Reversed .= operator

Hi all,

I'm sure we've all wished for a concatenation operator that would prepend a string to a string in the same way the .= operator appends.

So why isn't there one?

It's silly that you can write:

$foo .= 'bar';
[download]

But not:

$baz =. 'qux';
[download]

and instead have to do:

$baz = 'qux' . $baz;
[download]

Today I got to wondering if I had missed that such an operator had been introduced in some recent Perl version so I ran the code, and to my surprise Perl said:

Reversed .= operator at -e line 5.
syntax error at -e line 5, near "=."
[download]

Now, if Perl knows that this particular syntax error is a "reversed .= operator", and not, say, "some new operator I didn't know about" - i.e. the syntax is not in use for anything else - then why isn't it implemented?

Can any guts gurus shed any light?

The way forward always starts with a minimal test.

Comment on Reversed .= operator Select or Download Code

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Re: Reversed .= operator by LanX (Saint) on Apr 29, 2016 at 16:54 UTC
`=.` doesn't follow the rule Operator + Assignment like in `+=` or `*=` , so many mechanisms would fail like in `overload` where the overloaded `+=` can be automatically derived from overloaded `+` So you'd either need an explicit prepend operator like `..=` or you'd need to define your own helper function `sub prepend($$)` . Cheers Rolf _{(addicted to the Perl Programming Language and ☆☆☆☆ :) Je suis Charlie!}	[reply] [d/l] [select]
Re: Reversed .= operator by choroba (Cardinal) on Apr 29, 2016 at 21:40 UTC
substr can do that easily: `substr $string, 0, 0, $prefix;` [download] ($q=q:Sq=~/;[c](.)(.)/;chr(-\|\|-\|5+lengthSq)`"S\|oS2"`map{chr \|+ord }map{substrSq`S_+\|`\|}3E\|-\|`7**2-3:)=~y+S\|`+$1,++print+eval$q,q,a, [download]	[reply] [d/l] [select]
Re^2: Reversed .= operator by 1nickt (Canon) on Apr 29, 2016 at 22:08 UTC
Yes, of course, there is more than one way to do it. I would use `$string = $prefix . $string` [download] over `substr $string, 0, 0, $prefix` [download] myself (for readability, not because it's two fewer characters) ... but I want a (built-in) operator, dammit! The way forward always starts with a minimal test.	[reply] [d/l] [select]
Re^3: Reversed .= operator by choroba (Cardinal) on Apr 29, 2016 at 22:25 UTC
It's not the same. substr is a bit faster, probably because it doesn't create a new string? ($q=q:Sq=~/;[c](.)(.)/;chr(-\|\|-\|5+lengthSq)`"S\|oS2"`map{chr \|+ord }map{substrSq`S_+\|`\|}3E\|-\|`7**2-3:)=~y+S\|`+$1,++print+eval$q,q,a, [download]	[reply] [d/l]
Re^4: Reversed .= operator by pryrt (Abbot) on Apr 29, 2016 at 22:38 UTC
Re^5: Reversed .= operator by choroba (Cardinal) on Apr 29, 2016 at 22:44 UTC
Some notes below your chosen depth have not been shown here
Re: Reversed .= operator by Laurent_R (Canon) on Apr 30, 2016 at 09:42 UTC
Just create the operator: > multi sub infix:<=.> (Str $w1 is rw, Str $w2) { $w1 = $w2 ~ $w1;} sub infix:<=.> (Str $w1 is rw, Str $w2) { #`(Sub+{Precedence}\|14963240 +0) ... } > > my $c = "bar"; bar > $c =. "foo"; foobar > say $c; foobar [download] Oh, but I forgot to say, this is Perl 6, of course. Sorry about it, I couldn't resist. `;-)`	[reply] [d/l] [select]
Re^2: Reversed .= operator by 1nickt (Canon) on Apr 30, 2016 at 15:50 UTC
So in other words, "Perl6" doesn't have it either. The way forward always starts with a minimal test.	[reply]
Re^3: Reversed .= operator by Laurent_R (Canon) on Apr 30, 2016 at 16:16 UTC
Nope, but it does not really need it either: it takes just one code line to define it. Take a look at Perl 6 without prejudice, I'm sure you'll love it.	[reply]
Re^4: Reversed .= operator by BrowserUk (Patriarch) on Apr 30, 2016 at 19:27 UTC
Re^5: Reversed .= operator by Laurent_R (Canon) on Apr 30, 2016 at 21:24 UTC
Re^2: Reversed .= operator by raiph (Deacon) on May 03, 2016 at 06:26 UTC
I prefer your solution but for interest here's another using the arg swap metaop: `my $foo = 'foo'; $foo ~= 'bar'; say $foo; # 'foobar' $foo [R~]= 'bar'; say $foo; # 'barfoobar'` [download] »ö« ^{. o O ( "the celebrity tell-all of the Perl-6 cult?" )}	[reply] [d/l]
Re^3: Reversed .= operator by Laurent_R (Canon) on May 03, 2016 at 20:58 UTC
Thanks, raiph, I did not know about that reverse argument operator.	[reply]
Re: Reversed .= operator by karlgoethebier (Abbot) on May 03, 2016 at 07:38 UTC
"...prepend a string to a string" Seems to be an old debate: What operator prepends? Ibidem: "...It wasn't a useful enough operation to merit its own operator" Regards, Karl «The Crux of the Biscuit is the Apostrophe»	[reply]