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Date: Tue, 19 Jun 2001 10:11:11 +0200
From: =?iso-8859-1?Q?Fr=E9d=E9ric?= van der Plancke <fvdp@decis.be>
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CC: caml <caml-list@inria.fr>
Subject: Re: [Caml-list] Newbie: declarations
References: <20010616170909.B10922@jean>
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[Excuse-me if this is a duplicate. I haven't seen the previous instance
of this message on the list yet, I think I sent it to the wrong address.
This message contains additional thoughts anyway ;-)]

leary@nwlink.com wrote:
> More to the point, is there a way to declare a record variable without
> listing/initializing all the fields?  If yes, what are the default values?

You can create a default record and initialize new records from a copy
of the default record like this:

type datarec = {
         mutable name : string;
         mutable color : string;
         mutable value : int;
}

let default_datarec = { name = "?"; color = "?"; value = 0; }   

let x = { default_datarec with value = 113 }
let y = { default_datarec with name = "y"; color = "00FF00 FF00FF" }

Beware of aliasing though: the name string is mutable and is shared by
all instances for which no specific name was given, so:

# x.name.[0] <- '!';;
- : unit = ()
# default_datarec.name;;
- : string = "!"

Furthermore "someone" may directly modify the default_datarec in your
back.

This is not the safest way to do it.
Unless you do this:

let default_datarec () =
 { 
   name = String.copy "?"; (* Need to copy in order to avoid aliasing *)
   color = String.copy "?"; 
   value = 0; 
 }   

let x = 
    let d = default_datarec () in 
    { d with value = 113 } 

One can't write { default_datarec () with value = 113 } ...

And BTW, I like Python's immutable strings ! (Wouldn't be long to
implement an immutable-strings module in OCaml though.)

/Frederic vdP

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