Query.elm

module GraphQL.Query exposing (..)

import Json.Decode as Decode exposing (Decoder, (:=))
import Array exposing (Array)


type SelectionSet
    = SelectionSet (List Selection)


type Selection
    = FieldSelection Field


type Field
    = Field
        { name : String
        , selectionSet : SelectionSet
        }


type Decodable a result
    = Decodable a (Decoder result)


getAst : Decodable a result -> a
getAst (Decodable ast _) =
    ast


getDecoder : Decodable a result -> Decoder result
getDecoder (Decodable _ decoder) =
    decoder


string : Decodable SelectionSet String
string =
    Decodable (SelectionSet []) Decode.string


int : Decodable SelectionSet Int
int =
    Decodable (SelectionSet []) Decode.int


float : Decodable SelectionSet Float
float =
    Decodable (SelectionSet []) Decode.float


bool : Decodable SelectionSet Bool
bool =
    Decodable (SelectionSet []) Decode.bool


list : Decodable SelectionSet a -> Decodable SelectionSet (List a)
list (Decodable selectionSet decoder) =
    Decodable selectionSet (Decode.list decoder)


array : Decodable SelectionSet a -> Decodable SelectionSet (Array a)
array (Decodable selectionSet decoder) =
    Decodable selectionSet (Decode.array decoder)


object : (a -> b) -> Decodable SelectionSet (a -> b)
object constructor =
    Decodable (SelectionSet []) (Decode.succeed constructor)


field :
    String
    -> Decodable SelectionSet a
    -> Decodable SelectionSet (a -> b)
    -> Decodable SelectionSet b
field name child parent =
    let
        field =
            Field
                { name = name
                , selectionSet = (getAst child)
                }

        (SelectionSet parentSelections) =
            (getAst parent)

        selections =
            (FieldSelection field) :: parentSelections

        decoder =
            Decode.object2 (<|) (getDecoder parent) (getDecoder child)
    in
        Decodable (SelectionSet selections) decoder


type alias Project =
    { name : String }


type alias User =
    { name : String
    , projects : Connection Project
    }


type alias Connection a =
    { edges : List (Edge a)
    , pageInfo : PageInfo
    }


type alias Edge a =
    { cursor : String
    , node : a
    }


type alias PageInfo =
    { hasNextPage : Bool
    , hasPreviousPage : Bool
    }


edge child =
    object Edge
        |> field "cursor" string
        |> field "node" child


pageInfo =
    object PageInfo
        |> field "hasNextPage" bool
        |> field "hasPreviousPage" bool


connection child =
    object Connection
        |> field "edges" (list (edge child))
        |> field "pageInfo" pageInfo


connectionField :
    String
    -> Decodable SelectionSet a
    -> Decodable SelectionSet (Connection a -> b)
    -> Decodable SelectionSet b
connectionField name child =
    field name (connection child)


user : Decodable SelectionSet User
user =
    let
        project =
            object Project
                |> field "name" string
    in
        object User
            |> field "name" string
            |> connectionField "projects" project