Thanks Matthias, I am still a little unclear. Is this a logical result, or a result of the evaluator's rules? Or are they the same thing?
It wasn't until i followed the logic of (mk-length mk-length) outside the function that i saw it would replicate on and on - which i guess is what we want so we can measure lists of whatever length. Likely reflecting my immaturity, now i have that result in my head, i am unclear why references to the original function stop replication at a single copy. is this 'original function' the above level copy of: (lambda (mk-length) (Ie (lambda (x) ( (mk-length mk-length) x)))) I am asking as i am wondering where to focus my efforts - as this is not yet obvious to me. Perhaps i need to type up some notes to clear up my thinking. Best regards mj On 11/02/2014, at 3:08 AM, Matthias Felleisen <matth...@ccs.neu.edu> wrote: On Feb 10, 2014, at 4:52 PM, Matthew Johnson wrote: Why does (lambda (x) (e x)) make it evaluate once and stop? I mean how come when the evaluator gets to the call it doesn't get stuck in a loop? This explicit function gets "copied" over and over again during function calls (beta-value reductions) during an evaluation. When it is called the 'e' inside contains (references to) the original function so there is no danger that the infinite loop unfolds -- because it is waiting for the next explicit call. ;; --- but i really don't see why one would ever bother to do so. What are the benefits? the cost (hard to read / write / understand) seems high. CPS is a programming technique that occasionally comes up in program designs: -- if you must implement a recursive algorithm in a language w/o recursion, cps has eliminated it -- systematically [this situation used to be common when I started teaching; now it's rare] -- if you need to suspend a computation temporarily, the 'k' is what you don't call but stick into a known place from where to resume [web programs often have to obey this discipline: suspend k, hand control to user to fill out some form, resume k] cps provides this capability -- systematically -- if you need to write sophisticated interleaving of routines in a language that doesn't provide it, cps supports it -- systematically The 'systematically' says that a programmer can blindly use the transformation and then modify it at some places to get things right. ;; --- The transformation is also used inside of compilers as Yuhao mentions.
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