I palindromise the source code, you palindromise the input!

Code Golf

string array-manipulation source-rules add tag

Adám

Given a list or string (you can decide to handle only one of these), return it unmodified. However, if your source code is palindromised by appending its reverse to it, then the this new code must palindromise the given list or string.

For the purposes of this competition, you must count a trailing line break if it is necessary to make your code run when palindromised.

## Examples
Let's say your solution takes a list and your source code is
```
AB
DEF
```
and its input/argument is `[1,2,3]`. If I write
```
AB
DEFFED
BA
```
instead and run it, the output/result must be `[1,2,3,3,2,1]`.

Let's say your solution takes a string and your source code is
```
ABC

```
and its input/argument is `"42\n"`. If I write
```
ABC

CBA
```
instead and run it, the output/result must be `"42\n\n24"`.

APL (Dyalog Unicode) [APL (Dyalog Unicode)](https://www.dyalog.com/), 3 bytes^SBCS^

Bubbler

Python 3 [Python 3](https://docs.python.org/3/) REPL, 12 bytes

wizzwizz4

W [W](https://github.com/A-ee/w/) `t`, 4 bytes

liek

[Jelly](https://github.com/DennisMitchell/jelly), 3 bytes

xigoi

05AB1E [05AB1E](https://github.com/Adriandmen/05AB1E), 3 bytes

liek

Top Answer APL (Dyalog Unicode)

Bubbler

# [APL (Dyalog Unicode)], 3 bytes^SBCS^

    ⌽⊢,

[Try it online!][TIO-k89pozol]

[APL (Dyalog Unicode)]: https://www.dyalog.com/
[TIO-k89pozol]: https://tio.run/##SyzI0U2pTMzJT////1HP3kddi3T@pz1qm/Cot@9R31RP/0ddzYfWGz9qmwjkBQc5A8kQD8/g/2kK6olJyepcQNrA0MjYxNTM3MJSHQA "APL (Dyalog Unicode) – Try It Online"

## Palindromized

    ⌽⊢,,⊢⌽

[Try it online!!enilno ti yrT][TIO-k89ppz0r]

[APL (Dyalog Unicode)]: https://www.dyalog.com/
[TIO-k89ppz0r]: https://tio.run/##SyzI0U2pTMzJT////1HP3kddi3R0gASQ@T/tUduER719j/qmevo/6mo@tN74UdtEIC84yBlIhnh4Bv9PU1BPTEpW5wLSBoZGxiamZuYWluoA "APL (Dyalog Unicode) – Try It Online"

### How it works

    ⌽⊢,  ⍝ Original function; Input = s
    ⌽ ,  ⍝ Compute reverse(s) and flatten(s) (flatten is a no-op)
     ⊢   ⍝ Select the right one, i.e. flatten(s)
    
    ⌽⊢,,⊢⌽  ⍝ Palindromized function; Input = s
       ,⊢⌽  ⍝ Similar to the original, but select reverse(s)
     ⊢,     ⍝ Append the above to s, giving s+reverse(s)
    ⌽       ⍝ Reverse the entire string, which is no-op on a palindrome

Answer #2 Python 3

wizzwizz4

# [Python 3] REPL, 12 bytes

```
_#]1-::[_+_

```

[Try it online!][TIO-khffgwpk]

[Try it online!!enilno ti yrT][TIO-khffiaeg]

This is a pretty boring solution, but I can't see a way to do this that avoids comments altogether; Python requires brackets to do anything useful, and those don't like being re-ordered.

[Python 3]: https://docs.python.org/3/
[TIO-khffgwpk]: https://tio.run/##bVLBTsMwDL33K0x2aKt1kxC3SjsBV0DiyFDVZe4aLXWiJNVWIb69uC10TJBDFFt@7/nZsV2oDd31qrHGBdi1SgdFPop@XusCNqDItiFJo2iPFVTlEQuHVifS7DHNI@AjHgx6igOcjDtCZRyXkexgKLkRY0molWcuMUXoHAdPhnAMB4TKQCtCVgOktkFXBhw11t5qFZJ4S3H6rTcTLjcTCBZw/zKagT23wiQrq0uJ4INTdICmDWVQhv6gB9YBfaqROIkOYw@GdMcXgsOKMyRxfQG67tLDcCQbkaaxSmMysGZQibGnD/UpMog962uM0xmEZ4k2wGtHoTw/OsfeSw94zToNCK@VDPFOWrw2Mc/0n7lOcigTmcFBm12pfZL@evJOL/t0Qoi@WLzfrvL8rVgWUc@JNLI8wJCILT23gf9Bzp6KtK9Ra/MF "Python 3 – Try It Online"
[TIO-khffiaeg]: https://tio.run/##bVLBTsMwDL33K0x2aKt1kyZulTgBV0DiuE1Vl7lrtNSJklRbhfj24rawMUEOUZz4vednx3ahNnTfq8YaF2DXKh0U@Sj6OS0LeABFtg1JGkV7rKAqj1g4tDqRZo9pHgEv8WTQUxzgZNwRKuM4jWQHQ8qdGFNCrTxziSlC5zh4MYRjOCBUBloRshogtQ26MuCosfRWq5DEG4rTb70L4fxhAsEMHt9GM7DnUphkYXUpEXxwig7QtKEMytAf9MA6oE81El@iw9iDId3xhuCw4huSuLwCXXetYViSjUjTWKUxGVgzqMRY04f6FBnEnvU1xukFhGeJNsB7R6E8PzvH3ksPeMs6NQhvlQzxTFq8NXHp6T99neRQJjKDgza7Uvsk/XXkmV7n6YQQfTHbrhZ5vi7mRRTxts7zxWo7K3p@TCPLzQyJ2NBrG/hP5OyvSPsatTZf "Python 3 – Try It OnlineenilnO tI yrT – 3 nohtyP"

Answer #3 W

liek

# [W](https://github.com/A-ee/w/) `t`, 4 bytes
```
+_:a
```
# Explanation
```
+    % Join 2 (implicit) inputs
 _   % Reverse the joined input
  :  % Copy the input
   a % All of this is discarded after the input push
```
# Explanation-noitanalpxE
```
+_:a % Stack is as above

a    % Push the input
 :   % Copy the input
  _  % Reverse the copy
   + % Join to the original string
     % I.e. palindromizing the input
```

Answer #4

xigoi

# [Jelly], 3 bytes

Works for both lists and strings.

## Normal

    0m(

[Try it online!][TIO-khnmvshw]

[Jelly]: https://github.com/DennisMitchell/jelly
[TIO-khnmvshw]: https://tio.run/##y0rNyan8/98gV@P////RhjoKRjoKxrEA "Jelly – Try It Online"

## PalindromizeddezimordnilaP

    0m((m0

[Try it online!!enilno ti yrT][TIO-khnmwmig]

[TIO-khnmwmig]: https://tio.run/##y0rNyan8/98gV0Mj1@D////RhjoKRjoKxrEA "Jelly – Try It Online"

Answer #5 05AB1E

liek

## [05AB1E], 3 bytes

    «ÂI

[Try it online!][TIO-k78bvage]

[05AB1E]: https://github.com/Adriandmen/05AB1E
[TIO-k78bvage]: https://tio.run/##yy9OTMpM/f//0OrDTZ7//0cb6hjpGMcCAA "05AB1E – Try It Online"
## Explanation
```
«   Concatenate the (implicit) inputs
 Â  Bifuricate: Push a, reversed(a)
  I Push the input
    Implicitly output the top of the stack
```
## Explanation-noitanalpxE
```
«ÂI Same as above

I   Push the input
 Â  Bifuricate: Push a, reversed(a)
  « Concatenate them
    Implicitly output the top of the stack
```

5 Answers