“A guy walks into a bar...” is a typical form of what has been called the “bar joke.” A math and science version is:

“A mathematician walks into a bar. The punchline is left as an exercise for the reader.”

“Q: Why did the chicken cross the road? A: The answer is trivial and is left as an exercise for the reader” was cited in “13 Jokes That Every Math Geek Will Find Hilarious” by Walter Hickey on Business Insider on May 21, 2013. “Two mathematicians walk into a bar. One orders a drink. The punchline has been left as an exercise for the reader” was posted on Twitter on August 8, 2013.

“A mathematician, a college professor, and a textbook author walk into a bar. [The punchline is left as an exercise for the reader.]” was posted on Reddit on September 26, 2016.


Google Groups: rec.arts.sf.written.robert-jordan
TAN: A joke walked into a bar (was Re: TAN: Re: Wheel of Time
Tshen
11/20/95
(...)
Actually it was just another mutation of the old joke format “A ____ walks into a bar...”.  Such as the ever-popular “A string walks into a bar...”, “A horse walks into a bar...”, “A bear walks into a bar...”, “Three vampires walk into a bar...”, and “Two young musicians from the Two Rivers walk into a inn...” (Please note, this joke does not actually exist yet, so its body and punchline are left as an exercise for the reader.)

Business Insider
13 Jokes That Every Math Geek Will Find Hilarious
Walter Hickey
May 21, 2013, 5:38 AM
(...)
JOKE #6
Q: Why did the chicken cross the road?
A: The answer is trivial and is left as an exercise for the reader.
Source: Reddit

Explanation:
This is a common refrain found in mathematics texts.

It is widely considered a cruel professor’s malicious cop-out by particularly lazy students of mathematics.

Twitter
luis
‏@lquirosr
Two mathematicians walk into a bar. One orders a drink. The punchline has been left as an exercise for the reader.
1:52 PM - 8 Aug 2013

Twitter
Thomas Oléron Evans
‏@Mathistopheles
A mathematician walks into a bar. The punchline is left as an exercise for the reader. #mathsjoke
7:00 AM - 17 Mar 2015

Reddit
A mathematician walks into a bar and orders a drink.
submitted June 2, 2015 by thefran
The punchline has been left as an exercise for the reader.

Twitter
Terrible Jokes
‏@doyouevenjoke
A mathematician walks into a bar and orders a drink.
The punchline has been left as an exercise for the reader.
12:55 PM - 2 Jun 2015

Twitter
Stefan Jon Silverman
‏@sjs_sf
@attackerman Two Corinthians walk into a bar.... (left as an exercise for the reader to complete) 😈
7:21 PM - 1 Sep 2016

Reddit
A mathematician, a college professor, and a textbook author walk into a bar.
submitted September 26, 2016 by sloppyjoes7
[The punchline is left as an exercise for the reader.]