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3 points by shader 5213 days ago | link | parent

The colon is a standby from other lisps; Common Lisp would intern symbols starting with : to a special KEYWORD symbol table.

That was important because you could do things like keyword based arguments to functions, etc. i.e. if you had a function that was defined as

  (defun greet (&key (a "Hello") (b "World")) 
    (print a " " b))
then you could call it like:

  >(greet)
  Hello World
  >(greet :b "Bob")
  Hello Bob
  >(greet :b "Alice" :a "Yo")
  Yo Alice
etc. As you can see, it's a lot more flexible than "traditional" optional arguments, since you can specify any of them, and in any order.