On Thu, Mar 22, 2012 at 9:07 AM, Bruce Gray <bruce.g...@acm.org> wrote:
> On Mar 21, 2012, at 11:49 PM, Jonathan Lang wrote:
>
> What I want to know is whether there's a way to define a step function
>> that's based in part or in whole on the current term's index. For example,
>> how would I use infix:<...> to generate the perfect squares between 0 and
>> 100? Namely, '0,1,4,9,16,25,36,49,64,81,**100'. For example, is Perl 6
>> set up to try to pass the current element's index into the step function by
>> means of a named parameter?
>>
>> I would hope for something like:
>>
>> -> :index { $index * $index } ... 100
>>
>> or
>>
>> 1, *+:index ... * # 1,3,6,10,15,21,28,36,45,...
>>
>> (Note: I _don't_ expect the above to work as is; they're merely intended
>> to convey a rough idea of what I _do_ want.)
>>
>> If not, how _would_ I generate these lists?
>>
>
>
> In real life, you would just use this:
> my @squares = map { $_ * $_ }, 0..10;
> or, for an infinite list, use a binding instead of an assignment:
> my @squares := map { $_ * $_ }, 0..*;
>
True enough.
> But you were asking about ... specifically :^)
>
Yes, I was.
> On freenode/#perl6, I was pointed to part of the spec that I had
> overlooked.
> The "sequence operator" is defined in S03:
>
> http://perlcabal.org/syn/S03.**html#List_infix_precedence<http://perlcabal.org/syn/S03.html#List_infix_precedence>
> Buried in its definition is this gem:
>
> http://perlcabal.org/syn/S03.**html#line_1884<http://perlcabal.org/syn/S03.html#line_1884>
> The function may also be slurpy (n-ary), in which case all
> the preceding values are passed in (which means they must
> all be cached by the operator, so performance may suffer,
> and you may find yourself with a "space leak").
>
Interesting; it never occurred to me to try to retrieve the current index
by slurping in all of the previous elements and counting them.
> After writing all the above, it occurred to me that the use of @_ should
> implicitly define a closure as slurpy/n-ary. That would remove the need
> for the arrow, and make the code much less ugly.
> Also, the first value (0) might be unnecessary. The spec says that it
> should not be required when the closure is 0-ary, but I think that
> should also be true for slurpy/n-ary closures.
>
Agreed: starting values should only be mandatory when dealing with a step
function that definitely requires them.
> These work in Niecza:
>
> my @squares := { @_ ** 2 } ... *;
>
And this is rather elegant, syntactically (though not so much in the
performance department).
--
Jonathan "Dataweaver" Lang
