]> > Date: Sat, 6 Jul 2002 14:22:52 -0400 > From: William Lovas <wlovas@stwing.upenn.edu> > > On Sat, Jul 06, 2002 at 05:26:33PM +0200, Pierre Weis wrote: > > > Does this mean > > > that my "stream_cons" function is useless? > > > > > > Thanks in advance, > > > > > > Mike > > > > No. It means you cannot always use it in place of a constructor > > application. Note that a similar phenomenon occurs for pattern > > matching: constructors are mandatory, you cannot subtitute them by > > equivalent function calls! > > Actually, i think stream_cons may be useless, due to OCaml's eager > evaluation strategy. If you try to delegate the recursion to a function > call, the following happens: > > # let rec constant n = stream_cons n (constant n);; > val constant : 'a -> 'a stream = <fun> > # constant 1;; > Stack overflow during evaluation (looping recursion?). > > ... but the constructor still works: > > # let rec constant n = Cons (n, lazy (constant n));; > val constant : 'a -> 'a stream = <fun> > # constant 1;; > - : int stream = Cons (1, {contents = Lazy.Delayed <fun>}) > > The second argument needs to be treated lazily (which it is, explicitly, > using the Cons constructor call), but stream_cons will never achieve that, > unless i'm missing something. > > William You're right. I realized this shortly after sending out the question. Because of ocaml's eager evaluation, the whole point of 'stream_cons' is thwarted by writing it as a function, because by definition all the arguments to the function are evaluated. I got confused because I was trying to imitate some scheme code from SICP in ocaml, and there was (what looked like) a 'stream-cons' function in the SICP code. It turns out that 'stream-cons' has to be implemented as a macro in scheme (a point glossed over in SICP). > > P.S. Michael, why not call your type something like `lazy_list', instead of > `stream'? The latter conjures up images of stateful Stream.t's, for me at > least. Just a thought! Right again. This is SICP terminology. I'll change it if I post any more questions on this subject to this list. Thanks for your input (and to Pierre Weis for his explanation of the error message). Mike ------------------- To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/ Beginner's list: http://groups.yahoo.com/group/ocaml_beginners