Improve types for useErrorHandler plugin

[andrew0]

Is your feature request related to a problem? Please describe.

The context property passed as the payload to useErrorHandler callbacks is typed as Readonly<DefaultContext>:

envelop/packages/core/src/plugins/use-error-handler.ts

Line 11 in b7d0a94

context: Readonly<DefaultContext>;

I'm trying to use this plugin and this type doesn't seem to be very useful. Given that the useErrorHandler function takes a generic type for the context, I would expect the ErrorHandler type to also be generic instead of just using DefaultContext.

Additionally, depending on the phase, it seems like the type is actually different. The context value is passed to the error handler callback in the parse and validate phases, but in the execution phase it's passing a TypedExecutionArgs<ContextType>:

envelop/packages/core/src/plugins/use-error-handler.ts

Line 24 in b7d0a94

errorHandler({ errors: result.errors, context: args, phase: 'execution' });

. So even though the type of context is Readonly<DefaultContext>, I need to look in context.contextValue (untyped) to get the same context value as the parse and validate phases.

And in the "context" phase, the context value passed to the error handler callback is a OnPluginInitEventPayload<ContextType>. I don't really understand why this is being passed to error handlers - it would make more sense to me if the context value was pulled from this callback instead:

envelop/packages/core/src/plugins/use-error-handler.ts

Line 48 in b7d0a94

context.registerContextErrorHandler(({ error }) => {

But I'm not sure if it's intentional that the "onPluginInit" payload is being used instead.

Describe the solution you'd like

Make the ErrorHandler type a generic type with a tagged union:

export type ErrorHandler<ContextType extends Record<string, any> = DefaultContext> = ({
  errors,
  context,
  phase,
}: {
  errors: readonly Error[] | readonly SerializableGraphQLErrorLike[];
} & (
  | {
      context: Readonly<ContextType>;
      phase: 'parse' | 'validate';
    }
  | {
      context: OnPluginInitEventPayload<ContextType>;
      phase: 'context';
    }
  | {
      context: TypedExecutionArgs<ContextType>;
      phase: 'execution';
    }
)) => void;

Describe alternatives you've considered

In my useErrorHandler plugin, explicitly cast the type of context based on the phase.

Additional context