F#列表选择很多 [英] F# List SelectMany

问题描述

这是一个非常简单的问题，但我没有找到答案:

This is quite simple question but I didn't find an answer:

F#中是否有任何Seq/List操作与LINQ SelectMany相匹配?

Is there any Seq/List operation in F# to match the LINQ SelectMany?

• 我知道如果我可以在F#中使用System.Linq 想要.
• 我知道我可以做一个递归方法 并使用F#计算表达式 (并使功能更强大).

• I know I can use System.Linq in F# if I want to.
• I know I can make a recursive method and use F# Computation Expressions (and make even more powerful things).

但是，如果我试图证明F#List操作比LINQ更强大...

But if I try to prove that F# List operations are more powerful than LINQ...

• .Where = List.filter
• .选择= List.map
• .Aggregate = List.fold
• ...

在C#中，SelectMany的用法非常简单:

In C# SelectMany usage syntax is pretty simple:

var flattenedList = from i in items1
                    from j in items2
                    select ...

是否有任何简单的直接匹配项，List.flatten，List.bind或类似的东西?

Is there any easy direct match, List.flatten, List.bind or something like that?

SelectMany有几个签名，但是最复杂的签名似乎是:

SelectMany has a couple of signatures, but the most complex one seems to be:

IEnumerable<TResult> SelectMany<TSource, TCollection, TResult>(
    this IEnumerable<TSource> source, 
    Func<TSource, IEnumerable<TCollection>> collectionSelector, 
    Func<TSource, TCollection, TResult> resultSelector
);

用F#表示，这将是:

('a -> 'b list) -> ('a -> 'b -> 'c) -> 'a list -> 'c list

推荐答案

collect是SelectMany的F#等效项，但是它不提供所有重载.这是制作您所引用的内容的方法.

collect is the F# equivalent of SelectMany however it doesn't provide all the overloads. Here's how to make the one you referenced.

let selectMany (ab:'a -> 'b seq) (abc:'a -> 'b -> 'c) input =
    input |> Seq.collect (fun a -> ab a |> Seq.map (fun b -> abc a b))
// gives
// val selectMany : ('a -> seq<'b>) -> ('a -> 'b -> 'c) -> seq<'a> -> seq<'c>

我相信F#不会提供所有SelectMany重载，因为它们会给库增加噪音.这是Microsoft中 SelectMany 的所有四个重载命名.

I believe F# doesn't provide all the SelectMany overloads because they would add noise to the library. Here's all four overloads to SelectMany in Microsoft Naming.

let selectMany (source : 'TSource seq) (selector : 'TSource -> 'TResult seq) =
    source |> Seq.collect selector

let selectMany (source : 'TSource seq) (selector : 'TSource -> int -> 'TResult seq) =
    source |> Seq.mapi (fun n s -> selector s n) |> Seq.concat

let selectMany (source : 'TSource) 
               (collectionSelector : 'TSource -> 'TCollection seq)
               (resultSelector : 'TSource -> 'TCollection -> 'TResult) =
    source 
    |> Seq.collect (fun sourceItem -> 
        collectionSelector sourceItem 
        |> Seq.map (fun collection -> resultSelector sourceItem collection))

let selectMany (source : 'TSource) 
               (collectionSelector : 'TSource -> int -> 'TCollection seq)
               (resultSelector : 'TSource -> 'TCollection -> 'TResult) =
    source 
    |> Seq.mapi (fun n sourceItem -> 
        collectionSelector sourceItem n
        |> Seq.map (fun collection -> resultSelector sourceItem collection))
    |> Seq.concat

"F#列表操作比LINQ更强大..."虽然seq/list操作非常有用，但某些真正的"F#力量"来自咖喱.

"F# List operations are more powerful than LINQ..." While seq / list operations are great some real "F# power" comes from Function Composition and Currying.

// function composition
let collect selector = Seq.map selector >> Seq.concat

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