[dschafer]

It's passed to that function in the reference implementation, but a server would probably create a helper function that runs a query for a particular schema. For example, you can just pass a query to Facebook's GraphQL endpoint (without specifying a schema), and it will run the query on the Facebook schema.

[barakm]

This is also reasonable; but an example of how schema get created and discovered by a client would be a nice extension to the spec. :)