在F#中实施Haskell -Maybemonad-我们如何获得这个懒惰?
题
我们正在尝试从 http://www.haskell.org/all_about_monads/html/maybemonad.html 在f#中。
这个想法是在两个词典中搜索一个邮编。如果两个查找都返回结果,我们会查看第三个。
let bindM x k =
match x with
| Some value -> k value
| None -> None
let returnM x = Some x
type MaybeBuilder() =
member this.Bind(x, k) = bindM x k
member this.Return(x) = returnM x
member this.ReturnFrom(x) = x
member this.Delay(f) = f()
let maybe = MaybeBuilder()
//Sample dictionaries
let fullNamesDb =
[("Bill Gates", "billg@microsoft.com")
("Bill Clinton", "bill@hope.ar.us")
("Michael Jackson", "mj@wonderland.org")
("No Pref Guy", "guy@nopref.org")]
|> Map.ofList
let nickNamesDb =
[("billy", "billg@microsoft.com")
("slick willy", "bill@hope.ar.us")
("jacko", "mj@wonderland.org") ]
|> Map.ofList
let prefsDb =
[("billg@microsoft.com", "HTML")
("bill@hope.ar.us", "Plain")
("mj@wonderland.org", "HTML")]
|> Map.ofList
let mplus m1 m2 = if m1 <> None then m1 else m2
let (+) = mplus
let lookUp name = maybe {
let! combined = fullNamesDb.TryFind name + nickNamesDb.TryFind name
return! prefsDb.TryFind combined
}
let billGatesPref = lookUp "Bill Gates" |> printfn "%A" // Some "HTML"
let billyPref = lookUp "billy" |> printfn "%A" // Some "HTML"
let billClintonPref = lookUp "Bill Clinton" |> printfn "%A" // Some "Plain"
let steffenPref = lookUp "Steffen" |> printfn "%A" // None
let noPref = lookUp "No Pref Guy" |> printfn "%A" // None
System.Console.ReadKey() |> ignore
问题在于,即使第一个返回结果,我们也执行第二个查找。关于Haskell的好处在这里,它可以评估懒惰。现在,我们在F#中寻找类似的东西。我们尝试了以下操作,但看起来很丑陋,似乎打破了封装建筑商中逻辑的想法:
let mplus m1 m2 = if m1 <> None then m1 else m2()
let (+) = mplus
let lookUp name = maybe {
let! combined = fullNamesDb.TryFind name + fun _ -> nickNamesDb.TryFind name
return! prefsDb.TryFind combined
}
有更好的解决方案吗?
问候,福基
解决方案
您可以实现在Baybuilder中运行/组合的其他方法,因此结果应为以下内容:
let bindM x k =
match x with
| Some value -> k value
| None -> None
let returnM x = Some x
type MaybeBuilder() =
member this.Bind(x, k) = bindM x k
member this.Return(x) = returnM x
member this.ReturnFrom(x) = x
member this.Delay(f) = f
member this.Combine(a, b) = if Option.isSome a then a else b()
member this.Run(f) = f()
let maybe = MaybeBuilder()
//Sample dictionaries (the same with original sample)
let fullNamesDb = ...
let nickNamesDb = ...
let prefsDb = ....
let lookUp name =
let findName m = maybe {
let! v = Map.tryFind name m
return! prefsDb.TryFind v
}
maybe {
return! findName fullNamesDb
return! findName nickNamesDb
}
let billGatesPref = lookUp "Bill Gates" |> printfn "%A" // Some "HTML"
let billyPref = lookUp "billy" |> printfn "%A" // Some "HTML"
let billClintonPref = lookUp "Bill Clinton" |> printfn "%A" // Some "Plain"
let steffenPref = lookUp "Steffen" |> printfn "%A" // None
let noPref = lookUp "No Pref Guy" |> printfn "%A" // None
其他提示
总是有 Lazy
, ,这实际上是您在这里拥有的,但具有不同的语法:
let mplus m1 (m2 : Lazy<'a option>) =
match m1 with
| Some _ as m -> m
| None -> m2.Force()
let (+) = mplus
let lookUp name = maybe {
let! combined = fullNamesDb.TryFind name + lazy (nickNamesDb.TryFind name)
return! prefsDb.TryFind combined
}
不隶属于 StackOverflow