[REBOL] Re: CPS transformations on func tion! values
From: joel:neely:fedex at: 27-Feb-2003 14:07
Hi, Maarten,
I had thought about this, but hit the wall except for thoughts to
would be the subject of further articles in the series of articles
on REBOLforces (Recursion->Iteration->Objects->Tasks).
Maarten Koopmans wrote:
> I'd like to be able to suspend my REBOL at any time, serialize it
> and the resume at some other time. Perhaps on another REBOL
> somewhere in the wild wild REB.
>...
> Storing the state of a REBOL is easy ...
>
> Storing control flow is a challenge.
>
But to me, storing control flow is part of the state.
For some simple cases, such as the example below (forgive me for the
definition of VOWELS, speakers of something other than English, I'm
just trying to make a small example ;-)
vowels: charset "AEIOUaeiou"
count-vowels: func [s [string!] /local count] [
count: 0
parse s [
any [
vowels (count: count + 1)
|
skip
]
]
count
]
foreach message [
"Eat at Joe's!"
"Hello, world!"
"My dog has fleas."
][
yield reduce [count-vowels message tab message]
]
Let's imagine that the word YIELD somehow saves state in a place/way
that it can be RESUMEd later, and returns the value of the following
expression.
I can understand how to transform all of this into an object with a
STEP method, which returns for each call a single YIELDed result:
my-obj: make object! [
vowels: charset "AEIOUaeiou"
count-vowels: func [s [string!] /local count] [
count: 0
parse s [
any [
vowels (count: count + 1)
|
skip
]
]
count
]
messages: []
start: func [b [block!]] [messages: copy/deep b]
step: func [/local message] [
if not tail? messages [
message: first messages
messages: next messages
return reduce [
count-vowels message tab message
]
]
]
]
my-obj/start [
"Eat at Joe's!"
"Hello, world!"
"My dog has fleas."
]
print my-obj/step
; ... arbitrary other unrelated stuff occurs
print my-obj/step
; ... more unrelated stuff
while [found? answer: my-obj/step] [print answer]
but if the YIELD had been placed inside the PARSE, as in
vowels: charset "AEIOUaeiou"
yield-vowels: func [s [string!] /local vowel] [
parse s [
any [
vowel: vowels (yield first vowel)
|
skip
]
]
none
]
foreach message [
"Eat at Joe's!"
"Hello, world!"
"My dog has fleas."
][
yield reduce [count-vowels message tab message]
]
then implementing this would require substantial rewriting of the
use of PARSE (assuming that we don't have a way to get/set the
state of an in-process PARSE evaluation).
My conclusion was that if I were going to be able to model a
yield/resume scenario (within my lifetime, at least ;-), I'd be
better off writing each one individually rather than trying to
come up with an automatic rewriting mechanism. Of course, there
are folks who can probably do more with this idea than I...
-jn-
--
----------------------------------------------------------------------
Joel Neely joelDOTneelyATfedexDOTcom 901-263-4446
Counting lines of code is to software development as
counting bricks is to urban development.
