Guards -- not just for switch!

[Remi Forax]

----- Mail original -----
> De: "Guy Steele" <guy.steele at oracle.com>
> À: "Brian Goetz" <brian.goetz at oracle.com>
> Cc: "amber-spec-experts" <amber-spec-experts at openjdk.java.net>
> Envoyé: Mercredi 29 Novembre 2017 22:49:23
> Objet: Re: Guards -- not just for switch!

> Um, let’s be careful here.  See below.
> 
>> On Nov 29, 2017, at 4:54 PM, Brian Goetz <brian.goetz at oracle.com> wrote:
>> 
>> As we've swirled around the design space on pattern matching, it seems the
>> sensible place to land is to not provide explicit syntax for AND and OR
>> patterns, but to support guards on case labels:
>> 
>>     case Foo(String s)
>>         where (s.length() > 0): ...
>> 
>> In the course of a separate discussion, we realized that guards can profitably
>> go in some other places, too.  Like methods/constructors:
>> 
>>     public Range(int lo, int hi)
>>         where (lo <= hi) {
>>             this.lo = lo;
>>             this.hi = hi;
>>     }
>> 
> 
> The two situations are not analogous, because if a case clause fails you just
> try the next one, but if a constructor fails you don’t try another constructor.
> 
> If we use the term “where” on a constructor, then Joe Programmer (that's me)

i was thinking you were more a random Guy :)

> will probably be very surprised if
> 
>  switch (x) {
>      case Foo(String s)
>          where (s.length() > 0): ...
>      case Foo(String s)
>          where (s.length() == 0): ...
>  }
> 
> 
> works, but
> 
>    public Interval(int lo, int hi)
>        where (lo <= hi) {
>            this.lo = lo;
>            this.hi = hi;
>	    this.exterior = false;
>    }
> 
>    public Interval(int lo, int hi)
>        where (lo > hi) {
>            this.lo = lo;
>            this.hi = hi;
>	    this.exterior = true;
>    }
> 
> does not work.
> 
> It might make sense to use a word such as “requires” or “assert” rather than
> “where” in the constructor case.

yes, it's the whole design by contract thingy, i vote for "requires" like in Eiffel, 

record Apple(int seed)
  requires seed >= 0;

> 
> —Guy


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