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From: Goswin von Brederlow <goswin-v-b@web.de>
To: Yoann Padioleau <padator@wanadoo.fr>
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] pattern matching and records vs tuples
References: <87skkbuxx8.fsf@aryx.cs.uiuc.edu>
Date: Tue, 14 Apr 2009 18:40:40 +0200
In-Reply-To: <87skkbuxx8.fsf@aryx.cs.uiuc.edu> (Yoann Padioleau's message of
	"Tue, 14 Apr 2009 09:12:19 -0500")
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X-Spam: no; 0.00; compiler:01 compiler:01 foo:01 ocaml:01 'foo':01 mfg:98 caml-list:01 tuples:01 tuples:01 pairs:01 int:01 int:01 writes:01 partially:02 tuple:02 

Yoann Padioleau <padator@wanadoo.fr> writes:

> Hi, 
>
> I've found that while records provide advantages over tuples, 
> they also have disadvantages when it comes to evolution issues. 
> If I decide to evolve code using a tuple type, for instance adding
> new information and so extend a 4-uple in a 5-uple, then the compiler
> will tell me all the places that I need to update, which is good.
> If I use records instead, and have 4 fields, and I want to add again
> some new information in a new field, then the way the compiler works
> right now will not help me at all. E.g with this code 
>
>   type record = { 
>     field1: int;
>     field2: int;
>   }
>   let foo = function 
>    { field1 = v1; field2 = v2 } -> ...
>
>
> If I extend record with a new field   field3, then ocaml will
> not warn me and tell me to modify also the 'foo' function :(

On the other hand that is a verry good thing.

let set_field1 r x = { r with field1 = x }
let set_field2 r x = { r with field2 = x }

Try doing that with tuples and then add a 3rd, 4th, 5th field.

I have to say I didn't even know you could match a record partially
or have ever wanted to match a record at all. I use record only when I
have a collection of otherwise independent values. As such any
matching will be done on a single component of the record but never
pairs of them. At least I can't remember having done so before.

MfG
        Goswin