From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <maranget@yquem.inria.fr>
X-Original-To: caml-list@yquem.inria.fr
Delivered-To: caml-list@yquem.inria.fr
Received: by yquem.inria.fr (Postfix, from userid 18041)
	id 5F950BB9A; Thu, 27 Oct 2005 15:41:14 +0200 (CEST)
Date: Thu, 27 Oct 2005 15:41:14 +0200
To: "Alexander A. Vlasov" <zulu@galaradio.com>
Cc: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] Beginner's question.
Message-ID: <20051027134114.GB12868@yquem.inria.fr>
References: <1130419808.12902.13.camel@localhost.localdomain>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1130419808.12902.13.camel@localhost.localdomain>
User-Agent: Mutt/1.5.9i
From: maranget@yquem.inria.fr (Luc Maranget)
X-Spam: no; 0.00; caml-list:01 beginner's:01 maranget:01 maranget:01 ocaml:01 rec:01 rec:01 mismatch:01 transformers:01 avoiding:01 matchings:01 caml-list:01 beginner's:01 ocaml:01 beginners:01 
X-Spam-Checker-Version: SpamAssassin 3.0.3 (2005-04-27) on yquem.inria.fr
X-Spam-Level: 
X-Spam-Status: No, score=-2.8 required=5.0 tests=ALL_TRUSTED 
	autolearn=disabled version=3.0.3

> Hello.
> 
> I'm making my first steps in OCaml and trying (just for expirience) to
> implement a simple function which applies given function to all elements
> of given list.
> I successfully did it using if/then/else
> 
> let rec map func lst =
>         if lst = [] then []
>         else (func (List.hd lst)) :: (map func (List.tl lst))
> ;;
> 
> But I can't do it using pattern matching -- following function doesn't
> work
> 
> let rec mapm func lst = function
>         | ( _, [] ) -> []
>         | ( _, head::tail ) -> (func head) :: (mapm func tail)
> ;;
> 
> I get type mismatch error: characters 39-55:
> This expression has type 'a * 'b list -> 'c list but is here used with
> type  'c list
> 
> But wait, ``map func (List.tl lst)''  in first example has the same
> type, isn't it?
> Both right parts in first and second example are, say, transformers from
> a tuple ( something, list-of-something-else ) to list-of-something-else.
> 

Basically no, your function is defined as taking three arguments
func, lst and one anonymous argument to be matched,
where, in fact, you want to match over 'lst'.


I sugest that, to begin with caml, you stick to explict
match constructs, avoiding matchings introduced by function

 let rec mapm func lst = match lst with
         | [] -> []
         | head::tail -> (func head) :: (mapm func tail)
 ;;


Oh well, and with function :

let rec mapm func = function 
         | [] -> []
         | head::tail -> (func head) :: (mapm func tail)
 ;;
> 
> 
> 
> -- 
> Best regards,
> Alexander A. Vlasov.
> 
> _______________________________________________
> Caml-list mailing list. Subscription management:
> http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list
> Archives: http://caml.inria.fr
> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
> Bug reports: http://caml.inria.fr/bin/caml-bugs

-- 
Luc Maranget