Ah, but how would you easily access the number 6 out of the nested list '((((5 4...

ajuc · on Feb 20, 2015

    (def x ((((5 4 3 2 1) 6) 7) 8 9))

    (get-in x (0 0 0 1))

Probably botched quoting, don't know common lisp, only clojure.

GregBuchholz · on Feb 20, 2015

For the heck of it, I though I'd try it in scheme with pattern matching...

  (let ((x '((((5 4 3 2 1) 6) 7) 8 9)))
    (syntax->datum
     (syntax-case x ()
       ((((_ p) _) _ ...) #'p))))

chongli · on Feb 20, 2015

One word: lenses.

Flow · on Feb 20, 2015

And how would that look for this particular example?

chongli · on Feb 20, 2015

Definitely not as terse but far more general, flexible and powerful. With car and cdr you're restricted to working on cons cells while lenses are generic.

Some examples (from Haskell):

    >>> ((((5,4,3,2,1),6),7),8,9) & view (_1 . _1 . _2)
    6
    >>> ((((5,4,3,2,1),6),7),8,9) & (_1 . _1 . _2) .~ 4
    ((((5,4,3,2,1),4),7),8,9)
    >>> let foo = ((((5,4,3,2,1),6),7),8,9)
    >>> foo & view (_1 . _1 . _2)
    6
    >>> let six = _1 . _1 . _2
    >>> foo & view six
    6
    >>> foo & six .~ 4
    ((((5,4,3,2,1),4),7),8,9)
    >>> foo & six .~ "abc"
    ((((5,4,3,2,1),"abc"),7),8,9)