develooper Front page | perl.perl6.users | Postings from June 2018

Re: mixin syntax: does vs but

Thread Previous | Thread Next
From:
Brandon Allbery
Date:
June 12, 2018 19:01
Subject:
Re: mixin syntax: does vs but
Message ID:
CAKFCL4Wb3iE2nGEEq2Bs5aKzY+N13yZ1BbYtk=PvyGbKLv80OA@mail.gmail.com
.WHAT is a "macro"/shorthand, which is why it's uppercase. There's a
metamodel (the real meaning of the ^) version of it as well.

On Tue, Jun 12, 2018 at 2:59 PM Joseph Brenner <doomvox@gmail.com> wrote:

> >> say @y.^WHAT
>
> > If you want to print the name use `.^name`.
>
> > If you want the type object for more advanced usages use `.WHAT`.
>
> Sorry, typo on my part.
>
> Though that raises another syntactic oddity I might whine about: perl6
> code examples frequently use ".WHAT".  I was interested in getting a
> list of all available methods, so I started trying some guesses:
> ".METHODS", ".METHOD", ".methods".... But actually it's ".^methods".
> Okay, the caret is used for introspection... but then why isn't it
> ".^what"?
>
> (It also turns out that the list ".^methods" gives you is bizarre:
> hard to read, has many duplicates, unsorted, full of odd entries that
> look like internal use only widgets I don't care about just now...)
>
>
>
>
> On Tue, Jun 12, 2018 at 11:34 AM, Brad Gilbert <b2gills@gmail.com> wrote:
> > On Tue, Jun 12, 2018 at 12:55 PM, Joseph Brenner <doomvox@gmail.com>
> wrote:
> >> Thanks, both your suggestion and JJ Merelo's work, but I think I like
> >> yours for readability:
> >>
> >>   # # using binding, suggested by JJ Merelo <jjmerelo@gmail.com>
> >>   # my @y := @x but LookInside;
> >>
> >>   # suggested by Elizabeth Mattijsen liz@dijkmat.nl
> >>   my @y does LookInside = @x;
> >>
> >> I actually found the use of "but" in the objects docs to be
> >> tremendously confusing at first:  it looks like some sort of
> >> conditional check, like "unless".
> >
> > The reason `but` exists is basically for the following
> >
> >     my $v = 0 but True;
> >
> >     if $v { say $v } # prints 0
> >
> > In Perl 5 it is common to return the string `"0 but true"` for a value
> that is
> > both 0 and true.
> >
> > Since one of the design philosophies of Perl 6 is to reduce the number
> > of special cases this was made to be more generally useful.
> >
> > Note that you should not do the following
> >
> >     my $v = 0;
> >     $v does True;
> >
> >     say 'WTF!' if 0; # prints WTF!
> >
> > Basically you can use `but` anywhere you like, but be careful with
> `does`.
> >
> >> On Tue, Jun 12, 2018 at 1:01 AM, Elizabeth Mattijsen <liz@dijkmat.nl>
> wrote:
> >>>> On 12 Jun 2018, at 09:06, Joseph Brenner <doomvox@gmail.com> wrote:
> >>>>
> >>>> I thought this would work to make a copy of @x but with the role
> >>>> "LookInside" attached to it:
> >>>>
> >>>>   my @y = @x but LookInside;
> >>>>
> >>>> But that didn't add the role to @y. E.g.
> >
> > That is effectively the same as:
> >
> >     my @y = (@x but LookInside).map: *.self;
> >
> > That is @ sigiled variables tend to slurp in iterable values.
> >
> >>>>  say @y.^WHAT
> >
> > If you want to print the name use `.^name`.
> >
> > If you want the type object for more advanced usages use `.WHAT`.
> >
> >>>>
> >>>> Would just report (Array), not (Array+{LookInside}).
> >>>
> >>> What you’re doing here, is assigning the elements of an Array but
> Lookinside to a normal Array.  The elements of the Array do not have any
> roles mixed in, so you wind up with a normal Array with normal elements in
> them.  Perhaps you meant:
> >>>
> >>>     my @y does LookInside = @x;
> >>>
> >>> Here you create an Array @y with the role LookInside mixed in, and
> *then* assign the values from @x.
> >>>
>


-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b@gmail.com                                  ballbery@sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net

Thread Previous | Thread Next


nntp.perl.org: Perl Programming lists via nntp and http.
Comments to Ask Bjørn Hansen at ask@perl.org | Group listing | About