Question

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

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

• 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).

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

• .Where = List.filter
• .Select = List.map
• .Aggregate = List.fold
• …

In C# SelectMany usage syntax is pretty simple:

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

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

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
);

In F# terms this would be:

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

Answer 1

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>

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# 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