Is there a way to nest calls to F# active patterns?

Question

Is there a way to nest calls to active patterns?

Something like this:

type Fnord =
| Foo of int

let (|IsThree|IsNotThree|) x = 
  match x with
  | x when x = 3 -> IsThree
  | _ -> IsNotThree

let q n =
  match n with
  | Foo x ->
    match x with
    | IsThree -> true
    | IsNotThree -> false
  // Is there a more ideomatic way to write the previous
  // 5 lines?  Something like:
//  match n with
//  | IsThree(Foo x) -> true
//  | IsNotThree(Foo x) -> false

let r = q (Foo 3) // want this to be false
let s = q (Foo 4) // want this to be true

Or is the match followed by another match the preferred way to go?

The damn language is unreadable. Seriously - what can it do that Python cannot? — Hamish Grubijan, Jan 04 '10 at 01:24
@lpthnc: Type checking? Algebraic data types? Currying? etc etc ... Actually, I already answered this question: http://stackoverflow.com/questions/126790/if-you-already-know-lisp-why-would-you-also-want-to-learn-f/129402#129402 — Nathan Shively-Sanders, Jan 06 '10 at 15:13

score 12 · Accepted Answer · answered Jan 04 '10 at 01:34

12

It works. You just have the patterns backwards.

type Fnord =
| Foo of int

let (|IsThree|IsNotThree|) x = 
  match x with
  | x when x = 3 -> IsThree
  | _ -> IsNotThree

let q n =
  match n with
  | Foo (IsThree x) -> true
  | Foo (IsNotThree x) -> false

let r = q (Foo 3) // want this to be true
let s = q (Foo 4) // want this to be false

answered Jan 04 '10 at 01:34

gradbot

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@AlexeyRomanov Well, the ability to nest patterns in their main benefit over other forms of dispatch, e.g. virtual methods. – J D Mar 11 '13 at 21:06

Is there a way to nest calls to F# active patterns?

1 Answers1