Double-duty quine: quine!dlroW ,olleH

Question

Write a program that will output its own source code when run, and nothing else. Seems easy, right?

The catch is that when the source code is reversed, your program must output "Hello, World!" exactly, without the quotes.

This is code-golf, so lowest byte count wins.

Edit: Your quine must be a proper quine.

Answer 1

JavaScript (ES6), 42 38 bytes

f=_=>/\//g&&"f="+f||"!dlroW ,olleH">=_

Reversed

_=>"Hello, World!"||f+"=f"&&g//\/>=_=f

Explanation

When reversed, it becomes an anonymous function that returns the string Hello, World!.

The regex /\//g becomes a comment when it is reversed, which allows the syntactically invalid >=_=f to be commented out in the reversed code.

Answer 2

Y, 19 bytes

Upxp"!dlroW ,olleH"

U captures a string with U at the beginning until the next U is met, in this case, the source code. p prints the item, and x is a termination link. When reversed, this looks like:

"Hello, World!"pxpU

This captures the string and prints it with p, again terminating the program with x.

Try it here!

Answer 3

JavaScript (ES6), 71 bytes

trela=a=>alert("trela="+trela+"\ntrela\n``")//
`!dlroW ,olleH`
trela
``

How it Works:

Line 1 defines function trela that when run outputs the program's source code. Line 2 is an unassigned string, does nothing. Lines 3 and 4 call trela abusing the template string syntax.

Reversed:

``
alert
`Hello, World!`
//)"``n\alertn\"+alert+"=alert"(trela>=a=alert

How it Works:

Line 1 is an unassigned string, does nothing. Lines 2 and 3 abuse the template string syntax to print Hello, World!. Line 4 is a comment.

Answer 4

GolfScript, 33 bytes

Forwards

{`".;1$~]"}"!dlroW ,olleH".;1$~]

Try it online!

Backwards

]~$1;."Hello, World!"}"]~$1;."`{

Try it online!

Answer 5

GolfScript, 29 28 bytes

"`'.~]'#\"!dlroW ,olleH".~]

It has one trailing newline. Try it here.

Reversed:

]~."Hello, World!"\#']~.'`"

Try it here.

Answer 6

RETURN, 94 bytes

"34¤¤,,,,,,,,,,,,,% 'H'e'l'l'o',' 'w'o'r'l'd'!'"34¤¤,,,,,,,,,,,,,% 'H'e'l'l'o',' 'w'o'r'l'd'!'

Reversed:

'!'d'l'r'o'w' ','o'l'l'e'H' %,,,,,,,,,,,,,¤¤43"'!'d'l'r'o'w' ','o'l'l'e'H' %,,,,,,,,,,,,,¤¤43"

Try it here.

Outputs to STDOUT. Until I find a better quine framework, this will have to do for now.

Explanation

"34¤¤,,,,,,,,,,,,,% 'H'e'l'l'o',' 'w'o'r'l'd'!'"

This contains the quine string. In reverse, this is pushed to the stack but not outputted.

34¤¤,,,,,,,,,,,,,

This pushes a quote char to the stack and outputs the result twice until there is nothing left to output. In reverse, this will print the charcodes already on the stack.

% 'H'e'l'l'o',' 'w'o'r'l'd'!'

This one pops the top stack item (in reverse, this would pop a space char) and pushes a series of charcodes to the stack (in reverse, these charcodes would later be printed by the series of ,'s).

Answer 7

Fission 2, 42 bytes

Shameless adaptation of a excellent quine by @MartinBüttner in this answer

'!+O!'!d'!l'!r'!o'!W'! '!,'!o'!!l'!e'!H'R"

Try it online

And reversed

"R'H!'e!'l!!'o!',!' !'W!'o!'r!'l!'d!'!O+!'

Try it online

In the quine version the atom starts at the R heading right. The "starts print mode which wraps to the next " (itself). This prints everything out except the ". '!+ set the atom to char ". O prints it out and destroys the atom ending the program.

The reversed version starts at the R again and for each character in Hello, World set the atom and prints ! it out without destroying the atom. For the final character ! the print O destroys the atom.

Answer 8

Javascript ES6, 55 bytes

$=_=>`$=${$};$()//"!dlroW ,olleH"`;$()//"!dlroW ,olleH"

Quite simple, really.

Answer 9

Python 2, 60 bytes / Python 3.8 (pre-release), 64 bytes

Python 2: (60 bytes)

• Normal :

a="print'a=%r;exec a#\\42'%a#'!dlroW ,olleH'tnirp;";exec a#"

• Reversed:

"#a cexe;";print'Hello, World!'#a%'24\\#a cexe;r%=a'tnirp"=a

Try it online!

Python 3: (64 bytes)

• Normal:

exec(a:="print('exec(a:=%r)#\\42'%a)#)'!dlroW ,olleH'(tnirp;")#"

• Reversed:

"#)";print('Hello, World!')#)a%'24\\#)r%=:a(cexe'(tnirp"=:a(cexe

Try it online!

Explanation :

• I used a modification of the quine a="print'a=%r;exec a'%a";exec a adding some comments # to isolate the "hello world" part.
• I also used \\42 to get the char " without breaking my strings or having some slashes \ at the printing

• Thanks to the previous comments and a few well placed quotes, the reversed program is very straight forward

Answer 10

Python 2, 131 bytes

Forward:

print(lambda x:x+repr(x)+")#'!dlroW ,olleH' tnirp")('print(lambda x:x+repr(x)+")#\'!dlroW ,olleH\' tnirp")(')#'!dlroW ,olleH' tnirp

Reverse:

print 'Hello, World!'#)'()"print '\Hello, World!'\#)"+)x(rper+x:x adbmal(tnirp'()"print 'Hello, World!'#)"+)x(rper+x:x adbmal(tnirp

The first half is a one-line quine, followed by a # to form a comment separating the first half from the simpler second half.

Answer 11

C, 108 bytes

char*s="char*s=%c%s%c;main(){printf(s,34,s,34);}";main(){printf(s,34,s,34);}//};)"!dlroW ,olleH"(stup{)(niam

Answer 12

Python 2, 70 bytes

_='_=%r;print _%%_#"dlroW ,olleH"tnirp';print _%_#"!dlroW ,olleH"tnirp

Answer 13

><> (Fish), 25 bytes

"r00g>o<o>'Hello, World!'

Try it

Reversed

'!dlroW ,olleH'>o<o>g00r"

Try it