Add type parameters to allow for schema type constraints

[jamesmacaulay]

Concurrently with #17, we should investigate adding phantom type parameters to ObjectType and SelectionSpec representing a type marker corresponding to a type from the target schema:

type ObjectType schemaType
    = ObjectType

type SelectionSpec schemaType selectionType result vars
    = SelectionSpec AST.Selection (AST.SelectionSet -> Decoder result) (List (Variable vars)) (List AST.FragmentDefinitionInfo)

This would allow each module that provides helper functions for an object/interface/union type to constrain those functions such that they only operate in correct ways according to the source schema. For example:

module Schema.Types.User exposing (..)

import GraphQL.Request.Builder exposing (..)
import Schema.Types as Types

name : SelectionSpec Types.User String vars
name =
    field "name" [] string


projects :
    List ( String, Arg.Value vars )
    -> ValueSpec NonNull (ObjectType Types.Project) result vars
    -> SelectionSpec Types.User (List result) vars
projects args projectSpec =
    field "projects" args (list projectSpec)

[jamesmacaulay]

Note: instead of adding this parameter to ObjectType, it may be preferable to add it to ValueSpec, and allow it for non-object types as well.

[jamesmacaulay]

Closing for the foreseeable future along with #17.