A colleague has also been starting to learn F#, and today gave me a problem he’d been trying to solve in F# in a “functional style”. He wanted a function that took a string input, and returned a list of the words, tupled with the starting index.

I spent the whole of lunch staring at my screen, trying to wrap my head around List.fold, list.foldBack and other functions; but no joy.

While driving home, it occurred to me that I could do away with that nonsense, and just have a recursive function that passes all the values down, top-to-bottom!

Here’s my latest attempt… The results are correct, which is an improvement over my original attempt!

let splitOn f x = (Seq.takeWhile f x, Seq.skipWhile f x)
let isSpace c = c = ' '
let notSpace c = c <> ' '
let string (s : seq<char>) = new String(Seq.toArray s)

/// Get a list of all words in a string tupled with the starting index
let getWords input =

    let rec getWordsRec index results input =
        match input with
        | [] -> results
        | _ ->
            let white, rest = splitOn isSpace input
            let word, rest = splitOn notSpace rest
                (index + Seq.length white + Seq.length word)
                ((index + Seq.length white, string word) :: results)
                (Seq.toList rest)

    getWordsRec 0 [] (Seq.toList input) |> List.rev

let expected = [(0, "Hello"); (6, "test"); (12, "World")]
printfn "Result is %A" ((getWords "Hello test  World" = expected))

Can anyone come up with something more elegant?


I managed to simplify/flatten it a little, though I’m not convinced it’s any more readable!

let string (s : seq<char>) = new String(Seq.toArray s)

let getWords input =
    |> Seq.zip (Seq.initInfinite id) // Zip the list with indexes
    |> Seq.scan (fun (i1, x1) (i2, x2) -> // Scan; replacing  index with the previous index when not a space
        match x1 with
        | ' ' -> (i2, x2)
        | _ -> (i1, x2)
    ) (0, ' ')
    |> Seq.filter (fun (_, x) -> x <> ' ') // Strip the spaces
    |> Seq.groupBy (fun (i, _) -> i) // Group by the word start index
    |> Seq.map (fun (i, x) -> (i, x |> Seq.map snd |> string)) // Strip redundant indexes; convert to string

You may also like Chromebook Comparison Chart, Drone Comparison Chart and how to easily split Amazon referrals between Amazon US and Amazon UK.

Don't forget to check out my kids on YouTube! Amazon Echo Dot (2nd Generation) Review, GP Toys Luctan S912 RC Car Review, Syma X5HC-1 Drone/Quadcopter Review.