26 lines
817 B
Forth
26 lines
817 B
Forth
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namespace Sanchime.Monads
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[<AutoOpen>]
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module Reader =
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// 读取器单子是计算状态并产生一个值't的单子
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type Reader<'s, 't> = Reader of ('s -> 't)
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// 单位元不读取任何操作，因此产生Reader<unit, 'a>
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let unit v = Reader (fun () -> v)
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// 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>
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// 并在一个元组中产生一个读取两个状态值的Reader<('d * 'b), 'c>
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let bind f (Reader (g)) = Reader (fun (a, b) ->
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let v = g a in
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let (Reader h) = f v in
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h b)
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type ReaderBuilder () =
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member __.Return(v) = unit v
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member __.Bind(v, f) = bind f v
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member __.Delay(f) = Reader (fun s ->
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let (Reader h) = f ()
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h s)
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let reader = ReaderBuilder ()
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