Generics

Yet another point where GraphQL and Rust differs is in how generics work:

• In Rust, almost any type could be generic - that is, take type parameters.
• In GraphQL, there are only two generic types: lists and non-nullables.

This poses a restriction on what we can expose in GraphQL from Rust: no generic structs can be exposed - all type parameters must be bound. For example, we cannot expose Result<T, E> as a GraphQL type, but we can expose Result<User, String> as a GraphQL type.

Let's make a slightly more compact but generic implementation of the last schema error example:

extern crate juniper;
use juniper::{graphql_object, GraphQLObject};

#[derive(GraphQLObject)] 
struct User { 
    name: String, 
}

#[derive(GraphQLObject)] 
struct ForumPost { 
    title: String,
}

#[derive(GraphQLObject)]
struct ValidationError {
    field: String,
    message: String,
}

struct MutationResult<T>(Result<T, Vec<ValidationError>>);

#[graphql_object]
#[graphql(name = "UserResult")]
impl MutationResult<User> {
    fn user(&self) -> Option<&User> {
        self.0.as_ref().ok()
    }

    fn error(&self) -> Option<&[ValidationError]> {
        self.0.as_ref().err().map(Vec::as_slice)
    }
}

#[graphql_object]
#[graphql(name = "ForumPostResult")]
impl MutationResult<ForumPost> {
    fn forum_post(&self) -> Option<&ForumPost> {
        self.0.as_ref().ok()
    }

    fn error(&self) -> Option<&[ValidationError]> {
        self.0.as_ref().err().map(Vec::as_slice)
    }
}

fn main() {}

Here, we've made a wrapper around a Result and exposed some concrete instantiations of Result<T, E> as distinct GraphQL objects.

NOTE: The reason we needed the wrapper is of Rust's orphan rules (both the Result and Juniper's internal traits are from third-party sources).

NOTE: Because we're using generics, we also need to specify a name for our instantiated GraphQL types. Even if Juniper could figure out the name, MutationResult<User> wouldn't be a valid GraphQL type name. And, also, two different GraphQL types cannot have the same MutationResult name, inferred by default.