Why Use Comments?
Depending upon the capabilities of the target language, there are a number of ways to implement an implementation-first GraphQL server like Grats. Some libraries use introspection while others use macros. So why does Grats choose to use comments instead? Let's explore why these other approaches are not a good fit for TypeScript and Grats.
Why not use introspection?
Because TypeScript types are stripped at runtime, it's fundamentally impossible to use introspection to "see" the types of fields/arguments etc. at runtime. This means that introspection cannot be used to derive a GraphQL schema from TypeScript types at runtime.
That said, introspection can be used to derived pieces of a GraphQL schema. For example, names of type/class decorations and field names. TypeGraphQL is a library that is similar to Grats in many ways, but uses decorators and introspection instead of comments. However, since types are not visible at runtime, it ends up needing to fall back to a builder-like API to express things like field return types, and argument types. This tradeoff has the advantage of avoiding a build step, but at the cost of a more complex API.
For Grats, we wanted to optimize for the simplest possible API.
Why not use macros?
TypeScript does not currently support macros, and seems unlikely to add them.
Then why comments?
Given the limitations faced when trying to support an implementation-first API in TypeScript, Grats falls back to a lowest common denominator: static analysis. Grats infers GraphQL types from TypeScript at build time by analyzing the code rather than running it.
We chose doc-blocks because they accurately convey that the code within them does not impact the runtime, while still having a well defined structure that can be parsed. In particular, we're able to leverage the TypeScript compiler's JSDoc parsing, which aligns well with our design principle of "A few dependencies well leveraged".
Ideally Grats would be able to leverage either introspection or macros to derive a GraphQL schema. These are both very nice approaches to implementation-first GraphQL. However, the limitations of TypeScript preclude these options. Instead, Grats resorts to static analysis to infer a GraphQL schema from TypeScript types, and doc-blocks strike a nice balance of being structured enough to be machine readable, while clearly communicating that they are not part of the runtime.