BTW, this is guile version 1.8.7, for me. --linas
On 10 May 2010 11:26, Linas Vepstas <linasveps...@gmail.com> wrote: > On 7 May 2010 03:09, user8472 <head_over_he...@freenet.de> wrote: >> >> Please find the code for streams and the integration below. >> ;; The troublesome procedure >> (define (solve f y0 dt) >> (define y (integral (delay dy) y0 dt)) >> (define dy (stream-map f y)) >> y) >> >> ;; This works >> (define y (integral (delay dy) 1 0.001)) >> (define dy (stream-map (lambda (x) x) y)) >> (stream-ref y 1000) >> >> ;; This doesn't work >> ;(stream-ref (solve (lambda (x) x) 1 0.001) 1000) >> </code> > > Well, you modified your code enough so that let* now works fine, > at least for me: > > guile> (define (solve f y0 dt) > ... (let* ((y (integral (delay dy) y0 dt)) > ... (dy (stream-map f y)) > ... ) > ... y)) > guile> (stream-ref (solve (lambda (x) x) 1 0.001) 1000) > 2.7169239322359 > > Superficially, I want to say "this is why let* was invented, to > ensure ordering of definitions". In practice, I see that dy is > used in both, so there's a circular reference going on here. > The thing that saves you, and allows the let* to work, is the > (delay dy) which avoids evaluating dy (which is undefined, > at the time its encountered). Without the delay, the circular > references would cause the whole thing would fall apart, > and not even letrec would save you. > > As proof of this, here's another example, without let*, which > does enforce ordering (and which depends on the (delay dy) > to avoid the "use of variable dy before its defined" error): > > guile> (define (solve f y0 dt) > ... (define y (integral (delay dy) y0 dt)) > ... (let ((dy (stream-map f y))) > ... y)) > guile> (stream-ref (solve (lambda (x) x) 1 0.001) 1000) > 2.7169239322359 > guile> > > > I'm an amateur at scheme, but I beleive what I say is more > or less correct ... > > --linas >
