More Active Pattern Activity

Here are some more samples I came up with while practicing with active patterns. Again, I'm not sure I would ever implement any of these in particular; there may be better ways. But still, I could see where a couple of them would be handy, and my main purpose was to become comfortable with using active patterns.

Forgive my unexciting examples, it was late on a Sunday night, lol.

(As always, presented "as-is" and without warranty or implied fitness of any kind; use at your own risk.)

// Explicit empty; just for fun.

let inline (|Empty|NonEmpty|) (s:string) =

  match System.String.IsNullOrEmpty(s) with

  | true -> Empty

  | _ -> NonEmpty

// Return Some() if s starts with p, else None.

let inline (|StartsWith|_|) (p:string) (s:string) =

  match s with

  | Empty -> None

  | _ ->

    match s.StartsWith(p) with

    | true -> Some()

    | _ -> None

// Return Some(p,s') if s starts with p, else None,

// where s' is the remainder of s.

let inline (|BeginsWith|_|) (p:string) (s:string) =

  match s with

  | Empty -> None

  | _ ->

    match s.StartsWith(p) with

    | true -> Some((p,s.Substring(p.Length)))

    | _ -> None

// Return (p,s') if s starts with p, else ("",s),

// where s' is the remainder of s.

let inline (|SplitStart|) (p:string) (s:string) =

  match s with

  | Empty -> ("",s)

  | _ ->

    match s.StartsWith(p) with

    | true -> (p,s.Substring(p.Length))

    | _ -> ("",s)

// Return index if s contains p, else None,

let inline (|IndexOf|_|) (p:string) (s:string) =

  match s with

  | Empty -> None

  | _ ->

    match s.IndexOf(p) with

    | -1 -> None

    | i -> Some(i)

let test0 (s:string) =

  match s with

  | StartsWith "zero" -> "zero"

  | StartsWith "one" -> "one"

  | _ -> ""

let test1 (s:string) =

  match s with

  | BeginsWith "zero" (h,t) -> t+h

  | BeginsWith "five" (h,t) -> t+h

  | _ -> s

let test2 (s:string) =

  match s with

  | SplitStart "zero" (h,t) -> t+h

let test3 (s:string) =

  match s with

  | IndexOf "zero" (i) -> s.Substring(i)

  | IndexOf "seven" (i) -> s.Substring(i)

  | _ -> ""

let tests = [

  test0;

  test1;

  test2;

  test3 ]

let strings = [

  "zero two";

  "one two seven four";

  "five two";

  "three two" ]

let result =

  List.map 

    (fun t -> List.map t strings)

    tests 

printf "Done."