Hi Tomas! In some way what you write makes sense. Let me state here, that I did read that book and worked through it for a year though, even through the complicated parts like the y-combinator and some chapters I must have read like 4 or 5 times and discovered new aspects on each try.
What is typically the case in the book is a different situation though, than what was in Taylan's procedure. Usually it is the list you are working on in that iteration, which you check for being (null? ...), not the thing, that you give as argument to a recursive call or as a return value, which you add in some way to the result. Usually the questions from the quote are asked once the argument is received in the next iteration. That I definitely usually do, but in Taylan's answer there is an (if (null? ...) ...) for the `rest`, inside the case, where the usual (null? ...) check is already done on the subtree, which we recur on. Perhaps I did not recognize the similarity there. Perhaps the rule from TLS is applies more broadly, than I was aware of. And perhaps that is something, that I can take from your reply! Thanks! Anyway, it is at least also new aspect, that I got from Taylan's answer. Regards, Zelphir On 12/13/20 3:24 PM, to...@tuxteam.de wrote: > On Sun, Dec 13, 2020 at 01:29:31PM +0100, Zelphir Kaltstahl wrote: >> Hello Taylan! >> >> I tried your procedure and indeed it seems to work : ) >> >> I think what I had been missing before were 2 things: >> >> 1. I did not have the (if (null? rest) ...) parts, so I always tried to >> directly make a recursive call, perhaps wrapped into a cons, append or > "When recurring on a list of atoms, /lat/, ask > two questions about it: /(null? lat)/ and *else*. > When recurring on a number, /n/, ask two > questions about it: /(zero? n)/ and *else*. > When recurring on a list of S-expressions, /l/, > ask three question about it: /(null? l)/, /(atom? > ( car l))/, and *else*." > > Daniel P. Friedman and Matthias Felleisen: The Little Schemer [1] > "First Commandment". > > Now before this sounds arrogant or something: this is a botch > I'd be very likely to do myself. That's perhaps why this book, > which at first sight looks so harmless, was for me a joy to read. > > Cheers > > [1] > https://pdfs.semanticscholar.org/35d0/d5275a8390c351ce98fbdc2ad37d210ba63b.pdf > > - t -- repositories: https://notabug.org/ZelphirKaltstahl
