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I'm new to F#, so I've ported an old Python script I have to F#. It scans a directory for files named something like Game.Of.Thrones.S01E02.xvidblahblah.mp4 or The_Wire_s03e04.somethingelse.avi, and renames them to a nice form like TheWire_s03e04_Hamsterdam.avi (where Hamsterdam is the episode's title, found from a web api). I'd welcome any advice on how to make it more idiomatic or efficient.

open System.Text.RegularExpressions
open System
open System.IO
open FSharp.Data

let RegexPattern pattern input =
    let m = Regex.Match(input, pattern)
    if m.Success then Some(List.tail [for g in m.Groups -> g.Value])
    else None

let (|ShortShowCode|_|) input = RegexPattern @"(.*)[sS](\d{1,2})[^\d]?[eE](\d{1,2})" input
let (|LongShowCode|_|) input = RegexPattern @"(.*)Season ?(\d{1,2}) ?Episode ?(\d{1,2})" input

let uncamelcase phrase = Regex.Replace(phrase, @"(?=[A-Z0-9])", " ")
let compressSpaces phrase = Regex.Replace(phrase, @"[^a-zA-Z0-9]+", " ")
let trim (phrase:string) = phrase.Trim()

let prettify title = title |> uncamelcase |> compressSpaces |> trim

let compress title = Regex.Replace(title, @"\W", "")

let apiUrl showName = sprintf "http://api.tvmaze.com/singlesearch/shows?q=%s&embed=episodes" (Uri.EscapeUriString(showName))
type Shows = JsonProvider<"http://api.tvmaze.com/singlesearch/shows?q=The%20Sopranos&embed=episodes">

let extension filePath = Path.GetExtension(filePath).ToLower()
let fileNameOnly filePath = Path.GetFileName(filePath)

let rec walk dir =
    seq { yield! Directory.EnumerateFiles(dir, "*")
          for subDir in Directory.EnumerateDirectories(dir) do
              yield! walk subDir }

type VideoFileInfo (showName: string, season: int, number: int, fileName: string) =
    member this.ShowName = showName
    member this.Season = season
    member this.Number = number 
    member this.FileName = fileName
    member this.Extension = extension fileName

let videoInfo vidFileName =
    match vidFileName with
    | ShortShowCode [showName; season; number]
    | LongShowCode [showName; season; number] 
        -> Some <| new VideoFileInfo(prettify showName, season = Int32.Parse season, number = Int32.Parse number, fileName = vidFileName)
    | _ -> None

let episodeMatch (toMatch: VideoFileInfo) (episode: Shows.Episode) = 
    (episode.Season = toMatch.Season && episode.Number = toMatch.Number)

let episodeInfo (epData: VideoFileInfo) =
    let show = epData.ShowName |> apiUrl |> Shows.Load
    show.Embedded.Episodes |> Seq.find (episodeMatch epData)

let titleImprovement vidInfo =
    let epInfo = episodeInfo vidInfo
    let newName = sprintf "%s_s%02ie%02i_%s%s" (compress vidInfo.ShowName) epInfo.Season epInfo.Number (compress epInfo.Name) vidInfo.Extension
    if vidInfo.FileName = newName then None 
    else Some(vidInfo.FileName, newName)

let renameVideo (dir: string) (oldTitle: string) (newTitle: string) =
    File.Move(dir + oldTitle, dir + newTitle)
    // Prints the command to reverse this rename!
    sprintf "rename \"%s%s\" \"%s%s\"" dir newTitle dir oldTitle

let printeach list = list |> Seq.iter (printfn "%A")

let testfiledir = "C:\\Dummy\\vids\\"
let testfilenames = ["The.Sopranos.Season 5 Episode 6.mp4"; "I am not an episode.txt"; "GameOfThrones_s01e02_TheKingsroad.m4v"; "TheWire.S03E04.avi"]
let createTestFiles dir filenames = filenames |> Seq.iter (fun filename -> File.WriteAllText(dir + filename, ""))

[<EntryPoint>]
let main argv = 
    createTestFiles testfiledir testfilenames
    walk testfiledir 
    |> Seq.map fileNameOnly 
    |> Seq.choose videoInfo 
    |> Seq.choose titleImprovement 
    |> Seq.map (fun (oldTitle, newTitle) -> renameVideo testfiledir oldTitle newTitle)
    |> printeach
    0
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Looks good; there's nothing I'd really do differently. Good structure, no overly imperative code, appropriate use of pipes (including not piping for calling a single function with a single argument, which many people do, but I think reduces readability), pattern matching, even Active Patterns. Nice use of a type provider with a web data source too.

Two very minor things:

  • You don't need new when calling a constructor; the compiler only recommends it when creating an instance implementing IDisposable. (I'm very happy about not needing new, because I find it terribly annoying and redundant in C#.)
  • I've recently started either putting if-then-else on a single line if short enough, or giving the then part its own line as well because that better represents the structure of the construct.
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