setcdr! for lists is not available in the language, since lists are
immutable.  The underlying implementation model supports setcdr!
(at some cost, though, because of the generational garbage
collection), so we could have a "magic" implementation of List.map
that uses setcdr!, but it's always preferable to remain within what
the language can express.

Let me put this another way.  There are some library functions for
which no "one size fits all" implementation exist: i.e. the
implementation favors one style of use over others.  For List.map or
list concatenation, for instance, one has to choose between running at
full speed on small to medium-sized lists, or being tail-recursive and
handling arbitrarily large lists at a significant cost in speed and
clarity of implementation.  I chose the former approach.  So, if you
really need to handle large lists, you may have to write your own
(tail-rec and slow) map function.  It's not really code duplication,
in that your map function will have quite different running behavior
than the one in the standard library.  

I would also contend that if your program routinely manipulate
100000-element lists, then you're probably using the wrong data
structure anyway.  But that's a different issue.

Regards,

- Xavier Leroy