F# List SelectMany
f#, linq
Solution
`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
Problem
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 ```