On Thu, Apr 24, 2008 at 09:15:15PM +0200, TSa wrote:
> I had hoped that WHAT denotes a more specific type than HOW. E.g.
>    subset ThreeChars of Str where {$_.elems == 3}
>    my ThreeChars $x = 'xxx';
>    $x.WHAT;  # ThreeChars
>    $x.HOW;   # Str
>    $x === 'xxx'; # false because type i.e. the WHAT
>                  # has to be the same

You are confusing the container with the object.  .WHAT and .HOW are
both dynamically typed, and $x.WHAT returns Str, because objects do
not carry subtypes.  The container enforces the ThreeChars constraint,
but does not require a ThreeChars object.

And .HOW doesn't return Str, it returns a metaobject of some sort, which
might be the same as ^Str in a class-based system, but needn't.

In your drive to separate types from values, please note that Perl 6
considers all values to be subsets.  'xxx' is just a Str that is
constrained to be equal to three x's.  So please don't also try to
make subsets into real types, if you want to keep your types separate...


> It were interesting to know what $x.new returns. If that dispatches
> to Str the result is '' which is not a ThreeChars. So how does the
> subset declaration make it return e.g. '   '? Looks like one needs
> more than a subset declaration to make a proper subtype ;)
>> Using Str by itself is a listop with no parameters and returns the 
>> undefined prototype. So
>> say "10".WHAT === Str;
>> prints True. Both refer to the same value: the undefined prototype object 
>> of type Str. If the undefined prototype is guaranteed to be singular, then 
>> =:= would also show True.
> Sounds reasonable. The HOW behind Str might e.g. be PerlStr.
>> say "10".HOW === Str;
>> prints False. The left side is a Metaobject, the right a Str mixed in with 
>> something that delegates class methods and overrides stringification etc. 
>> The things you can call on them are different. They could not be 
>> implemented as the same object.
> How would Str be implemented then? It is more concrete than a role but
> more abstract than a class.
> Regards, TSa.
> -- 
> "The unavoidable price of reliability is simplicity"
>   -- C.A.R. Hoare

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