Subject:
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Re: Forth question
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Newsgroups:
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lugnet.robotics.rcx.pbforth
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Date:
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Wed, 24 May 2000 11:09:57 GMT
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Reply-To:
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SJM@JUDGEMENTnomorespam.COM
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Viewed:
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1531 times
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Ralph Hempel wrote:
>
> > I'm trying to define a constant in a compiled word.
> >
> > : zzz 1 CONSTANT ONE ;
> >
> > I know its not right. It tries to compile "ONE" which is not
> > what is wanted. What is the right way to say this?
>
> 1 CONSTANT ONE
>
> Recall that CONSTANT is a *defining* word, just like the ":"
I still want to create a constant from within a definition. I
have an initializing word that will calculate the value of
the constant. For various reasons I can't do this calculation
except within the runtime behavior of the word.
This is not critical. I have since replaced the constant
with a variable that I define in advance and then set
within my word, but then I have to use variable semantics
instead of constant semantics to access the value, which is
not what I wanted. I get around that by defining a new
word which leaves the value of the "constant" on the stack
": fake_constant varname @ ;" However this is a hack
and I was trying to understand how to do it right, i.e.
create a constant during the runtime behavior of a word.
I guess that the reason that this is hard is that I
effectively want my word to be a defining word. I guess
I need a new word that behaves like:
1 CONSTANT" name"
which saves the string at compile time and then uses it at
runtime to create the constant.
or perhaps one that behaves like:
1 S" name" SCONSTANT
Which expects a string on the stack instead of parsing
forward through the input stream.
As I said this is not important. I have a workaround.
It is more a matter of self education. I am trying to
learn to create new compile time behavior.
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Message has 3 Replies:
 | | Re: Forth question
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| (...) Actually the various reasons are simple so I might as well state them. The serial port is not available at the time I want to create a constant because the serial port will be broken and therefor can't be used as the forth input stream. I am (...) (25 years ago, 24-May-00, to lugnet.robotics.rcx.pbforth)
| | | CONSTANTS and VALUES in pbForth
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| (...) There are a couple of things in this note. One is an introduction to CREATE DOES> and the other is a solution to your actual problem. Basically, what CREATE DOES> does is separate the run-time and compile time semantics of a word. This is (...) (25 years ago, 24-May-00, to lugnet.robotics.rcx.pbforth)
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Message is in Reply To:
| | RE: Forth question
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| (...) 1 CONSTANT ONE Recall that CONSTANT is a *defining* word, just like the ":" It makes a name in the dictionary, in this case "ONE" and then links to a special bit of code that is executed every time ONE is interpreted. It grabs the value out of (...) (25 years ago, 24-May-00, to lugnet.robotics.rcx.pbforth)
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