Message-ID: <394DD1B8.9635C9AB@forth.org>
Date: Mon, 19 Jun 2000 10:54:32 +0300
From: Michael Gassanenko <mlg@forth.org>
To: ansforth-real@minerva.com
Subject: BEHAVIOR -- rephrase 1

I try to rephrase M.Milendorf's proposal about BEHAVIOR so that it would
be in ANS style and ready-to-copy-and-paste.

*Any* comments would be appreciated.
If you do not want to create noise in ansforth-real@minerva.com ,
use personal mail ( mlg@forth.org ).

Best regards, Michael Gassanenko

-- changes ---

  Removed the brief summary line.

  Added the description, including the ambiguous condition.

  Moved the typical use description to rationale.

  Explained in the rationale what happens if a deferred word is
  assigned to a deferred word ( BEHAVIOR does not look for the
  end of chain).

--- questions ---

Do we need a special exception number or
> -12   argument type mismatch
is ok?


-------------- rephrase 1 ------------

Title:        Add BEHAVIOR to the CORE EXT wordset

Justification:
  This word provides a read access to deferred words.

Problem:
  Without BEHAVIOR, we have only write and execute access to deferred words.

Proposal:

?.?.???? BEHAVIOR                       CORE EXT
      ( xt1 -- xt2 )

Retrieve _xt2_, the execution token of the word currently associated
with the deferred word whose execution token is _xt1_.

An ambiguous condition exists if the word identified by _xt1_
is not a deferred word.

Rationale.

A.??.?.???? BEHAVIOR

This command is used to obtain the execution contents of a deferred
word. A typical use would be to retrieve and save the execution
behavior of the deferred word, set the deferred word to a new behavior,
and then later restore the old behavior.

If the deferred word identified by _xt1_ is associated with some
other deferred word, _xt2_ is the execution token of that other
deferred word. To retrieve the execution token of the word currently
associated with that other deferred word, use the phrase BEHAVIOR BEHAVIOR .

Experience:
  Many years of use in OpenBoot and OpenFirmware systems.

Comments:
  See PROPOSALS for TO, DEFER.

  Michael Gassanenko: I began to use it in practice only after
  M.Milendorf's proposal in 1999, and found this word very useful.
  I especially appreciate that one can, as a last resort,
  use the xt's that may be stored into a deferred word
  as an enum type (unaesthetic, but cheap, safe and practical).

=== the original proposal =================================

Title:        Add BEHAVIOR word to the CORE EXT wordset

Justification:BEHAVIOR word along with DEFER and TO constitute a wordset to
              support run-time modifying words in the Forth Programming Language.

Problem:      If DEFER is included in the standard, BEHAVIOR shall be included
              as well.

Proposal:     BEHAVIOR      ( defer-xt -- contents-xt )

              Retrieve execution behavior of a defer word.

              This command is used to obtain the execution contents of a DEFER
              word. A typical use would be to retrieve and save the execution
              behavior of the defer word, set the defer word to a new behavior,
              and then later restore the old behavior.

              Used as: ['] defer-name behavior ( contents-xt -- )

Examples:     ok defer foo
              ok ' 1 to foo
              ok ' foo behavior >r
              ok ' 2 to foo
              ok foo .
              2
              ok r> to foo
              ok foo .
              1
              ok

Remarks:      The current wording is an exact copy of IEEE 1275-1994 definition.

Experience:   Many years of use in OpenBoot and OpenFirmware systems.

Comments:     See PROPOSALS for TO, DEFER.