I may have made a mistake under time pressure: [else (+ (force (sum (car x))) (force (sum (cdr x))))]
On Jul 7, 2012, at 10:19 AM, Sam Tobin-Hochstadt wrote: > On Sat, Jul 7, 2012 at 10:14 AM, Matthias Felleisen > <matth...@ccs.neu.edu> wrote: > >> p.s. Sam's version forces too early because strict functions should >> force not variable lookups. This is a common misconception among programmers. >> I suspect it goes back to Abelson and Sussman getting it wrong (see what >> happens if you don't understand the foundations). > > I still don't think my version forces too early -- your alternative > contains no calls to `force` at all. Are you suggesting that I should > `force` around the recursive calls to `sum`? In that case, clients > are exposed to promises, and the result type is (U Integer (Promise > Integer)). I can't tell from your description if you think that's an > acceptable result. > -- > sam th > sa...@ccs.neu.edu _________________________ Racket Developers list: http://lists.racket-lang.org/dev
