Re: [Scheme-reports] Legacy caar to cddddr

[Andre van Tonder <andre@x>]

On Mon, 24 Oct 2011, Alex Shinn wrote:

> I'd be interested to see real-world examples of (set-car! (c[ad]{3,}r x) y).
> My guess is these are examples of abusing lists as records.

Whether to call this an abuse is very subjective, depending very much on the 
type system you are imposing in your mind on your Scheme program.

In my opinion, lists provide a wonderfully elegant representation 
of anonymous product types a la Hindley-Milner.  They have the additional 
advantage that you can portably READ and WRITE them and debug them easily, 
whereas records require a bunch of annoying boilerplate to do this.

As for which composite selectors should be provided by default, I agree with 
yout that one can probably suppress some of the selectors deeper than the spine 
of the list.  But maybe one could expand the number of selectors along the 
spine itself.  I have often run into the following:

     (define binding-type           car)
     (define binding-name           cadr)
     (define binding-levels         caddr)
     (define binding-mutable?       cadddr)
     (define (binding-dimension b)  (car (cddddr b))) ; UGLY

The last line is not a tragedy, but it is ugly.  Also, some optimizing 
implementations might be able to provide random access to more 
than 4 initial elements of short lists.


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