type Person = { FirstName: string; LastName: string }
let people = [ 
    { FirstName = "Jane"; LastName = "Milton" }
    { FirstName = "Travis"; LastName = "Smith" } ]

open FSharp.Data.GraphQL
open FSharp.Data.GraphQL.Types

// GraphQL type definition for Person type
let Person = Define.Object("Person", [
    Define.Field("firstName", String, fun ctx p -> p.FirstName)
    Define.Field("lastName", String, fun ctx p -> p.LastName)  
])

// each schema must define so-called root query
let QueryRoot = Define.Object("Query", [
    Define.Field("people", ListOf Person, fun ctx () -> people)
])

// then initialize everything as part of schema
let schema = Schema(QueryRoot)

open FSharp.Data.GraphQL.Execution
let query = """
    query Example {
        people {
            firstName
        }
    }
    """
async {
    let! response = schema.AsyncExecute(query) // Here's the error.
    printf "%A" response
}