CAR, CDR, and Lisp...

Regarding the "hunks" he mentioned, here's what Guy Steele had to say about them when I interviewed him for Coders at Work: "I implemented a new data type called a 'hunk,' which was probably the biggest disaster in language design we ever put in. It was essentially a cons cell with more than two pointers. It was a desperation move because we were running out of address space on the PDP-10. Recall that the -10 only had an 18-bit address space. In a list, 50 percent of the pointers were dedicated to just maintaining the structure of the list, whereas with a threaded set of hunks, maybe only one-eighth of the pointers were dedicated to the threading of the chunks so you’d get better memory usage that way."

Of course since Clojure is built upon abstractions, and because it's simple to introduce your own protocols and extend them to existing types w/o conflicts - it's straightforward to reintroduce the concept of a "pair" where the right-hand side is not a sequence. I recently encountered this challenge working on a Clojure port of miniKanren (the relational programming system described in the The Reasoned Schemer).

In miniKanren the concept of a "pair" is useful because the right hand side of a sequence in miniKanren might be a logic variable and not a sequence.

What's lovely is how simple this is. I can define a custom iteration protocol (LogicSeq) which define two functions - lfirst, lnext. Then I create a custom sequence type that returns a legitimate Sequence or logic variable for lnext.

To bring all the other collections into this protocols is 4 lines of code:

  (extend-protocol LConsSeq
    clojure.lang.IPersistentCollection
    (lfirst [this] (clojure.core/first this))
    (lnext [this] (clojure.core/next this)))

Voila! Custom iteration!

What's even more amazing is that there's hardly a perf hit. The JVM sees it's just a pass through call for real Clojure collections.

Fun stuff :)

Why are people so offended by syntax? So many people complain about parentheses or names like car and cdr. Is it the programming equivalent of being shallow or judging by appearance?

When I first started learning scheme I was much more challenged by the rest of the cognitive assault - tail recursion is "free", call/cc or continuations, the shift to thinking in "higher order" mode, macros, being able to effectively define your own syntax.

I'm surprised cadr, caddr, cddr, etc. were not mentioned when comparing car/cdr with first/rest.

See http://www.franz.com/support/documentation/8.2/ansicl/dicten...

car and cdr must have something going for them that first and rest don't, or else people would have abandoned them decades ago. The author mentions one point in their favor: generality. I'd add another two: brevity, and symmetry.

It's also worth noting that the primary disadvantage of car and cdr, their arbitrariness, fades as one becomes familiar with the language.