On Tue, 08 Nov 2011 23:14:59 +0100 Thorsten <quintf...@googlemail.com> wrote:
> José Romero <jose.cyb...@gmail.com> > writes: > > > 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. > > great, thank you - thats probably what one would call "dense code". > > what I did not understand (or remembered) in the beginning was that > symbols evaluate to their VAL, a number (the second 'and') and that a > final '.' immediatly followed by a closing parenthesis indicates a > circular list. > > a lot of things going on in one single line of code. in picolisp its > really helpfull to be aware whats going on under the hood. > > > 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): > > that looks a bit easier ... > > > (and 5 . ((prinl @) (gt0 (dec @)) .)) > > let me try this one: > > 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 number i was passed, i see a number 5, > i evaluate it, a number came out, it's not NIL, so i shove it in @ and > look at the next cadr. > > here I get stuck - this looks like a dotted pair cell, but thats a > list and not an atom in the CDR? or is this list evaluated until it > return an atom as an result? otherwise its clear that doAnd evaluates > the circular list until NIL is returned. > > Cheers If it helps, 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 @ (I hope email keeps it's formatting) Cheers -- UNSUBSCRIBE: mailto:picolisp@software-lab.de?subject=Unsubscribe
