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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