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From: Pierre Weis <Pierre.Weis@inria.fr>
Message-Id: <199809080642.IAA27315@pauillac.inria.fr>
Subject: Re: declaration of an infix operator in a .mli file
To: lcheno@club-internet.fr (Laurent Ch=?ISO-8859-1?Q?éno?=)
Date: Tue, 8 Sep 1998 08:42:38 +0200 (MET DST)
Cc: caml-list@inria.fr
In-Reply-To: <199809071512.RAA29206@front1.grolier.fr> from "Laurent Ch=?ISO-8859-1?Q?éno?=" at Sep 7, 98 05:15:07 pm
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> Dans foo.ml, j'ai les définitions suivantes de nouveaux opérateurs infixes :
> 
> (***)
> let epsilon = 1.0e-4 ;;
> 
> let (prefix =?) = fun x y -> abs_float (x -. y) <. epsilon
> and (prefix <?) = fun x y -> x <. y +. epsilon
> and (prefix >?) = fun x y -> y <. x +. epsilon ;;
> (***)
> 
> Quel fichier foo.mli dois-je écrire pour les exporter ?

value prefix =? : float -> float -> bool
and prefix <? : float -> float -> bool
and prefix >? : float -> float -> bool;;

> Et, au fait, pourquoi
> 
> let (prefix =?) x y = abs_float (x -. y) <. epsilon ;;
> 
> est-il incorrect ? (J'obtiens une Erreur de syntaxe.)

Il n'y a pas de bonne raison évidemment. Mais l'idée est que le
mot-clé prefix suffit à introduire la version prefixe d'un opérateur
infixe et que les parenthèses sont inutiles. On écrirait donc plutôt

let prefix =? = fun x y -> abs_float (x -. y) <. epsilon
and prefix <? = fun x y -> x <. y +. epsilon
and prefix >? = fun x y -> y <. x +. epsilon ;;

et donc

let prefix =? x y = abs_float (x -. y) <. epsilon
and prefix <? x y = x <. y +. epsilon
and prefix >? x y = y <. x +. epsilon ;;

-----
> I wrote this litte file foo.ml :
> 
> (***)
> let epsilon = 1.0e-4 ;;
> 
> let (prefix =?) = fun x y -> abs_float (x -. y) <. epsilon
> and (prefix <?) = fun x y -> x <. y +. epsilon
> and (prefix >?) = fun x y -> y <. x +. epsilon ;;
> (***)
> 
> I do'nt know write the file foo.mli.

value prefix =? : float -> float -> bool
and prefix <? : float -> float -> bool
and prefix >? : float -> float -> bool;;

> Another question :  why is the following wrong ?
> 
> let (prefix =?) x y = abs_float (x -. y) <. epsilon ;;
> 
> (I obtain a Syntax error)

No really good reason. In fact the prefix keyword if used to introduce
the prefix version of an infix operator. Hence the parens are useless
and redundant. If you remove them then you can write

let prefix =? = fun x y -> abs_float (x -. y) <. epsilon
and prefix <? = fun x y -> x <. y +. epsilon
and prefix >? = fun x y -> y <. x +. epsilon;;

and thus

let prefix =? x y = abs_float (x -. y) <. epsilon
and prefix <? x y = x <. y +. epsilon
and prefix >? x y = y <. x +. epsilon;;

Pierre Weis

INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://cristal.inria.fr/~weis/