Re^4: Hash/Array slice : how to exclude items?

> Shouldn't this work?:

> print Dumper(do { delete local @$hashref{ @$bad_keys }, $hashref} ); or this print Dumper(do { delete local @$hashref{ @$bad_keys }; $hashref} );

yes it's surprising.

One needs to remember that local is protecting certain entries and not the whole hash (The hash could be a lexical variable, local wouldn't do anyway)

Since you are returning a reference, you'll see the restored entries again after leaving the block.

consider what's happening without delete

  DB<91> use Data::Dump qw/pp/ 

  DB<92> ; {local @hash{qw/log schema/};say pp \%hash}; say pp \%hash
{ a => 1, b => 2, c => 3, log => undef, schema => undef }
{ a => 1, b => 2, c => 3, log => "l", schema => "s" }
[download]

you can still have the desired effect by just returning a list or a new anonymous hash

  DB<96>  say pp do {delete local @hash{qw/log schema/}; %hash }; say 
+pp \%hash
("a", 1, "c", 3, "b", 2)
{ a => 1, b => 2, c => 3, log => "l", schema => "s" }

  DB<97>  say pp do {delete local @hash{qw/log schema/}; +{%hash} }; s
+ay pp \%hash
{ a => 1, b => 2, c => 3 }
{ a => 1, b => 2, c => 3, log => "l", schema => "s" }

  DB<98>
[download]

if you shy away from the overhead to return a new hash, just do the dumping inside the block.

  DB<99> ; {delete local @hash{qw/log schema/};say pp \%hash}; say pp 
+\%hash
{ a => 1, b => 2, c => 3 }
{ a => 1, b => 2, c => 3, log => "l", schema => "s" }

  DB<100>
[download]

HTH! :)

Cheers Rolf
_{(addicted to the 𐍀𐌴𐍂𐌻 Programming Language :)

Wikisyntax for the Monastery}

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Re^5: Hash/Array slice : how to exclude items? by bliako (Abbot) on Jan 25, 2023 at 09:01 UTC
`if you shy away from the overhead to return a new hash,` yes, that's one of my fetishes. But as you and hv said: do the printing in the block. It sounds a bit excessive but it seems a good candidate now, along with the slice-with-grep-keys. Given that I may use a number of "printers" : print, say, Dumper, Mojo::Log, then I would not want a special sub which hardcodes the logger. One purpose of this post was to nudge Perl language developers to consider exclude-slices, with maximum elegance and minimum overhead (Logging like this happens every few lines of code for me). Btw, the negation with `!` looks nice. Btw2, is anyone aware of another language implementing this? I googled pithon and did not see something.	[reply] [d/l] [select]
Re^6: Hash/Array slice : how to exclude items? by LanX (Saint) on Jan 25, 2023 at 14:26 UTC
> Btw, the negation with ! looks nice. I'm not sure about this anymore, it's already legal syntax and could conflict with older code. `DB<33> $h{!A}=1 DB<34> $h{!(A)}=1 DB<35> $h{!("A")}=1 DB<36> x \%h 0 HASH(0x32dad38) '' => 1 DB<37>` [download] One could consider something like `@h!{LIST}` `DB<40> @h!{"A"} = (1) syntax error at ...` [download] But still, is adding another obscure operation to Perl worth it? > One purpose of this post was to nudge Perl language developers to consider exclude-slices, with maximum elegance and minimum overhead (Logging like this happens every few lines of code for me). I think we need a meta discuss, what "good" syntax is. "Elegance" alone doesn't help if the use case is so obscure that you never use it. For instance I saw `delete local SLICE` before, but didn't remember or ever used it till now. I'm a fan of orthogonality, the combination of syntax should be productive and easily predictive instead of being limited to isolated edge-cases. This special case interferes somehow with Perl's lacking abilities for set-operations. Probably a combined approach would be a better return of investment. edit Furthermore: IMHO A named "speaking" operator like `->excl` would be a better choice than investing into a short symbol like '!'. Don't forget that various parsers like perl-tidy need to be updated if `!` was introduced. Cheers Rolf _{(addicted to the 𐍀𐌴𐍂𐌻 Programming Language :) Wikisyntax for the Monastery}	[reply] [d/l] [select]

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