6 Glossary

MARKER

( "<spaces>name" -- )

Skip leading space delimiters. Parse name delimited by a space. Create a definition for name with the execution semantics defined below.

name Execution:

( -- )

Restore all dictionary allocation and search order pointers to the state they had just prior to the definition of name. Remove the definition of name and all subsequent definitions. Restoration of any structures still existing that could refer to deleted definitions or deallocated data space is not necessarily provided. No other contextual information such as numeric base is affected.

See:

Rationale:

As dictionary implementations have become more elaborate and in some cases have used multiple address spaces, FORGET has become prohibitively difficult or impossible to implement on many Forth systems. MARKER greatly eases the problem by making it possible for the system to remember "landmark information" in advance that specifically marks the spots where the dictionary may at some future time have to be rearranged.

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JennyBrienavatar of JennyBrien Extending MARKERRequest for clarification2020-09-16 15:33:45

Many years ago, when I was modifying F-83 to be ANS-compilant, I had to temporarily patch several parts of the core. I used:

   CHANGED \  n addr -- ;  add addr and old value to a linked list and then store n at addr

I then modified FORGET to run down the list and restore any values changed after that point.

Is there any common practice on how to add similar further "landmark information" that MARKER should restore?

MarcelHendrixavatar of MarcelHendrix

As iForth supports FORGET and making executables, it has lots of support for saving 'structural' details of which MARKER is just one example. Of the top of my head: Each word has a FORGET and REVISION fields, and each word has the option of defining a FORGET> section with code that is executed when it is forgotten (like the DOES> construct). Words can be hooked into several lists ( chains ) that are walked through when certain events occur (like COLD). Included files can be marked as a revision that is automatically unhooked when the file is reloaded. When Forth is used as an application shell and is allowed to compile, one frequently wants to clean up the dictionary when the user wants to work on another file.

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