Add more information in conversion error

[domi7777]

I've often had this error: Cannot return null for non-nullable field x.

I find it easier to understand why the error occurred with a bit more of information, eg:

Cannot return null for non-nullable field Transition.date. Value 2018-11-20T14:45:19.182Z cannot be converted to DateTime.

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[domi7777]

The pipeline failed but I'm not sure why. TBH it's the first time I do a PR for a flow project (I'm usually using Typescript).

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[IvanGoncharov]

You can use + to produce better formatting:

`Cannot return null for non-nullable field ` +
`${info.parentType.name}.${info.fieldName}. ` +
`Value ${inspect(result)} cannot be converted to ${inspect(returnType)}`

[IvanGoncharov]

I also think converted is not a correct term here, the spec calls it coercion:
https://facebook.github.io/graphql/draft/#example-cb7e7
Also, see 4a here:
https://facebook.github.io/graphql/draft/#CompleteValue()

[IvanGoncharov]

Cannot return null for non-nullable field DataType.test. Value null cannot be converted to [Int!]! is strange error.
You can do:

if (result === null) {
  throw new Error(`Cannot return null for non-nullable field ${info.parentType.name}.${info.fieldName}.`);
} else {
  throw new Error(`<Another error specifically about coercion>`);
}

[domi7777]

Thanks for the advice, I'll do that.

[domi7777]

How does this look?

if (result === null) {
  throw new Error(
    `Cannot return null for non-nullable field ` +
      `${info.parentType.name}.${info.fieldName}.`,
  );
} else {
  throw new Error(
    `Cannot coerce '${inspect(result)}' into type ` +
      `${inspect(returnType)} for non-nullable field ` +
      `${info.parentType.name}.${info.fieldName}`,
  );
}

[IvanGoncharov]

@domi7777 I still not sure about couple edge cases that could mismatch with this error message. I need to refresh my knowledge of this code to be sure so I will try to do that on weekend.

[Neitsch]

@IvanGoncharov - mind sharing some edge cases you can imagine?

[IvanGoncharov]

@Neitsch @domi7777 Hi sorry for the radio silence.
I have some project that I need to finish.
I will try to review and merge all stale PRs in the next couple of weeks.

mind sharing some edge cases you can imagine?

To be honest I already forgot. I will try to find some time next week to properly review it. Also feel free to ping me.

[mjmahone]

This seems like a positive improvement from my end.