The discussion took an interesting route. :)
The problem with an array in a hash is the problem of hashes implicitly
itemizing values. Consider the example:
⇒ raku -e 'my %h = a => [1,2,3,4]; say %h<a>.raku; say (%h<a> ==> map({ sqrt $_
})); %h<a> := [1,2,3,4]; say %h<a>.raku; say (%h<a> ==> map({ sqrt $_ }))'
$[1, 2, 3, 4]
(2)
[1, 2, 3, 4]
(1 1.4142135623730951 1.7320508075688772 2)
Sorry for one-liner. The point of the example is that binding to a key avoids
itemization of its RHS if it is a non-itemized object.
Because itemizing in fact means putting things into a Scalar container, then
the fastest thing which resolves the issue with ==> would be de-containerize a
value:
⇒ raku -e 'my %h = a => [1,2,3,4]; say %h<a>.raku; say (%h<a> ==> map({ sqrt $_
})); say (%h<a><> ==> map({ sqrt $_ }));'
$[1, 2, 3, 4]
(2)
(1 1.4142135623730951 1.7320508075688772 2)
<> here is the decontainerization operator. And it would be the fastest
solution because most others are coercers meaning that the most likely outcome
of .Slip, .List, pipe operator, etc. – would be creation of a new object. With
<> we just pull out the Array from the container with no modifications or
copying.
Note that I say nothing to justify the use of ==>. I simply don't use it.
Best regards,
Vadim Belman
> On Jul 14, 2021, at 8:13 PM, Brad Gilbert <[email protected]> wrote:
>
> Honestly I would advise against using ==> at the moment.
>
> For one thing it doesn't even work like it is intended.
> Each side of it is supposed to act like a separate process.
>
> There are also issues with the syntax that are LTA.
> The fact that you have to tell it the left side is actually a list is one
> such issue.
>
> It isn't even all that clearer than just using a method call
>
> > %a<column1> .map({.sqrt});
>
> ---
>
> Using ｢.Slip｣ or ｢|｣ prefix works, but is the wrong thing.
> You need to tell it that it is a list, so use ｢.List｣ or ｢@(…)｣
>
> > @(%a<column1>) ==> map({.sqrt})
> > %a<column1>.List ==> map({.sqrt})
>
> Since a Slip is a type of List, using it works, but for the wrong reasons.
>
>
> On Wed, Jul 14, 2021 at 2:58 PM Aureliano Guedes <[email protected]
> <mailto:[email protected]>> wrote:
> thank
>
> It is now more clear.
> And I like this notation |%a<column1> ==> map({.sqrt});
> less is more sometimes
>
>
>
> On Wed, Jul 14, 2021 at 4:41 PM Daniel Sockwell <[email protected]
> <mailto:[email protected]>> wrote:
> To expand slightly on what Clifton said, the reason that
>
> > %a<column3> = %a<column1>.map: { .sqrt };
> > # (1 1.4142135623730951 1.7320508075688772 2 2.23606797749979)
>
> does what you mean but
>
> > %a{'column1'} ==> map( { .sqrt } )
> > # (2.23606797749979)
>
> does not is that the method .map maps over *each item* in the Array, whereas
> ==> map maps over the Array as *one collection*. When taking the square root,
> an Array needs to be treated as an number, which for Raku means treating it
> as
> a count of how many elements it has (i.e., its length).
>
> So `%a{'column1'} ==> map({.sqrt})` is the same as
> `%a{'column1'}.elems.map({.sqrt})`
>
> If want to map over each item in the Array when using the ==> operator, you
> need to
> slip the items out of the Array before feeding them on. You can do that with
> either
> of the following (equivalent) lines:
>
> > %a{'column1'}.Slip ==> map({.sqrt});
> > |%a{'column1>'}==> map({.sqrt});
>
> (Also, you may already know this, but when the keys of your hash are strings,
> you
> can write %a<column1> instead of %a{'column1'} )
>
> Hope that helps!
>
> –codesections
>
>
> --
> Aureliano Guedes
> skype: aureliano.guedes
> contato: (11) 94292-6110
> whatsapp +5511942926110
