[IDL] Support accepts_definitions functions in IDL

[rmosolgo]

In the DSL, the best way to customize parts of a GraphQL schema is with custom definitions.

It would be nice to have a similar feature for the IDL, why not use the same feature?

Shout out to @theorygeek for first suggesting the parse -> apply -> remove pipeline for extracting behavior from IDL descriptions.

See to-dos and caveats below.

On this branch, these are equivalent:

Field

DSL

field :thing,  types.String, "A thing", property: :other_thing, authorize: :admin

IDL

# A thing 
# @property :other_thing
# @authorize :admin 
thing: String

Type

DSL

Types::ThingType = GraphQL::ObjectType.define do 
  name "Thing" 
  description "A thing"
  access :viewer
  # ...
end 

IDL

# A thing 
# @access :viewer
type Thing {
  # ...
}

Schema

DSL

MySchema = GraphQL::Schema.define do 
  max_complexity 1000
  object_from_id Helpers::ObjectFromId
  use GraphQL::Batch
end 

IDL

# @max_complexity 1000 
# @object_from_id Helpers::ObjectFromId
# @use GraphQL::Batch
schema {
  query Query 
}

Caveats

• Input is passed to instance_eval which is unsafe. This is ok for safe input, but it's a foot-gun if turn user input into a schema. I should lock down the parser a bit.
• TODO: lossless schema dump (ie, .to_definition should include @-definitions)
• TODO: somehow give a context-aware backtrace if something fails.
• TODO: support all customizable objects (eg GraphQL::Argument for prepare: handlers)
• TODO: don't reuse instrument -- find a new name for this
• TODO: don't use @, since it's already used for directives?

[ThisIsMissEm]

Supporting declarations through IDL would be very nice!

[rmosolgo]

I really like this approach, but I'm going to put it on the backburner and try #871 first.