Re: [Chicken-users] list vs dotted-list

[John Cowan]

Yi DAI scripsit:

> Why do we have both list and dotted-list, given that the former is just a
> special case (terminated by nil) of the latter? Can we simply live with the
> latter?

Conceptually, lists are the basic functional programming data structure:
sequential with O(n) access and shareable (in Lisp/Scheme, also mutable).
For historical reasons, lists are not primitive in Scheme but are
implemented using pairs, and this fact is exposed to Scheme programmers.

Dotted-lists are basically an artifact of that implementation strategy.
In statically typed functional languages like ML or Haskell, they are
not permitted.  Even in Lisp/Scheme, they are not often used (even the
use of plain pairs is relatively uncommon except in a-lists).

> I see it is fairly easy to write common list procedures, like
> `length', `map', `append' etc that could handle dotted-list very well, with
> the predicate `pair?'. 

Not so much.  Map and append operate on every element of a list, but
is the dotted element of a list part of it or not?  What is the proper
value of (append '(a b . c) '(d e . f))?  Not (a b . c d e . f), for
that is meaningless.

In Common Lisp there is a predicate "endp" (which would be spelled "end?"
in Scheme) that returns true on (), false on a pair, and signals an
error on any other argument.  It's convenient for dealing with lists
element by element that should not be dotted but still might be.

-- 
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