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2value.html
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<title>2VALUE</title>
<h3>2VALUE</h3>
<h4>Poll standings</h4>
See <a href="#voting">below</a> for voting instructions.
<h5>Systems</h5>
<pre>
[ ] conforms to ANS Forth.
Firmworks Open Firmware implementation (Mitch Bradley)
iForth (Marcel Hendrix)
[ ] already implements the proposal in full since release [ ].
Firmworks Open Firmware implementation (Mitch Bradley) a long time ago
[ ] implements the proposal in full in a development version.
[ ] will implement the proposal in full in release [ ].
[ ] will implement the proposal in full in some future release.
iForth (Marcel Hendrix)
[ ] There are no plans to implement the proposal in full in [ ].
4th 3.5d rel. 2 (Hans Bezemer)
[ ] will never implement the proposal in full.
</pre>
<h5>Programmers</h5>
<pre>
[ ] I have used (parts of) this proposal in my programs.
Marcel Hendrix
Gerry Jackson
[ ] I would use (parts of) this proposal in my programs if the systems
I am interested in implemented it.
Marcel Hendrix
David N. Williams
[ ] I would use (parts of) this proposal in my programs if this
proposal was in the Forth standard.
Marcel Hendrix
David N. Williams
[ ] I would not use (parts of) this proposal in my programs.
Hans Bezemer
Hugh Aguilar
</pre>
<h5>Informal results</h5>
<h4>Proposal</h4>
<pre>
Problem
-------
The word VALUE was considered useful enough to be included in
ANS94 Forth. An RfD for FVALUE stated that a search through 4827
source files shows 532 occurrences of VARIABLE versus 4241 uses
of VALUE. It would be obviously useful to have a variant of
VALUE that works for cell pairs and double values. Also, if the
proposal for FVALUE is accepted, acceptance of 2VALUE will
complete the family of such words and maintain consistency with
the rest of of the data defining words and many others.
The name 2VALUE is preferred to the alternative DVALUE as it
follows existing practice for data defining words e.g. VARIABLE
2VARIABLE and FVARIABLE. Furthermore the obvious pronunciation
of DVALUE is an English word (devalue) with a totally different
meaning.
The main idea behind 2VALUE (VALUE, FVALUE) is that fetching a
variable is more frequent than changing it. Therefore both
readability of source code and better efficiency of execution
can be achieved by using words defined by 2VALUE rather than
2VARIABLE.
Current practice
----------------
Win32Forth supports it, iForth supports it under the name
"DVALUE", the extent of support for it or DVALUE in other
systems is unknown.
Solution
--------
This solution follows existing practice with VALUE by defining
2VALUE as a parsing word, and is a straightforward extension of
VALUE to a cell pair or double number.
The syntax is:
: s S" some string" ;
s 2VALUE stringref
to define <stringref> as a cell pair holding (caddr u) for the
string.
Alternatively, one can use
123. 2VALUE ddata
to define <ddata> as a double value initialized to 123..
To access the value of stringref and ddata:
stringref TYPE <cr> some string ok
ddata D. <cr> 123 ok
The word TO is used to change stringref and ddata:
: s2 S" another string" ; s2 TO stringref
456. TO ddata ddata D. <cr> 456 ok
As there is no standard way to obtain the address of a 2VALUE,
this definition does not require the cell pair or double value
to be stored in any particular order in memory.
Proposal
--------
Add the following definition to section 8.6.2 Double-Number
extension words.
8.6.2.xxxx 2VALUE "two-value" DOUBLE EXT
( x1 x2 "<spaces>name" -- )
Skip leading space delimiters. Parse name delimited
by a space. Create a definition for name with the
execution semantics defined below, with an initial
value of x1 x2.
name is referred to as a "two-value".
name Execution: ( -- x1 x2 )
Place cell pair x1, x2 on the stack. The value of
x1, x2 is that given when name was created, until the
phrase x3 x4 TO name is executed, causing a new cell
pair x3, x4 to be associated with name.
TO name Run-time: ( x1 x2 -- )
Associate the cell pair x1, x2 with name.
See: 3.4.1 Parsing and 6.2.2295 TO.
Rationale:
Typical use:
: fn1 s" filename" ;
fn1 2VALUE myfile
fn1 included
: fn2 s" filename2" ;
fn2 TO myfile
<a href="tests/2value.fs">Testing</a>
-------
t{ 1 2 2VALUE t2val -> }t
t{ t2val -> 1 2 }t
t{ 3 4 TO t2val -> }t
t{ t2val -> 3 4 }t
: sett2val t2val 2SWAP TO t2val ;
t{ 5 6 sett2val t2val -> 3 4 5 6 }t
Notes
-----
1) This proposal does not address the issue of local 2VALUEs as
this depends on the outcome of another RfD on the Locals word
set and could require an extension to that RfD.
Reference Implementation
------------------------
The implementation of TO to include 2VALUEs requires detailed
knowledge of the host implementation of VALUE and TO, which is
the main reason why 2VALUE should be standardized. The order in
which the two cells are stored in memory is not specified in the
definition for 2VALUE but this reference implementation has to
assume one ordering - this is not intended to be definitive.
: 2VALUE ( x1 x2 -- )
CREATE , ,
DOES> 2@ ( -- x1 x2 )
;
The corresponding implementation of TO disregards the issue that
TO must also work for integer VALUEs and locals.
: TO ( x1 x2 "<spaces>name" -- )
' >BODY
STATE @
IF
POSTPONE 2LITERAL POSTPONE 2!
ELSE
2!
THEN
; IMMEDIATE
</pre>
<h4><a name="voting">Voting instructions</a></h4>
Fill out the appropriate ballot(s) below and mail it/them to me
<anton@mips.complang.tuwien.ac.at>. Your vote will be published
(including your name (without email address) and/or the name of your
system) here. You can vote (or change your vote) at any time by
mailing to me, and the results will be published here.
<p>Note that you can be both a system implementor and a programmer, so
you can submit both kinds of ballots.
<h5>Ballot for systems</h5>
If you maintain several systems, please mention the systems separately
in the ballot. Insert the system name or version between the brackets
or in the line below the statement. Multiple hits for the same system
are possible (if they do not conflict).
<pre>
[ ] conforms to ANS Forth.
[ ] already implements the proposal in full since release [ ].
[ ] implements the proposal in full in a development version.
[ ] will implement the proposal in full in release [ ].
[ ] will implement the proposal in full in some future release.
[ ] There are no plans to implement the proposal in full in [ ].
[ ] will never implement the proposal in full.
</pre>
If you want to provide information on partial implementation, please do
so informally, and I will aggregate this information in some way.
<h5>Ballot for programmers</h5>
Just mark the statements that are correct for you (e.g., by putting
your name on the line below the statement you agree with). If some
statements are true for some of your programs, but not others, please
mark the statements for the dominating class of programs you write.
<pre>
[ ] I have used (parts of) this proposal in my programs.
[ ] I would use (parts of) this proposal in my programs if the systems
I am interested in implemented it.
[ ] I would use (parts of) this proposal in my programs if this
proposal was in the Forth standard.
[ ] I would not use (parts of) this proposal in my programs.
</pre>
If you feel that there is closely related functionality missing from the
proposal (especially if you have used that in your programs), make an
informal comment, and I will collect these, too. Note that the best time
to voice such issues is the RfD stage.
<hr><a href="http://www.complang.tuwien.ac.at/anton/">Anton Ertl</a>