15.25.6.4 ’for-as-on-list’

This is the third of seven ‘for/as’ syntaxes

for var [type-spec] on expr1 [by step-fun]
 as var [type-spec] on expr1 [by step-fun]

This construct iterates over the contents of a list (using the successive tails of the list each iteration).

• It checks for the end of the list as if using the Common Lisp function endp.
• The variable var is bound to the successive tails of the list expr1.
• At the end of each iteration, the function step-fun is called on the list and is expected to produce a successor list; the default value for step-fun is the cdr function.
• The loop keywords ‘on’ and ‘by’ serve as valid prepositions in this syntax.
• The ‘for’ or ‘as’ construct causes termination when the end of the list is reached.

;;; Collect successive tails of a list.
(loop for sublist on '(a b c d)
      collect sublist)

;;; Print a list by using destructuring with the loop keyword ON.
(loop for (item) on '(1 2 3)
      do (print item))  ;Prints 3 lines