Elegant way to write when inside a for loop?

Question

I can write something like this (elem here is an XML::Element but it doesn't really matter):

for $elem.nodes { when XML::Element { ... } when XML::Text { ... } ... default { note qq{Ignoring unknown XML node "$_".} } }

which looks nice, but doesn't give me a readable name for $_ inside the code using it, which is why I'd prefer to write this:

for $elem.nodes -> $child { when XML::Element { ... } when XML::Text { ... } ... default { note qq{Ignoring unknown XML node "$child".} } }

but this doesn't work because now $_ isn't set, and so I actually need to write

for $elem.nodes -> $child { given $child { when XML::Element { ... } when XML::Text { ... } ... default { note qq{Ignoring unknown XML node "$child".} } } }

which is a bit redundant and adds an extra level of indentation.

It's definitely not the end of the world, but am I missing some simple way to have both a readable variable name and avoid given?

I dream of a "forgiven"... who knows maybe some day...

Mustafa Aydın
– Mustafa Aydın

2023-01-25 21:34:15 +00:00
Commented Jan 25, 2023 at 21:34 — Mustafa Aydın
– Mustafa Aydın, Commented Jan 25, 2023 at 21:34
Is smartmatching elegant enough? :-)

jubilatious1
– jubilatious1

2024-02-28 19:46:31 +00:00
Commented Feb 28, 2024 at 19:46 — jubilatious1
– jubilatious1, Commented Feb 28, 2024 at 19:46

Rawley Fowler · Accepted Answer · 2023-01-20 19:43:05Z

9

You can bind the variable above the when statements, it's a little uglier but it does the job.

for $elem.nodes { my $child = $_; when XML::Element { say 'I am XML!' } when XML::Text { say 'I am text!' } default { say "I am default: $child" } }

Edit: In Raku I think it is perfectly reasonable to stick to using $_ seeing as the idea of $_ has been around for quite some time.

edited Jan 20, 2023 at 19:43

answered Jan 20, 2023 at 17:00

Rawley Fowler

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3 Comments

librasteve Over a year ago

should this be $child := $_;?

Rawley Fowler Over a year ago

@p6steve it can be, it shouldn't make a difference in this case though.

VZ. Over a year ago

Thanks, I guess this is indeed the most reasonable answer, I just hoped there might be some magic way to bind both $child and $_ to the loop value at the same time, somehow -- but if there is none, this is the most clear and concise way to do what I want.

librasteve · Accepted Answer · 2023-01-20 22:34:22Z

8

for @a -> $x { { when 2 { say "$x ... a" } when 4 { say "$x ... b" } } given $x }

naughty double curlies and a post-given?

answered Jan 20, 2023 at 22:34

librasteve

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2 Comments

librasteve Over a year ago

my only defence of this shameless use of double curlies is JavaScript thing(function() { ... }); style code

VZ. Over a year ago

Thanks for the idea, it's nice to know that it works, but I wouldn't want to defend writing this in a code review...

jubilatious1 · Accepted Answer · 2024-02-28 20:28:44Z

Smartmatching the when condition(s):

my @a = <cat rabbit 2 dog>; say @a; say "____\n"; for @a -> $x { when $x ~~ 2 { say "$x ... a" }; when $x ~~ 4 { say "$x ... b" }; #default { say $_ if $x.defined; }; #uncomment to catch fall-through; } ;

Returns:

[cat rabbit 2 dog] ____ 2 ... a

Note: default above returns (Any) for any fall-through, which tells us that $_ is unset. You can change default to something more useful like:

default { say "$x is default" if $x.defined };

Longer Example:

% cat ~/exemel_text.xml <?xml version="1.0"?> <root> <file>text1</file> <file>text2</file> <file>text3</file> <file>text4</file> </root> % raku -MXML -e 'my $mydocument=open-xml($*ARGFILES.Str); my @array = $mydocument.root.nodes[1,3,5,7...*]; #odd elements only for @array -> $scalar { when $scalar ~~ XML::Element {say "$scalar is an XML::Element!"}; when $scalar ~~ XML::Node {say "$scalar is an XML::Node!"}; default { say $_ if $scalar.defined }; };' ~/exemel_text.xml <file>text1</file> is an XML::Element! <file>text2</file> is an XML::Element! <file>text3</file> is an XML::Element! <file>text4</file> is an XML::Element!

https://github.com/raku-community-modules/XML

This is also nice, thanks, although I'm not sure if it's better to repeat $var ~~ in every line or just do $var = $_ once, as in the other answer.
Thx! I think given serves to topicalize each when statement, which you can circumvent by an explicit smartmatch. So you can do a for/when loop without too much inconvenience.
FYI, if you have any further interest in manipulating XML in Raku, please peruse this repo (any comments/suggestions appreciated!): github.com/raku-community-modules/XML
Thanks, I'll keep this in mind when I have to do something with XML the next time (hopefully not too soon :-)

Mustafa Aydın · Accepted Answer · 2024-08-05 16:11:10Z

With a third-party library:

use Slang::Forgiven; forgiven $elem.nodes -> $child { when XML::Element { ... } when XML::Text { ... } ... default { note qq{Ignoring unknown XML node "$child".} } }

It introduces a keyword called "forgiven" to the grammar such that it is like the good-old for loops but also topicalizes the parameter(s) of the following block, i.e., makes $_ an alias to them, allowing, e.g., both $child and $_ to be used for the same thing.

Disclaimer: author here.

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