Is there any difference between the "for" and "as" keywords in the loop macro?

Question

In Common Lisp, in the loop macro, the and and as keywords appear to function identically:

(loop for i from 1 to 3 do (print i))
(loop as i from 1 to 3 do (print i))

Is there some subtle difference I'm missing? If not, why bother with two different yet identical keywords?

Answer 1

6.1.2.1 Iteration Control:

The for and as keywords are synonyms; they can be used interchangeably. ... By convention, for introduces new iterations and as introduces iterations that depend on a previous iteration specification.

E.g.:

(loop for x from 1 to 10
  as x2 = (* x x)
  as x4 = (* x2 x2)
  for y from 10 downto 1
  as y2 = (* y y)
  as y4 = (* y2 y2)
  sum (* x4 y4))

Why?!

Tradition! :-)

And, also...

"... a computer language is not just a way of getting a computer to perform operations, but rather ... it is a novel formal medium for expressing ideas about methodology" Abelson/Sussman "Structure and Interpretation of Computer Programs".

IOW, we write code for people (including you 6 months from now) to read, not just for computers to execute.

Everything that makes your code more readable is "fair game".