quotations-v1.txt

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From: "Alex McDonald" <blog@rivadpm.com>
Newsgroups: comp.lang.forth
Subject: RfD: Quotations
Date: Tue, 28 Jul 2015 21:22:14 +0100
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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.

Quotations
==========

Discussion
----------
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.

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>.

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

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 ;]

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.

Specification
-------------
The following notations have been seen in the wild, oldest first.
    :| ... |;
    :[ ... ]:
    [: ... ;]

[: ( -- c: nested-sys )
Compilation: suspends compiling to the current definition, starts a
new nested definition, and compilation continues with this nested
definition. It is an ambiguous condition if outer locals are visible
and referred to in the nested definition. Locals may be defined in
the nested definition. Inside the nested definition RECURSE applies
to the nested definition.

;] ( c: nested-sys -- )
Compilation: Ends the current nested definition, and resumes
compilation to the previous current definition. This includes ending
the action of DOES> if any. It appends the following run-time to the
current definition:

run-time: ( -- xt )
xt is the execution token to execute the nested definition.

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

It is not possible to define quotations in ISO Forth. The following
is an outline definition.

: [: ( -- c: nested-sys )
    postpone ahead :noname ; immediate

: ;] ( c: nested-sys -- ) ( -- xt )
    postpone ; postpone then ; immediate