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Date: -- (:)
From: christo@n...
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