Shadow the Seq module's iter and map functions to include type test

gistfile1.fs

module Seq
    // Shadowing the iter function is probably ok.
    let iter f (seq : seq<'a>) =
        match seq with
        | :? array<'a> -> Array.iter f (seq :?> array<'a>)
        | :? list<'a> -> List.iter f (seq :?> list<'a>)
        | _ -> Seq.iter f seq

    // Though you probably shouldn't shadow the map function!!
    let map f (seq : seq<'a>) =
        match seq with
        | :? array<'a> -> Array.map f (seq :?> array<'a>) :> seq<'a>
        | :? list<'a> -> List.map f (seq :?> list<'a>)    :> seq<'a>
        | _ -> Seq.map f seq

    let toArray (seq : seq<'a>) =
        match seq with
        | :? array<'a> -> seq :?> array<'a>
        | _ -> Seq.toArray seq

    let toList (seq : seq<'a>) =
        match seq with
        | :? list<'a> -> seq :?> list<'a>
        | _ -> Seq.toList seq


#load "Module1.fs"

let n = 10000000
let doNothing _ = ()
let incr x = x + 1

let arr = Array.init n (fun _ -> 0)

let rec buildList n acc i = if i = n then acc else buildList n (0 :: acc) (i + 1)
let lst = buildList n [] 0

#time

arr |> Array.iter doNothing             // 14ms
arr |> Seq.iter doNothing               // 17ms
arr |> Collections.Seq.iter doNothing   // 56ms

lst |> List.iter doNothing              // 19ms
lst |> Seq.iter doNothing               // 19ms
lst |> Collections.Seq.iter doNothing   // 89ms

arr |> Array.map incr                   // 31ms
arr |> Seq.map incr |> Seq.toArray      // 21ms
arr |> Seq.map incr 
    |> Collections.Seq.toArray          // 39ms
arr |> Collections.Seq.map incr 
    |> Collections.Seq.toArray          // 233ms

lst |> List.map incr                    // 776ms
lst |> Seq.map incr |> Seq.toList       // 768ms
lst |> Seq.map incr 
    |> Collections.Seq.toList           // 853ms
lst |> Collections.Seq.map incr 
    |> Collections.Seq.toList           // 1683ms

Just checked and FSharp.Core 4.3.0.0 does not specialize Seq.head or Seq.map

That's a shame, but easy enough to shadow those operations anyway, think it's worth adding to fsharpx?

I think it's definitely worth checking if these optimizations really improve the performance in most cases.And if they do, then of course they would be a great match to add to FSharpx!