Wacky stuff with circular lists in PicoLisp

Example 1 with walk-through

(and and (@ (min @ 5) (prinl @) (gt0 (dec @)) .))

I'm eval. I see a list, look inside, i see a symbol called 'and. i look at it's val. It's a number, thus a function pointer, i call it with the rest of the list unevaluated.

I'm doAnd, i look at the list i was passed, i see a symbol called 'and, i evaluate it, a number came out, it's not NIL, so i shove it in @ and look at the next cadr. It's a list, i evaluate it, has a symbol called @ in it's car, that symbol resolves to a number, a pointer to doAnd, i call it with the rest of the list unchanged.

I'm doAnd, i look at the list i was passed, the first argument, evaluating it results in a function call that returns 5, it's not nil, so i shove it to @ and go on. The next element is another list, a call to prinl happens, it returned 5, it's not nil, so i shove it to @ and go on. Look at the next element, a call to (gt0 (dec @)), returns 4, that is not nil, so i shove the 4 in @ and go on. Looking at the next cadr i see @ again (but i don't realize, because i don't know wether a list is circular or not), it evaluates to 4, so i shove it to @ keep going.

I see (min @ 5) again.........
 ... 3
 ... 2
 ... 1
 ... (gt0 (dec @)) returns NIL here

so i stop evaluating and return NIL

Back in the first doAnd, i see the previous call returned NIL, so i stop evaluating right there and return NIL.

Example 2 with graphical depiction

Example 1 could be written with only one 'and using the right syntax (and losing part of it's rube-goldberg appeal):

(and 5 . ((prinl @) (gt0 (dec @)) .))

This is how that S-expression should look in memory, I omitted the technically correct representation of numbers and symbols to keep it simple:

                      ,-----------------------,
 +---+---+  +---+---+ '->+---+---+  +---+---+ |
 |   | ---->|     ------>|   | ---->|   | ----'
 + | +---+  + | +---+    + | +---+  + | +---+
   v          v            | ,--------'
  and         5            | |  +---+---+  +---+---+
                           | '->|   | ---->|   | / |
               ,-----------'    + | +---+  + | +---+
               |                  v          |
               |                 gt0         |  +---+---+  +---+---+
               |                             '->|   | ---->|   | / |
               |   +---+---+  +---+---+         + | +---+  + | +---+
               '-->|   | ---->|   | / |           v          v
                   + | +---+  + | +---+          dec         @
                     v          v
                   prinl        @

http://picolisp.com/wiki/?circularlists

10nov11

tj