RFD: Quotations V2 update

From	"Alex McDonald" <blog@rivadpm.com>
Newsgroups	comp.lang.forth
Subject	RFD: Quotations V2 update
Date	2015-08-04 11:05 +0100
Organization	A noiseless patient Spider
Message-ID	<mpq2n2$85u$1@dont-email.me> (permalink)

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For discussion. Tests for Annex F have not been developed yet.

------>

RfD: Quotations
---------------

Change history
--------------
28Jul2015 Alex McDonald v1 initial draft
04Aug2015 Alex McDonald v2 update
    added & extended "Rationale", "Discussion"
    formalised "Specification"
    corrected "Implementation"
    added "References"

Quotations
==========

The essence of quotations is to provide nested colon definitions,
in which the inner definition(s) are nameless. The expression

: foo ... [: some words ;] ... ;

is equivalent to

:noname some words ; Constant #temp#
: foo ... #temp# ... ;

A simple quotation is an anonymous colon definition that is defined
inside a colon definition or quotation.

Rationale
---------

There is no situation where quotations cannot be implemented by
existing Forth words. Specifcially, :NONAME and : provide all the
features proposed for quotations. Their advantage is as a syntactic
"sugar" that permits a nameless definition in close proximity to its
use; and that it avoids generating one-use names only for the purpose
of refering to the definition inside another word.

One example use of quotations is to provide a solution to the use
of CATCH in a form close to other languages' TRY ... EXCEPT
blocks.

: foo ( i*x -- j*x )
    setup
    [: fee fi fo fum ;] catch
    if ... then
    teardown
    ( throw again ) ;

Under the term "Phrases", quotations have been in MacForth since at
least 1987. They are documented in the infamous "missing chapter" of
the MacForth manual.

Implementations exist in (at least) VFX, Gforth and iForth, and in
the iForth case appear to have some significant use and history.

Discussion
----------

If quotations are to have the same privileges as words defined with :
and :NONAME they should be able to use local variables and use
RECURSE and DOES>.

RECURSE: Recursion is defined as recursing to the start of the
quotation, not the enclosing definition.

DOES>: As with a normal definition that closes the DOES> action with ;
(semi-colon), a DOES> in a quotation is closed at the terminating ;]

While implementing support for DOES> and RECURSE, it has been
proposed to remove the following inconsistency in the definition of 
6.1.2120 RECURSE; "An ambiguous condition exists if RECURSE appears in
a definition after DOES>." The proposal is to remoive that sentence
and permit a RECURSE to the start of the DOES> definition to be
consistent with :NONAME : and the proposed [: .

The above features are illustrated here.

: foo create [: , does> @ dup if . recurse then ;] swap 0= ;
^A           ^B   ^C    ^x                      ^D         ^E

There are 3 definitions: A-E, B-D and C-D.
 The definition of FOO is the code between A and E.
 The quotation returned by executing FOO is the code between B and D
 The DOES> clause is the code between C and D

RECURSE calls the code immediately after C (marked x) as it is the
latest definition, and not the DOES> as it is consumed during
compilation.

Locals: A quotation may not be able to access the locals of the outer
word because it has no knowledge of when it might be executed and
hence whether outer locals are still alive. It does permit defining and
accessing its own locals.

: foo ... [: {: a b :} a b + ;] ... ; \ unambiguous
: bar {: a b :} ... [: a b + ;] ... ; \ ambiguous

There has been some discussion about providing higher-order or
embedded functions that are able to refer to their parent's locals
during their execution. This is a middle ground between the proposal
here and full closures that (almost always) require garbage
collection, and it would permit execution of the quotation in the
execution scope of the parent; in the BAR example above, if the
quotation was executed prior to the return of BAR to its caller.

There are two schools of thought.

1. Define [: ;] such that a standard program may not access
   local variables of the outer definition inside the quotation.

2. Permit such a feature, but restrict valid execution of the
   quotation to the lifetime of the parent's execution.

Solution 2 requires some form of locals frame, and may be excessively
complex to implement; it appears to have few advantages over
the simpler quotations as proposed here.

Specification
-------------

The following notations have been seen in the wild, oldest first.
    :| ... |;
    :[ ... ]:
    [: ... ;]
    
The notation [: ;] appears to have popular support.

[: ( c: -- quotation-sys )
  Interpretation: Interpretation semantics for this word are undefined.
  Compilation: ( c: -- quotation-sys ) suspends compiling to the
  current definition, starts a new nested definition, and compilation
  continues with this nested definition. Locals may be defined in the
  nested definition. It is an ambiguous condition [note 1] if outer
  locals are visible and referred to in the nested definition.

;] ( c: quotation-sys -- ) ( s: -- xt )
  Interpretation: Interpretation semantics for this word are undefined.
  Compilation: Ends the current nested definition, and resumes
  compilation to the previous current definition. It appends the
  following run-time to the current definition:
  run-time: ( -- xt )
  xt is the execution token to execute the nested definition.

6.1.2120 RECURSE CORE
  Interpretation: Interpretation semantics for this word are undefined.
  Compilation: ( –– )
  Append the execution semantics of the current definition to the
  current definition.
  See: 6.1.1250 DOES>, 6.1.2120 RECURSE, A.6.1.2120 RECURSE.

Implementation
--------------

It is not possible to define quotations in ISO Forth. The following
is an outline definition, where save-definition-state and restore-
definition-state require carnal knowledge of the system and are left
to the implementor.

: [: ( c: -- quotation-sys )
  postpone ahead save-definition-state :noname ; immediate
: ;] ( c: quotation-sys -- ) ( s: -- xt )
  postpone ; >r restore-definition-state
  postpone then r> postpone literal ; immediate

Compliance Tests
----------------

tbd

References
----------

Stephen Pelc wrote this; see

http://newsgroups.derkeiler.com/Archive/Comp/comp.lang.forth/2012-07/msg00791.html

I (Alex McDonald) have taken it and modified it as a proposed RfD,
and have removed the discussion of closures. http://www.forth200x.org/

The following notes on quotations and closures incorporate material
from several people, including Anton Ertl, Andrew Haley, Marcel
Hendrix, Ward Mcfarland, Bernd Paysan, and Elizabeth Rather. If you
have been left out and want acknowledgement, please let me know.

Additional contributors; Hugh Aguilar and Gerry Jackson.

[Note 1]
The phrase "ambiguous condition" has a specific meaning in Forth
standards. An ambiguous condition is "a circumstance for which this
standard does not prescribe a specific behavior". The RfD process
also states: "If you want to leave something open to the system
implementor, make that explicit, e.g., by making it an ambiguous
condition."

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Thread

RFD: Quotations V2 update "Alex McDonald" <blog@rivadpm.com> - 2015-08-04 11:05 +0100
  Re: RFD: Quotations V2 update Mark Wills <markwills1970@gmail.com> - 2015-08-04 04:09 -0700
    Re: RFD: Quotations V2 update Lars Brinkhoff <lars.spam@nocrew.org> - 2015-08-04 13:35 +0200
      Re: RFD: Quotations V2 update "Alex McDonald" <blog@rivadpm.com> - 2015-08-04 13:54 +0100
        Re: RFD: Quotations V2 update "Alex McDonald" <blog@rivadpm.com> - 2015-08-04 14:01 +0100
    Re: RFD: Quotations V2 update "Alex McDonald" <blog@rivadpm.com> - 2015-08-04 13:38 +0100
      Re: RFD: Quotations V2 update Mark Wills <markwills1970@gmail.com> - 2015-08-04 07:37 -0700
  Re: RFD: Quotations V2 update anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-08-04 14:23 +0000
    Re: RFD: Quotations V2 update "Alex McDonald" <blog@rivadpm.com> - 2015-08-04 16:09 +0100
      Re: RFD: Quotations V2 update anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-08-04 15:38 +0000
        Re: RFD: Quotations V2 update Bernd Paysan <bernd.paysan@gmx.de> - 2015-08-04 22:30 +0200
          Re: RFD: Quotations V2 update "Alex McDonald" <blog@rivadpm.com> - 2015-08-04 23:09 +0100
            Re: RFD: Quotations V2 update Bernd Paysan <bernd.paysan@gmx.de> - 2015-08-05 01:45 +0200
              Re: RFD: Quotations V2 update anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-08-05 07:23 +0000
                Re: RFD: Quotations V2 update "Alex McDonald" <blog@rivadpm.com> - 2015-08-05 09:55 +0100
          Re: RFD: Quotations V2 update Gerry Jackson <gerry@jackson9000.fsnet.co.uk> - 2015-08-05 12:03 +0100
            Re: RFD: Quotations V2 update "Alex McDonald" <blog@rivadpm.com> - 2015-08-05 13:50 +0100
              Re: RFD: Quotations V2 update anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-08-05 16:07 +0000
              Re: RFD: Quotations V2 update Gerry Jackson <gerry@jackson9000.fsnet.co.uk> - 2015-08-05 20:23 +0100
  Re: RFD: Quotations V2 update Bernd Paysan <bernd.paysan@gmx.de> - 2015-08-04 22:00 +0200
    Re: RFD: Quotations V2 update "Alex McDonald" <blog@rivadpm.com> - 2015-08-04 21:07 +0100
  Re: RFD: Quotations V2 update Gerry Jackson <gerry@jackson9000.fsnet.co.uk> - 2015-08-04 21:53 +0100
    Re: RFD: Quotations V2 update "Alex McDonald" <blog@rivadpm.com> - 2015-08-04 23:22 +0100
      Re: RFD: Quotations V2 update Gerry Jackson <gerry@jackson9000.fsnet.co.uk> - 2015-08-05 12:38 +0100
        Re: RFD: Quotations V2 update "Alex McDonald" <blog@rivadpm.com> - 2015-08-05 13:41 +0100
          Re: RFD: Quotations V2 update anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-08-05 12:46 +0000
            Re: RFD: Quotations V2 update Bernd Paysan <bernd.paysan@gmx.de> - 2015-08-06 02:12 +0200
              Re: RFD: Quotations V2 update "Alex McDonald" <blog@rivadpm.com> - 2015-08-06 14:16 +0100
              Re: RFD: Quotations V2 update anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-08-06 13:46 +0000
    Re: RFD: Quotations V2 update anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-08-05 08:15 +0000
      Re: RFD: Quotations V2 update Gerry Jackson <gerry@jackson9000.fsnet.co.uk> - 2015-08-05 20:31 +0100
        Re: RFD: Quotations V2 update peter.m.falth@gmail.com - 2015-08-05 14:41 -0700
          Re: RFD: Quotations V2 update "Elizabeth D. Rather" <erather@forth.com> - 2015-08-05 18:39 -1000
            Re: RFD: Quotations V2 update peter.m.falth@gmail.com - 2015-08-05 23:22 -0700
        Re: RFD: Quotations V2 update anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-08-06 13:19 +0000
          Re: RFD: Quotations V2 update Gerry Jackson <gerry@jackson9000.fsnet.co.uk> - 2015-08-07 22:28 +0100
            Re: RFD: Quotations V2 update Gerry Jackson <gerry@jackson9000.fsnet.co.uk> - 2015-08-07 22:34 +0100
              Re: RFD: Quotations V2 update anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-08-08 15:53 +0000
                Re: RFD: Quotations V2 update Gerry Jackson <gerry@jackson9000.fsnet.co.uk> - 2015-08-11 21:16 +0100
            Re: RFD: Quotations V2 update anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-08-08 15:39 +0000
              Re: RFD: Quotations V2 update Gerry Jackson <gerry@jackson9000.fsnet.co.uk> - 2015-08-10 12:55 +0100
                Re: RFD: Quotations V2 update anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-08-10 12:08 +0000
  Re: RFD: Quotations V2 update hughaguilar96@gmail.com - 2015-08-04 18:09 -0700
    Re: RFD: Quotations V2 update "Alex McDonald" <blog@rivadpm.com> - 2015-08-05 09:22 +0100
    Re: RFD: Quotations V2 update "Alex McDonald" <blog@rivadpm.com> - 2015-08-05 09:53 +0100

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