Date: Fri, 6 Sep 1996 18:20:31 +0900
From: christo@nextsolution.co.jp (Frank Christoph)
Message-Id: <9609060920.AA00629@sparc3.nextsolution.co.jp>
To: e-posse@uniandes.edu.co
In-Reply-To: <322F0B89.553@isis.uniandes.edu.co> (message from Ernesto Posse on Thu, 05 Sep 1996 12:19:05 -0500)
Subject: Re: Pattern Matching
> I have a question about pattern matching. I need a function which
> binds an identifier to a value in certain data structure (which is
> called "heap" here). This is actually a (string * record) list. It is
> not suppose to be a mutable data structure, so the binding just creates
> a
> new list copying the same values as the old one with the exception of
> the
> record to be changed. For this purpose I am using List.map (I am using
> O'Caml 1.01 for Windows 95):
...
> However the compiler tells me that line marked (* 1 *) is an unused case
> of the match expression. I thought that maybe the problem was that the
> inner id variables (the ones in the function passed to map) are
> identifier
> patterns therefor they are different from the parameter of the assign
> function.
There is no equality relationship between pattern variables and identifiers
in enclosing scopes unless you make it explicit with a boolean guard.
> So I tried to fix it like this:
> let assign heap id obj =
> List.map
> ( function
> (name,{ vartype = vt; contents = v;
> constraints = c; dependencies = d })
> when name = id ->
> (name,{ vartype = vt; contents = obj;
> constraints = c; dependencies = d })
> | (name,_) as r when name <> id -> r )
> heap
> And now I get the warning "this pattern-matching is not exhaustive".
It's probably because the compiler doesn't know that ((x = y) or (x <> y))
is always true. The following function definition also generates a complaint:
#let f = function x when x = 1 -> x | x when x <> 1 -> x+1;;
Warning: this pattern-matching is not exhaustive
val f : int -> int = <fun>
In every compiler I know of, patterns are matched sequentially. So if you
drop the guard and change the second match to:
(name,_) as r -> r
it should compile without a warning and have the same semantics, unless you
add a new match case.
------------------------------------------------------------------------
Frank Christoph Next Solution Co. Tel: 0424-98-1811
christo@nextsolution.co.jp Fax: 0424-98-1500