Re: [Caml-list] variant with tuple arg in pattern match?

[Alain Frisch <frisch@clipper.ens.fr>]

Hello,

On Wed, 4 Apr 2001, Chris Hecker wrote:

> I expected ii to be bound to the int * int tuple (1,2) in this pattern match:
> 
> # type foo = Foo of int * int;;
> type foo = Foo of int * int

You define Foo to be a constructor with two arguments ...

(the star here is not the same as the one for tuple types)

> #   match Foo (1,2) with Foo ii -> ii ;;

and you use it as an unary constructor. Hence the error:

> Characters 23-29:
> The constructor Foo expects 2 argument(s),
> but is here applied to 1 argument(s)

The solution is to declare:

type foo = Foo of (int * int)

The drawback is that the internal representation of the value Foo (1,2)
uses two heap allocated blocks (one for the constructor, one for the
tuple). A flat representation, as is the case with your declaration, may
be more efficient, but then you have to explicitely reconstruct the tuple:

match Foo (1,2) with Foo (a,b) -> (a,b);;


The issue probably deserves a few words in the FAQ or in the manual.


Hope this helps.

-- 
Alain Frisch
-------------------
To unsubscribe, mail caml-list-request@inria.fr.  Archives: http://caml.inria.fr