One possible source of confusion: begin that appears at the top-level (of a module or in the REPL) or in a definition context (like the body of a let expression or lambda expression) is treated specially from other begins; specifically, it is treated as if the contents of the begin were separate expressions, syntactically spliced directly into the context (or the top-level). This is to facilitate writing macros that expand into multiple definitions, etc.
A separate rule takes place for top-level expressions, which each get their own prompt (as Matthew pointed out). hth, Robby On Mon, Aug 2, 2010 at 8:04 AM, Matthew Flatt <mfl...@cs.utah.edu> wrote: > At Mon, 02 Aug 2010 21:54:54 +0900 (JST), Keiko Nakata wrote: >> Actually, I didn't see. >> >> Why this code >> >> (let ([d (box #f)] [a (box 0)]) >> (print (+ (begin (let/cc k (begin (set-box! d k) (set-box! a 2) 3))) (unbox >> a))) >> ((unbox d) 9)) >> >> prints 511111111, rather than 59999999? > > I'm not sure I'm following. Can you say more about why you expect > infinite "9"s instead of infinite "11"s? > > My reading is that a 2 gets installed into the box `a' once and for > all, and then it's repeatedly added to 9. Nothing aborts the `let/cc' > body before the `(set-box! a 2)' is evaluated. Also, the `+' is outside > the `let/cc', so it is captured in the continuation along with `print'. > > > In case it's a question of prompts: There is only one continuation > prompt in the example, which is around the entire expression. In > particular, the `begin' above doesn't introduce a continuation prompt. > The prompt in your earlier example wasn't from `begin', but from the > top level into which the `begin' forms were spliced. > > _________________________________________________ > For list-related administrative tasks: > http://lists.racket-lang.org/listinfo/users > _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
