Re: Improper lists Marc Feeley (29 Jun 1999 15:42 UTC)
Re: Improper lists Marc Feeley 29 Jun 1999 15:42 UTC
> > (let recur ((x x))
> > (if (pair? x)
> > (cons (car x) (recur (cdr x)))
> > y))
> But this borders on the absurd, because under this definition,
> (append '(a b c . d) '(e f g)) => (a b c e f g)
> that is, the d is silently dropped. What kind of sense does that make?
> Plenty of sense -- the D isn't doing anything except terminating that
> list. Once you get used to thinking of D as an empty list, Lars, you're going
> to love it.
I agree with Lars et al. An improper list is not a list!!! If someone
wants to mix them, explicit conversions should be performed. In the
early days Gambit's append (and other functions) silently treated
non-pairs as the empty list as in
(append '(a b c . d) '(e f g)) => (a b c e f g)
but some people complained that this did not help them catch errors
early in their programs. Now I have tightened up the type checks to
signal an error.