Some of IRB's command-line options can be called idiosyncratic as well. Take math mode¹ as an example: It will require the infamous mathn library on start up:

$ irb -m >> Math #=> CMath >> 3/2 #=> (3/2) >> !!defined?(Vector) #=> true

¹ Math mode actually has been removed in Ruby version 2.5

And another one surprised me: You can pass custom inspectors to IRB, for example, yaml:

$ irb -f --inspect yaml >> [1,2,3] => --- - 1 - 2 - 3

Or marshal:

$ irb -f --inspect marshal >> 42 => i/

But you can also define your own inspectors on the fly:

$ irb -f --inspect "{ |r| r.to_s.reverse }" >> [1,2,3] => ]3 ,2 ,1[

It will be eval'd (!) as the block part of a new proc .

Resources

More Idiosyncratic Ruby