Hi. Was going over Jay's Towers of Hanoi implementation that makes use of Racket streams. I find this comment confusing and would appreciate a more elaborate explanation:
> It’s important, using racket/stream, to use the stream-cons because if you > wrote a stream-append with three arguments, then the second recursive call > wouldn’t be lazily evaluated and you’d save nothing by using streams over > lists. That’s because stream-append is a function, while stream-cons is a > macro that thunks its arguments. Ok, so I assume what he means is that procedure arguments get evaled during application. Since (stream-append ..) is a procedure, the result of the second recursive call (last arg) would be forced. Docs for (stream-append ..) however say this: > new stream is constructed lazily, while the last given stream is used in the > tail of the result. I find this confusing. What does "costruct lazily" mean then? Does this phrase say that (n-1) args to it aren't forced, but the last one is? But append is a procedure, so all args should be evaled. As a side thought, I think I could cheat (like stream-cons) by thunking stuff I pass to append, thunk is a wrapping macro, right? > (stream-append (thunk (towers (sub1 n) from extra to)) > (thunk (move from to)) > (thunk (towers (sub1 n) extra to from))) thx! --- Vlad Kozin <[email protected]>
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