On Tue, 08 Nov 2011 21:47:04 +0100 Thorsten <quintf...@googlemail.com> wrote:
> José Romero <jose.cyb...@gmail.com> > writes: > > > 'text also uses @ http://software-lab.de/doc/refT.html#text > > It's also useful to note that in short-circuit evaluation style > > functions like 'and, @ always holds the result of the previously > > evaluated argument, as you can see in this typical PicoLisp line: > > > > (and and (@ (min @ 5) (prinl @) (gt0 (dec @)) .)) > > > > ;) > > ok, I added these two cases to the table (see below). > > Could you give a little 'walk-through' for the example line, its not > that obvious (to me) how this works (but it does) ;) > > min, prinl, gt0 and dec are obvious. > the double 'and and' and the '.' at the and are a bit strange. Where > does the initial @=5 come from - because @ is initially T and T is > greater than everything else? > > cheers Pretty silly and badly written explanation of what goes on there: 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. Then I realized that it 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 @)) .)) Cheers :3 -- UNSUBSCRIBE: mailto:picolisp@software-lab.de?subject=Unsubscribe
