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
getWordsRec
(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?

Update

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 =
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

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