Date: Wed, 13 Nov 1996 22:33:04 +0000
From: Quercia <querciam@l-carnot1.ac-dijon.fr>
To: caml-list <caml-list@inria.fr>
Subject: match values
If you can't read french, please do skip the 51 following lines.
En francais :
Le code Caml suivant ne fonctionne pas comme on pourrait le vouloir :
let f(x,y,z) = match x with
| y -> ...
| z -> ...
| _ -> ...
;;
le compilateur signale que les cas 2 et 3 sont inutiles car le "y"
du match est pris comme un identificateur nouveau et non comme le
deuxieme parametre de "f".
La solution est d'ecrire :
let f(x,y,z) =
if x = y then ...
else if x = z then ...
else ...
;;
ou aussi :
let f(x,y,z) = match 0 with
| _ when x=y -> ...
| _ when x=z -> ...
| _ -> ...
;;
mais ces deux formes ne me plaisent pas !
Peut-on envisager ?
1. de modifier la syntaxe de "match" de sorte que les identificateurs
soient evalues s'ils ne sont pas precedes de "as" comme dans :
match x with
| y -> ... (* comparaison des valeurs x et y *)
| _ as t -> ... (* definition de l'identificateur t *)
1'. sans pour autant modifier les autres formes de filtrage comme :
let (u,v) = ... in ... (* definition des identificateurs u et v *)
let f(u,v) = ... (* idem *)
Il est probable que 1 et 1' sont inconciliables ?
2. d'introduire une nouvelle forme de filtrage "matchval" qui evaluerait
les motifs avant de proceder a la comparaison, en conservant la
possibilite
de definir un nouvel identificateur par "_ as <ident>" ?
==============================================================================
In english :
The following Caml code does not work as one would expect :
let f(x,y,z) = match x with
| y -> ...
| z -> ...
| _ -> ...
;;
the compiler warns that cases #2 & #3 are useless because "y" in the
match
construct is interpreted as a new identifier and not as the second
parameter
of "f".
The solution is to write :
let f(x,y,z) =
if x = y then ...
else if x = z then ...
else ...
;;
or :
let f(x,y,z) = match 0 with
| _ when x=y -> ...
| _ when x=z -> ...
| _ -> ...
;;
but I don't like these constructs !
May I dream about ?
1. modify "match" syntax so that identifiers are evaluated unless they
are
precededed by "as" as in :
match x with
| y -> ... (* comparison of the values of x and y *)
| _ as t -> ... (* definition of identifier t *)
1'. without modifying other patern-matching forms as :
let (u,v) = ... in ... (* definition of identifiers u and v *)
let f(u,v) = ... (* idem *)
I think that 1 and 1' are incompatible ?
2. introduce a new form of pattern-matching "matchval" which would
evaluate
the patterns before comparison, preserving the possibility to define a
new
identifier with a "_ as <ident>" construct ?
-- Michel Quercia Lycee Carnot 16 bd Thiers 21000 Dijon e-mail = querciam@l-carnot1.ac-dijon.fr