namespace Sanchime.Monads

[<AutoOpen>]
module Reader =

    // 读取器单子是计算状态并产生一个值't的单子
    type Reader<'s, 't> = Reader of ('s -> 't)

    // 单位元不读取任何操作，因此产生Reader<unit, 'a>
    let unit v = Reader (fun () -> v)

    // 绑定操作接受Reader和一个读取状态另一部分的函数'a -> Reader<'b, 'c>
    // 并在一个元组中产生一个读取两个状态值的Reader<('d * 'b), 'c>
    let bind f (Reader (g)) = Reader (fun (a, b) ->
        let v = g a in
        let (Reader h) = f v in
        h b)

    type ReaderBuilder () =
        member __.Return(v) = unit v
        member __.Bind(v, f) = bind f v
        member __.Delay(f) = Reader (fun s ->
            let (Reader h) = f ()
            h s)

    let reader = ReaderBuilder ()