“A guy walks into a bar…” is a typical form of what has been called the “bar joke.” A math version is:
 
“f(x) walks into a bar and asks for a drink. The bartender says, ‘Sorry, we don’t cater for functions.’”
 
The joke has been cited in print since at least 2004.
 
   
Google Groups: alt.ozdebate
Got any nails?
Typhoid
9/24/04
f(x)=6x+3 walks into a bar. “Got any sandwiches? f(x)=6x+3 asks the barman.
 
“Sorry,” he replies, “We don’t cater for functions.”
 
Google Groups: uk.rec.humour
Yet another bar joke…
Hyphen
6/22/05
f(x) went into a bar and asked for a drink.
 
The bartender said “We don’t cater for functions”...
     
Webdeveloper.com
PhP Joke
nshiell
09-28-2005, 06:59 AM
So
 
function test(v1, v2)
{
global $hello;
$hello = "1";
echo $hello + $hello;
}
 
walks into a bar and asks for a pint of beer and the bar tender says “sorry we don’t cater for functions”
   
Hoops (Australia)
Matt Ryan
23:21 31 Oct 05
“f(x) = 3x+2” walks into a city pub and says “Listen, I’m turning 21 and I would like to hold my birthday party here.”
 
The owner says “I’m sorry, but we don’t cater for functions!”
     
LiveJournal   
Monument (marnanel) wrote,
2006-11-05 21:33:00
Fireworks Night
(...)
f(x)=3x² walks into a bar and asks for a sandwich. “Sorry,” says the barman, “we don’t cater for functions.”
 
the observers hunch
TUESDAY, MAY 08, 2007
Poor Physics Jokes
(...)
z=x^2+3xy Walks into a pub, sits down on the barstool and orders a pint.
The barman looks up from the glass he’s polishing and says “Sorry mate, you’re gonna have to leave. We don’t cater for functions”.
 
Twitter
Becky Haycox
‏@hambox
sin(x) walks into a bar and asks for a drink. Bartender says, “Sorry, we don’t cater for functions.”
12:32 PM - 28 Jul 2008
 
Straight Dope Message Board
Geeky Jokes
VunderBob
11-19-2008, 07:59 AM
Two spiral galaxies go into a pub. “Get out” says the landlord; “you’re barred”.
 
f(x) goes into a pub. The landlord says “Sorry, we don’t cater for functions”.
 
f’(x) goes into a pub. The landlord says “Sorry, we don’t cater for functions”. (This may look like the same joke, but it’s not; it’s derivative humour.)
This one took me a minute, until I realized my typeface and astigmatism obscured the apostrophe.
 
f ‘(x) goes into a pub… “.
   
Twitter
Jemima Kiss
‏@jemimakiss
f(x)=2x+3 walks into a bar.  The barman says, “Sorry, we don’t cater for functions.” (@liquidindian )
8:48 AM - 17 Dec 2008
 
Google Books
Extremely Embarrassing Dad Jokes:
Because Dads Don’t Know When to Stop
By Ian Allen
New York, NY: Pavilion Books
2015
Pg. ?:
F(x) walks into a bar and asks for a sandwich. The landlord says, ‘I’m sorry, we don’t cater for functions.’
   
Google Books
Algebra for Applications:
Cryptography, Secret Sharing, Error-Correcting, Fingerprinting, Compression
By Arkadii Slinko
New York, NY: Springer
2015
Pg. 132:
A polynomial walks into a bar and asks for a drink. The barman declines: “We don’t cater for functions.”
An old math joke.