locals-v5.txt

RfD - Enhanced local variable syntax, v5b
====================================
Stephen Pelc - 22 October 2009

20091022 Discussion about { and {:.
20090830 Tidied.
20090325 Updated with some renaming.
20080811 Removed references to local buffers as appropriate.
20070914 Moved local buffers to separate proposal.
20070607 Wordsmithing. Corrected reference implementation.
20060822 Added explanatory text.
         Corrected reference implementation.
         Updated ambiguous conditions.

Problem
=======
1) The current LOCALS| ... | notation explicitly forces all locals
to be initialised from the data stack.
2) 1) The current LOCALS| ... | notation defines locals in reverse
order to the normal stack notation.

This proposal is derived from implementations that have existed for
more than 15 years.


Solution
========
Base version
------------
The following syntax for local arguments and local values is
proposed. The sequence:
  {: arg1 arg2 ... | lv1 lv2 ... -- o1 o2 ... :}
declares local arguments, local values, and dummy outputs.

The local arguments are automatically initialised from the data
stack on entry, the rightmost being taken from the top of the data
stack. Local arguments and local values can be referenced by name
within the word during compilation. The output names are dummies
to allow a local declaration to be read as a stack comment.
  The items between {: and | are local arguments.
  The items between | and -- are local values.
  The items between -- and :} are outputs for formal comments only.

The -- to :} section is optional, i.e. | or {: direct to :} is
permitted.

The outputs are provided in the notation so that complete stack
comments can be produced. However, all text between -- and } is
ignored. This facility is there to permit the notation to form a
complete stack comment, which eases documentation.

Local arguments and values return their values when referenced,
and must be preceded by TO to perform a store.

In the example below, a and b are local arguments, and c and
d are local values.

: foo  {: a b | c d -- :}
  a b + to c
  a b * to d
  cr c .  d .
;

Local types and extensions
--------------------------
Although out of the scope of this proposal, it should be noted
that some current Forth systems use indicators to define local
values of sizes other than a cell. To avoid issues when porting
code to such systems, names ending in a ':' (colon) should be
avoided.

: foo  {: a b | F: f1 F: f2 -- c :}
  ...
;

At least one Forth implementation uses local value names ending in
the '[' character to indicate local buffers. This character should
be avoided to prevent disenfranchising implementations that implement
the behaviour. For similar reasons the use of '[' and '\' as local
argument or value names should also be avoided.


Discussion
==========
The phrase { ... } rather than {: ... :} has been used by systems
for over 15 years. However, it conflicts with existing practise in
other Forth systems. Choosing the new name allows both practises
to coexist.

The '|' (ASCII 0x7C) character is widely used as the separator
between local arguments and local values. Other characters
accepted in current Forth implementations are '\' (ASCII 0x5C) and
'¦' (0xA6). Since the ANS standard is defined in terms of 7-bit
ASCII, and with regard to internationalistion, we propose only to
consider the '|' and '\' characters further. Only recognition of
the '|' separator is mandatory.

Some current systems permit TO to be used with floats (children of
FVALUE) and other data types. Such systems often provide additional
operators such as +TO (add from stack to item) for children of VALUE
and FVALUE.


Forth 200x text
===============
13.6.2.xxxx {:
brace-colon LOCAL EXT

Interpretation: Interpretation semantics for this word are undefined.

Compilation:
( ... "<spaces>:}" -- )

Create local arguments by repeatedly skipping leading spaces,
parsing name (arg1 to argn), and executing implementation defined
actions. The list of local arguments to be defined is terminated
by "|", "--" or ":}". Append the run-time semantics for local
arguments given below to the current definition.

If a space delimited '|' (ASCII code 0x7C) is encountered, create
local values by repeatedly skipping leading spaces, parsing name
(lv1 to lvm), and creating the local element. The list of local
values to be defined is terminated by "--" or ":}". Append the
run-time semantics for local values given below to the current
definition. If "--" has been encountered, further text between "--"
and ":}" is ignored.

Systems should permit at least eight local arguments and eight
local values to be declared. A program that requires more has an
environmental dependency.

Local argument declaration run-time: ( x1 ... xn -- )
Local value declaration run-time: ( -- )

Initialise local arguments from the data stack. Local argument
arg1 is initialized with x1, arg2 with x2 up to argn from xn,
which is on the top of the data stack. When invoked, each local
argument returns its contents. The contents of a local argument may be
changed using 13.6.1.2295 TO.

Set up local values. The initial contents of local values are
undefined. When invoked, each local value returns its contents. The
contents of a local value may be changed using 13.6.1.2295 TO. The
size of a local value is a cell.

Local argument run-time: ( -- x )
Local value run-time: ( -- x )
TO local argument run-time: ( x -- )
TO local value run-time: ( x -- )

Ambiguous conditions:
a) The {: ... :} text extends over more than one line.
b) {: ... :} is used more than once in a word.


Reference implementation
=========================

0 [if]
BUILDLV    c-addr u +n mode
When executed during compilation, BUILDLV passes a message to the
system identifying a new local argument whose definition name is
given by the string of characters identified by c-addr u. The size
of the data item is given by +n address units, and the mode
identifies the construction required as follows:
  0 - finish construction of initialisation and data storage
      allocation code. C-addr and u are ignored. +n is 0
      (other values are reserved for future use).
  1 - identify a local argument, +n = cell
  2 - identify a local value, +n = cell
  3+ - reserved for future use
  -ve - implementation specific values

The result of executing BUILDLV during compilation of a definition
is to create a set of named local arguments and values, each of
which is a definition name, that only have execution semantics
within the scope of that definition's source.

Note that it is often useful to accumulate and store the size of
locals storage (0=none), so that compilers for EXIT can easily
detemine if locals clean up code is required.
[then]

VARIABLE #LVS   \ -- addr
\ Holds size of locals storage required.

: BUILDLV	\ c-addr u +n mode --
\ Dummy for testing
  OVER #LVS +!
  CR 2SWAP TYPE  SPACE  SWAP . .
;

: TOKEN         \ -- caddr u
\ Get the next space delimited token from the input stream.
\ Can be extended to permit multiple line declarations.
  PARSE-NAME
;

: LTERM?        \ caddr u -- flag
\ Return true if the string caddr/u is "--" or ":}"
  2DUP S" --" COMPARE 0= >R
  S" :}" COMPARE 0=  R> OR
;

: LSEP?         \ caddr u -- flag
\ Return true if the string caddr/u is the separator between
\ local arguments and local values or buffers.
  2DUP S" |" COMPARE 0= >R
  S" \" COMPARE 0=  R> OR
;

: {:            \ --
\ Parse the locals declaration up to the closing ":}".
  0 #LVS !                              \ indicate no locals yet
  0 >R                                  \ indicate arguments
  BEGIN
    TOKEN 2DUP LTERM? 0=
  WHILE                                 \ -- caddr len
    2DUP LSEP? IF                       \ if '|'
      R> DROP  1 >R                     \ change to vars and buffers
    ELSE
      R@ 0= IF                          \ argument?
        CELL 1
      ELSE                              \ value
        CELL 2
      THEN
      BUILDLV
    THEN
  REPEAT
  BEGIN
    S" :}" COMPARE
   WHILE
    TOKEN
  REPEAT
  0 0 0 0 BUILDLV
  R> DROP
; IMMEDIATE

Tests
=====

: TEST1  {: a b | c -- f :}
  CR ." Hello1 " CR  a  ;
\ 3 4 TEST1 -> 3

: TEST2  {: a | b c e :}
  CR ." Hello2 " CR  ;
\ 3 TEST2 ->

: TEST3  {: a b c -- :}
  CR ." Hello3 " CR  ;
\ 3 4 5 TEST3 ->

: TEST4  {: a b c :}
  CR ." Hello4 " CR  ;
\ 3 4 5 TEST4 ->

: TEST5  {: a | b c d -- e f g :}
  CR ." Hello5 " CR
  a 2* to b  b 2* to c  c 2* to d
  b c d  ;
\ 3 TEST2 -> 6 12 24