From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: X-Spam-Status: No, score=-1.3 required=5.0 tests=BAYES_00,INVALID_MSGID autolearn=no autolearn_force=no version=3.4.4 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: 103376,47def5aa7b3182bd X-Google-Attributes: gid103376,public From: Matthew Heaney Subject: Re: How to write TYPECASE in Ada 95? Date: 1999/02/18 Message-ID: #1/1 X-Deja-AN: 445542852 Sender: matt@mheaney.ni.net References: <79fct8$9k3$1@murdoch.acc.Virginia.EDU> <1103_918264881@DZOG-CHEN> <36cb2712.1390453@news.pacbell.net> NNTP-Posting-Date: Wed, 17 Feb 1999 17:59:31 PDT Newsgroups: comp.lang.ada Date: 1999-02-18T00:00:00+00:00 List-Id: tmoran@bix.com (Tom Moran) writes: > Dispatching is certainly better than a case statement - if you use it. > > Consider an existing package of the form: > > package Grocer is > type Fruit is abstract tagged ... > type Apple is new Fruit ... > ... > type Watermelon is new Fruit ... > ... > function Best(Budget : in Money) return Fruit'class; > end Grocer; > > And now you are writing a new package which, among other things, does > > with Grocer; > use Grocer; > package Meal is > procedure Serve(Dish : in Apple); > ... > procedure Serve(Dish : in Watermelon); > end Meal; > > Suppose a user wants, not unreasonably, to write > ... > Meal.Serve(Dish => Grocer.Best(Budget=>1.00)); > > Unless you can go back and modify Grocer, it appears you must include > in Meal > procedure Serve_Fruit(Dish : in Fruit'class); > with a body like > procedure Serve_Fruit(Dish : in Fruit'class) is > begin > if Dish in Apple'class then Serve(Apple(Dish)); > ... > elsif Dish in Watermelon'class then Serve(Watermelon(Dish)); > else raise Heck; > end if; > end Serve_Fruit; > > Is there a better way? It seems like the real problem in your example is that Fruit doesn't include any primitive operations for Serve'ing. If you want to extend a type with arbitrary behavior, you could take a look at using the Visitor pattern. Ada implementations of this pattern are available at the ACM patterns archive.