Proposed change to libraries
Richard A. O'Keefe
Tue Feb 8 01:42:30 CET 2005
> Scheme insists on the list argument of map being a proper list, and it
> seems to cause no trouble there.
I am not sure I get this, because map naturally protects itself from
inproper lists in its second argument.
It is not true that "map naturally protects itself. In Scheme,
(define (map F L)
(if (pair? L)
(cons (F (car L)) (map F (cdr L)))
is the most obvious way to implement map. You have to go out of your way
to explicitly check for an empty list. And that's my point: in a case
where it would not just be _as_ easy to be "generous" but it would actually
be _easier_ to be "generous", Scheme demands a proper list.
Can it be that you are mixing
map with some other function like append?
No. append does *not* require its second argument to be a proper list in
Scheme implementations. There is an interesting but obvious reason:
- in cases where the only burden is one extra check "now that this
argument isn't a pair, is it an empty list?", the check is done.
- in cases where the burden would be traversing a list that would
otherwise _not_ have been traversed, the check is not done.
That is, checks are made that don't change the expression inside the big Oh.
Not a bad rule for a library implementor.
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