15.25.6.5 ’for-as-equals-then’

This is the fourth of seven ‘for/as’ syntaxes.

for var [type-spec] = expr1 [then expr2]
 as var [type-spec] = expr1 [then expr2]

This construct

• initializes the variable var by setting it to the result of evaluating expr1 on the first iteration,
• then setting it to the result of evaluating expr2 on the second and subsequent iterations.
• If expr2 is omitted, the construct uses expr1 on the second and subsequent iterations. When expr2 is omitted, the expanded code shows the following optimization:

;;; Sample original code:
(loop for x = expr1 then expr2 do (print x))

;;; The usual expansion:
(tagbody
      (setq x expr1)
  tag (print x)
      (setq x expr2)
      (go tag))

;;; The optimized expansion:
(tagbody
  tag (setq x expr1)
      (print x)
      (go tag))

• The loop keywords ‘=’ and ‘then’ serve as valid prepositions in this syntax.
• This construct does not provide any termination conditions.

;;; Collect some numbers.
(loop for item = 1 then (+ item 10) repeat 5 collect item)