I wrote this macro for explaining 'MAPCAR in terms of 'LOOP to

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I wrote this macro for explaining 'MAPCAR in terms of 'LOOP to
pizzapal deep within one of James' mastodon threads.

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(defmacro loopcar (fun &rest lists
&aux (syms (loop for x in lists
collect (gensym))))
`(loop ,@(loop for list in lists
for sym in syms
nconcing (list :for syn :in list))
collecting (funcall ,fun ,@syms)))

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We were also talking about how big the common lisp standard is: But
there's so much useful stuff in there (that went into my explanation
of 'MAPCAR). I had actually been hoping to find a simple definition of
mapcar's behaviour analogous to this definitionally correct
description of mapcon in terms of maplist:

(mapcon f x1 ... xn)
:= (apply #'nconc (maplist f x1 ... xn))

but mapcar and maplist get prose definitions. Maybe

(labels
((%cars (lists)
(if (null lists) nil
(cons (caar lists) (%cars (cdr lists)))))
(%cdrs (lists)
(if (null lists) nil
(cons (cdar lists) (%cdrs (cdr lists)))))
(%mapcar (f &rest rest)
(if (some 'null rest) nil
(cons (apply f (%cars rest)) (%mapcar f (%cdrs rest))))))
(%mapcar f x1 ... xn))

Though we would never see this in common lisp because we have the
mapcar family in the standard.

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