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. -- With techies, I've generally found John Cowan If your arguments lose the first round http://www.ccil.org/~cowan Make it rhyme, make it scan co...@ccil.org Then you generally can Make the same stupid point seem profound! --Jonathan Robie _______________________________________________ Chicken-users mailing list Chicken-users@nongnu.org http://lists.nongnu.org/mailman/listinfo/chicken-users
