Variable pattern matching tuple

Variable pattern matching tuple

[Denis Bueno]

The following is not intuitive to me.

# type 'a withint = Withint of 'a * int;;
type 'a withint = Withint of 'a * int
# let foo = Withint ("hi there", 4);;
val foo : string withint = Withint ("hi there", 4)
# match foo with Withint _ -> ();;
- : unit = ()
# match foo with Withint d -> d;; (* expression A*)
Characters 15-24:
  match foo with Withint d -> d;;
                 ^^^^^^^^^
The constructor Withint expects 2 argument(s),
but is here applied to 1 argument(s)
#

What is the reason that expression A won't compile? Can't one variable
stand for a tuple in a pattern-matching clause?

-Denis

Re: [Caml-list] Variable pattern matching tuple

[Pietro Abate]

On Mon, Sep 18, 2006 at 09:26:15PM -0400, Denis Bueno wrote:
> What is the reason that expression A won't compile? Can't one variable
> stand for a tuple in a pattern-matching clause?

somebody was discussing a similar issue on the beginners 
mailing list this week:

type 'a withint = Withint of ('a * int);;
type 'a withint = Withint of ('a * int)
let foo = Withint ("hi there", 4);;
val foo : string withint = Withint ("hi there", 4)
 match foo with Withint d -> d;; ;;
- : string * int = ("hi there", 4)

if you want to pattern match the tuple you need to add parenthesis 
in the type declaration.

have a look here for a detailed explanation:
http://www.archivesat.com/Ocaml_Beginners/thread1620469.htm

:)
p

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