evaluation - What are the cool kids talking about when they use ##&[]?

All the cool kids are apparently using

##&[]

for

Unevaluated @ Sequence[]

but I have no idea what either means.

Please explain what these things are so I can be a cool kid!

Answer

Try this:

Map[If[#==1,Unevaluated@Sequence[],#]&,{1,2,3}]

Note the output. The 1 is gone. That's because Unevaluated@Sequence[] puts the empty sequence there, that is, "nothing".

##&[] is a shorthand that can be used in most places for same - ## is the sequence of arguments, & makes it a function to apply to something, [] is that something - an empty argument list, so the result is... a sequence that is empty.