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I was curious what it is like on Maclisp. Here is a complete telnet session with Lars Brinkhoff's public ITS: $ telnet its.pdp10.se 10003 Trying 88.99.191

• dokyun 2y ago
  I recall reading that in early versions of Maclisp, taking the CAR or CDR of NIL worked differently: Taking its CAR would signal an error as you would expect, however taking its CDR would return the symbol plist of NIL, as internally the operation of CDR on the location of a symbol would access its plist, and that's how it was commonly done before there was a specific form for it (and it actually still worked that way into Lisp Machine Lisp, provided you took the CDR of the locative of a symbol). Apparently the behaviour of the CAR and CDR of NIL being NIL was from Interlisp, and it wasn't until the designers of Maclisp and Interlisp met to exchange ideas that they decided to adopt that behaviour (it was also ostensibly one of the very few things they actually ended up agreeing on). The reason they chose it was because they figured operations like CADR and such would be more correct if they simply returned NIL if that part of the list didn't exist rather than returning an error, otherwise you had to check each cons of the list every time. (If somebody can find the source for this, please link it!)
  • pfdietz 2y ago
    But of course cadr still has to check each access, to see if its of type (or cons null). So I don't see what was saved.
    • dokyun 2y ago
      It would be considered "the right thing" to do something that's so common you probably want it without asking. I don't think CADR would check for NIL since it's meant to be equivalent to (car (cdr x)), so if you wanted a safe list operation you would have to check it like this: (I'll use CADADR because it makes the issue more apparent) (and (car x) (cadr x) (cadar x) (cadadr x)) You would have to write this every time you want to see if there's a really CADADR, whereas if CAR and CDR can return NIL then you can just write (cadadr x) and CADADR can still be defined as (car (cdr (car (cdr x)))) and have the desired behaviour.
      • pfdietz 2y ago
        Any argument of the form "you have to write this idiom" is covered by "so either it doesn't happen often, or one can use a macro". There's cognitive overhead to using a macro, but there's also cognitive overhead to remembering car and cdr work on nil. The latter is already paid for, so changing Common Lisp now doesn't make sense, but in an alternate world with a different design it would be a point.
        • dokyun 2y ago
          There's more 'cognitive overhead' to making CADADR etc. a macro that expands to the above when CAR and CDR don't work that way, since then its implementation isn't consistent with what it's meant to be. If you made it a macro with a different name then you have two slightly different versions of CADADR and every other accessor, which is even more overhead. Apparently this idiom happened often enough that it was deemed desirable to simply make it the default behaviour. Accommodating common idioms is a pattern of "the right thing" design, from which Lisp is heavily derived, and if you're not a Lisp programmer then keeping this philosophy in mind is a good way to not have misconceptions about the design of the system. However, thinking in terms of 'cognitive overhead' for a very minor design choice is very silly. I don't suffer any 'cognitive overhead' from having CAR and CDR work on NIL when I write Common Lisp because I'm used to it, but I do suffer 'cognitive overhead' when they don't in Scheme, which is the 'alternate world with a different design'. I am incredulous to the idea that one is actually superior to the other, and suppose that it is simply a matter of preference.