[tpm] Answers to some questions for tonight's talk

[zoffix at zoffix.com]

Turns out there's a much simpler way to check if a value fits a  
subset: just smartmatch against it!

     subset Even where * %% 2;
     say 3 ~~ Even;
     say 42 ~~ Even

     # OUTPUT:
     # False
     # True

Hope this helps.

Cheers,
ZZ




Quoting zoffix at zoffix.com:

> Hey,
>
> Thanks again for having me.
>
> Now that I got a Perl 6 compiler at my hands to play with, I can  
> answer a couple of questions that were asked:
>
> 1)
>
> Better messages for where { ... } subsets. The code in the `where`  
> can be anything you want, so you can `warn` or `fail` inside the  
> check to get a better error message. Once caveat: the argument given  
> to `callframe` might be different depending on where you're  
> performing the check. Try adjusting it:
>
>
>     subset Foo of Int where {
>         $_ > 10_000
>             or fail "You need a number more than 10,000 on "
>                 ~ "line {(callframe 4).line}, but you passed $_";
>     };
>
>     my Foo $x = 1000;
>
>     # OUTPUT:
>     #  You need a number more than 10,000 on line 7, but you passed 1000
>     #  in block <unit> at test.p6 line 2
>
> 2)
>
> As far as testing whether something fits the subset, you can use
> this trick with trying to assign to a variable and catching the  
> exception. It feels a bit like a hack, but I'm unsure if there's a  
> better way:
>
>     subset Foo of Int where {
>         $_ > 10_000
>             or fail "You need a number more than 10,000 on "
>                 ~ "line {(callframe 4).line}, but you passed $_";
>     };
>
>     my $value = 42;
>     try { my Foo $x = $value; CATCH { fail "It's no good" }; };
>     say "It's fine";
>
>     # OUTPUT:
>     #  It's no good
>     #    in block  at test.p6 line 8
>     #    in block <unit> at test.p6 line 8
>
> 3)
>
> "Can you have an infinite Set?"
>
> No, it tries to actually create one. Makes sense, since a set cares  
> about the elements. Sure, it's possible to special-case some forms  
> of sequences to figure out whether an element is part of the  
> sequence or not, but it's probably not worth it. In a more general  
> case, you are faced with the Halting Problem. Speaking of which,  
> here is a gotcha with the sequence operator and the upper limit:
>
>     my @seq = 0, 2 ... * == 1001;
>
> Here, I'm using the sequence operator to create a sequence of even  
> numbers, and I'm limiting the upper bound by when it'd be equal to  
> 1001. But it won't ever be equal to that. To human brain, it might  
> seem obvious that once you're over 1001, you should stop here, but  
> to a computer it's a Halting Problem and it'll keep trying to find  
> the end point (so it'll never complete here).
>
> 4)
>
> Places to learn Perl 6: along with http://perl6intro.com/ that I  
> mentioned during the talk, there's also Learn X in Y Minues Perl 6  
> page, which I personally found very useful when just starting out  
> with Perl 6: https://learnxinyminutes.com/docs/perl6/
>
> I'm also including the link to the Ecosystem:  
> http://modules.perl6.org/  you should have `panda` program  
> installed, and you can install modules from the Ecosystem by typing  
> `panda install Foo::Bar`
>
>
> These are all that I can remember being asked.
>
> Hope it helps.
>
> Cheers,
> ZZ
>
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