C. EXPERIMENTAL PROPOSALS
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APPENDIX C. EXPERIMENTAL PROPOSALS
Since FORTH is an extensible language and subject to evolution,
the Standard contains a section describing experimental
proposals. FORTH users are encouraged to study, implement, and
try these proposals to aid in the analysis of and the decision
for or against future adoption into the Standard. Readers are
cautioned that these proposals contain opinions and conclusions
of the authors of the proposals and that these proposals may
contain non-standard source code.
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C. EXPERIMENTAL PROPOSALS
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SEARCH ORDER SPECIFICATION AND CONTROL
WILLIAM F. RAGSDALE
1 INTRODUCTION
The method of selecting the order in which the dictionary is
searched has grown from unchained vocabularies to the present use
of chained vocabularies. Many techniques are in use for
specification of the sequence in which multiple vocabularies may
be searched. In order to offer generality and yet get precision
in specification, this proposal is offered.
2 DESCRIPTION
The following functions are required:
1. Two search orders exist. CONTEXT is the group of
vocabularies searched during interpretation of text from the
input stream. CURRENT is the single vocabulary into which
new definitions are compiled, and from which FORGET
operates.
2. Empty CONTEXT to a minimum number of system words. These
are just the words to further specify the search order.
3. Add individual vocabularies into CONTEXT. The most recently
added is searched first.
4. Specify which single vocabulary will become CURRENT.
The following optional functions aid the user:
1. Display the word names of the first vocabulary in the
CONTEXT search order.
2. Display the vocabulary names comprising CURRENT and CONTEXT
search orders.
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C. EXPERIMENTAL PROPOSALS
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3 ADVANTAGES
Use over the past year has demonstrated that the proposed
methods may emulate the vocabulary selection of all other
systems. The order is explicit by execution, may be interpreted
and compiled, and is obvious from the declaration. The search
order is specified at run-time rather than the time a new
vocabulary is created.
4 DISADVANTAGES
By migrating to a common structure, vendors give up one
point at which they may claim their product is better than
others. Another drawback is that the number of CONTEXT
vocabularies is fixed; older methods had an indefinite 'tree'
structure. In practice, the branching of such a structure was
very rarely greater than four.
Forth words operate in a context sensitive environment, as
word names may be redefined and have different definitions in
different vocabularies. This proposal compounds the problem. By
displaying the search order names, the user at least can readily
verify the search order.
5 IMPACT
The text of the Forth 83 Standard has been carefully chosen
for consistency and generality. However, no specification on how
the search order is developed by the user is given. This
omission is unavoidable, due to the diversity of contemporary
practice. This proposal is intended to complete the Forth 83
requirements in a fashion that exceeds all other methods.
Previously standardized words continue in their use:
VOCABULARY, FORTH, DEFINITIONS, and FORGET. However, this
proposal assumes that vocabulary names are not IMMEDIATE .
6 DEFINITIONS
Search order:
The sequence in which vocabularies are selected when
locating a word by name in the dictionary. Consists of one
transient and up to three resident vocabularies.
Transient order:
Execution of any vocabulary makes it the first vocabulary
searched, replacing the previously selected transient
vocabulary.
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C. EXPERIMENTAL PROPOSALS
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Resident order:
After searching the transient order, up to three additional
vocabularies may be searched. The application program
controls this selection.
7 GLOSSARY
ONLY -- ONLY
Select just the ONLY vocabulary as both the transient
vocabulary and resident vocabulary in the search order.
FORTH -- ONLY
The name of the primary vocabulary. Execution makes FORTH
the transient vocabulary, the first in the search order, and
thus replaces the previous transient vocabulary.
ALSO -- ONLY
The transient vocabulary becomes the first vocabulary in the
resident portion of the search order. Up to the last two
resident vocabularies will also be reserved, in order,
forming the resident search order.
ORDER -- ONLY
Display the vocabulary names forming the search order in
their present search order sequence. Then show the
vocabulary into which new definitions will be placed.
WORDS -- ONLY
Display the word names in the transient vocabulary, starting
with the most recent definition.
FORGET -- ONLY
Used in the form:
FORGET <name>
Delete from the dictionary <name> and all words added to the
dictionary after <name> regardless of the vocabulary.
Failure to find <name> is an error condition. An error
condition also exists upon implicitly forgetting a
vocabulary (due to its definition after <name>).
DEFINITIONS -- ONLY
Select the transient vocabulary as the current vocabulary
into which subsequent definitions will be added.
SEAL -- ONLY
Delete all occurances of ONLY from the search order. The
effect is that only specified application vocabularies will
be searched.
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C. EXPERIMENTAL PROPOSALS
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8 TYPICAL SOURCE CODE
0 ( ALSO ONLY 82jun12 WFR )
1 ( note the systems -FIND searches 1 to 5 vocabs in CONTEXT )
2 VOCABULARY ONLY ONLY DEFINITIONS
3 : ALSO ( slide transient into resident )
4 CONTEXT DUP 2+ 6 CMOVE> ;
5
6 HERE 2+ ] ( alter run time from usual vocabulary )
7 DOES> CONTEXT 8 ERASE DUP CONTEXT ! CONTEXT 8 + !
8 ALSO EXIT [
9 ' ONLY CFA ! ( Patch into ONLY; make NULL word )
10 CREATE X ' EXIT >BODY X ! 41088 ' X NFA ! IMMEDIATE
11 : FORTH FORTH ;
12 : DEFINITIONS DEFINITIONS ; : FORGET FORGET ;
13 : VOCABULARY VOCABULARY ; : ONLY ONLY ;
14 : WORDS WORDS ;
15
0 ( ORDER 82jun12 WFR )
1 : ORDER ( show the search order )
2 10 SPACES CONTEXT 10 OVER + SWAP
3 DO I @ ?DUP 0= ?LEAVE ID. 2 +LOOP
4 10 SPACES CURRENT @ ID. ;
5
6 ONLY FORTH ALSO DEFINITIONS
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8
9
10
11
12
13
14
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9 EXAMPLES OF USE
ONLY reduce search order to minimum
FORTH search FORTH then ONLY
ALSO EDITOR search EDITOR, FORTH then ONLY
DEFINITIONS new definitions will be added into the EDITOR
The same sequence would be compiled:
: SETUP ONLY FORTH ALSO EDITOR DEFINITIONS ;
10 REFERENCES
W. F. Ragsdale, The 'ONLY' Concept for Vocabularies, Proceedings
of the 1982 FORML Conference, pub. Forth Interest Group.
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C. EXPERIMENTAL PROPOSALS
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W. F. Ragsdale, fig-FORTH Installation Manual, Forth Interest
Group.
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C. EXPERIMENTAL PROPOSALS
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DEFINITION FIELD ADDRESS CONVERSION OPERATORS
by
Kim R. Harris
A. INTRODUCTION
The standard provides a transportable way to obtain the
compilation address of a definition in the dictionary of a FORTH
system (cf., FIND and ' ). It also provides an operator to
convert a compilation address to its corresponding parameter
field address. However, the standard does not provide a
transportable way to convert either of these addresses to the
other fields of a definition. Since various FORTH
implementations have different dictionary structures, a standard
set of conversion operators would increase transportability and
readability.
A set of words is proposed which allows the conversion of any
definitions field address to any other.
B. GLOSSARY
In the following words, the compilation address is either the
source or the destination, so it is not indicated in the names.
>BODY addr1 -- addr2 "to-body"
addr2 is the parameter field address corresponding to the
compilation address addr1.
>NAME addr1 -- addr2 "to-name"
addr2 is the name field address corresponding to the
compilation address addr1.
>LINK addr1 -- addr2 "to-link"
addr2 is the link field address corresponding to the
compilation address addr1.
BODY> addr1 -- addr2 "from-body"
addr2 is the compilation address corresponding to the
parameter field address addr1.
NAME> addr1 -- addr2 "from-name"
addr2 is the compilation address corresponding to the name
field address addr1.
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LINK> addr1 -- addr2 "from-link"
addr2 is the compilation address corresponding to the link
field address addr1.
The previous set of words is complete, but may be inefficient for
going between two fields when one is not the compilation address.
For greater efficiency, additional operators may be defined which
name both the source and destination fields.
N>LINK addr1 -- addr2 "name-to-link"
addr2 is the link field address corresponding to the name
field address addr1.
L>NAME addr1 -- addr2 "link-to-name"
addr2 is the name field address corresponding to the link
field address addr1.
C. DISCUSSION
The previous words provide a complete, consistent, and efficient
set of definition field address conversion operations. They can
be implemented in a FORTH system which uses any combination of
the following options for its dictionary structure:
Link fields first or second.
Fixed or variable length name fields.
Additional fields in the definitions structure.
Heads contiguous or separated from bodies.
Indirect, direct, subroutine, or token threaded code.
The words are compatible with this standard; their inclusion
would not require other changes to be made to the standard.
Disadvantages to including them in the standard include:
They add 6 to 8 more words to the standard.
A standard program may not use all of them since it is not
allowed to access the name or link fields. However, this
does not disqualify them from being in the standard.
If a definition's head is not in the dictionary, an error
condition would exist. In this case, what action should the
words take in an implemented system?
The author of this experimental proposal recommends that FORTH
system implementors try them and that they be included in the
System Word Set of the next FORTH standard.
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D. SOURCE CODE EXAMPLE
High level source code is shown below for a very simple
dictionary structure. This code assumes a FORTH system which
uses indirect threaded code, heads contiguous to bodies, and a
definition structure of the following format:
Name field, 4 bytes long, fixed length.
Link field, 2 bytes long.
Code field, 2 bytes long.
Parameter field, variable length.
: >BODY ( acf -- apf ) 2+ ;
: BODY> ( apf -- acf ) 2- ;
: >LINK ( acf -- alf ) 2- ;
: LINK> ( alf -- acf ) 2- ;
: >NAME ( acf -- anf ) 6 - ;
: NAME> ( anf -- alf ) 6 + ;
: N>LINK ( anf -- alf ) 4 + ;
: L>NAME ( alf -- anf ) 4 - ;
E. EXAMPLES OF USE
No examples are given because their use should be obvious.
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