products") {
    pageInfo
    edges {
      node {
        id
      }
    } 
  }
}

The GraphiQL interface was able to automatically correct a similar errors with edges or node, but there’s some magic1 missing from my implementation of pageInfo which doesn’t allow for the same.

  1. “Any sufficiently advanced technology is indistinguishable from magic.” ↩