Question

I’m reading A Gentle Introduction to Haskell (which is not so gentle) and it repeatedly uses the : operator without directly explaining what it does.

So, what exactly does it do?

Answer 1

: is the “prepend” operator:

x : xs

Returns a list which has x as first element, followed by all elements in xs. In other functional languages, this is usually called cons, because it “cons”tructs a list recursively by repeated application from an empty list:

1 : 2 : 3 : 4 : []

is the list [1, 2, 3, 4].