WordSets in External Modules
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## WordSets in External Modules

Part of the LGPL:ed PFE

### External Modules

The PFE has a generic extension system that a programmer can use to write his own forth extension in C. The lower C-level part is described on the page "how to write a PFE module". This external module contains a wordset loadlist table, and in fact the same tables are used internally in PFE. Traditionally, in PFE one did load the external modules via LOADM like

     LOADM gforth

however this will only work if the resp. module gforth.so had been living in a real external module. On some platforms however there is no good binary module provided by the operating system, so that the external modules are pre-linked to the PFE and all that is needed is to make the wordset entries available to the PFE runtime. This is not just an activation, the wordset table can have contructor instructions that can heavily modify the current PFE process instance.

The pre-linking does not bind anything of the external module to the basic PFE runtime system, so that (using the resp. LGPL exception rule about relinkable process binaries) these modules do not fall under LGPL themselves. The only reference is made in dl-internal to reference the data-like wordset-table of the extra module, and the table is searched later on, it is not activated in the default boot phase of PFE.

Apart from using this scheme for system that do not support a good dynaload system for binary modules, it can also be used for optimization of the system - for instance the zchar-ext wordset is usually pre-linked to the PFE core library. It implements the zero-terminated strings that are widely used in C, and it brings about the backslashstring definitions that are needed to use C-like strings with embedded newlines and other control-characters.

### WordSet Activation

To make it easier to provide for the different activation routines for non-core wordsets of PFE, the PFE of 31.x and higher is now using the EXTENSION-query mechanism to activate wordsets - this follows the guidelines of the ANS Forth standard to activate additinal wordsets. Forth applications shall check the environment for a specific wordset with a query. The wordset might not be present before this call, so that references to the words in that wordset will fail (with an `undefined` exceptioin), but after a query to the environment did return true, it must stay that way and a wordset shall not be unloaded/deactivated thereafter.

The PFE adopts this mechanism with some small restrictions. First of all, the ENVIRONMENT may contain a lot more definitions than just wordset assertions - if a query is not yet defined in the ENVIRONMENT, it is hard to decide whether it is part of another wordset or a reference to a wordset assertion itself that should trigger an implicit LOADM call. Therefore, if an application writer wants the PFE to check for a wordset then it should always be done for the -EXT variant. If the resp. wordset is already loaded, it will return the value of its assert constant. Otherwise, the PFE will replace the "-EXT" part with the module file extension being used on the current operation system, and if there is such an external module, it will loaded. When loaded succesfully, the environment-query will check again whether the newest module has been pushing an assert constant into ENVIRONMENT. If yes, then this value is returned, if no then an assert constant is generated that contains null as the value but makes the next environment query to succeed.

This scheme is used for internal and external modules, it even applies to the floating wordset. Since 31.x, the floating wordset is normally compiled as a wordset module, and the basic pfe runtime system does not contain the floating definitions after boot. They have to be activated. Try the following lines to see what it does:

     : EXIST? IF ." EXIST=" .  ELSE ." NOT EXIST!" THEN CR ;

     s" floating" ENVIRONMENT? EXIST? \ no
     s" floating-ext" ENVIRONMENT? EXIST? \ yes, loaded on the fly
     s" floating" ENVIRONMENT? EXIST? \ yes, matches floating-ext

Note that there is not even a floating-stack before the floating-ext wordset has been loaded, - it gets some memory from the forth dictionary via a constrctor, and it places a deconstructor, and it places a hook into the ABORT cleanup routine to have the floatingstack cleared when an error occurs.

### Available Modules

The pfe searches its lib-directory under "$prefix/lib/pfe/" for available modules - as an example, the activation of the FLOATING-EXT wordset is looked for in the binary "$prefix/lib/pfe/floating.so". The main LGPL:ed pfe core does ship with a number of external modules, and with each new generation more modules become available. These modules use the C compiler to generate optimized native code for the primitives they provide, and they can be easily used to interface with any other C-level code.

Among the wordset modules that are currently available, the application programmer can use these for example:

ZCHAR-EXT
  usually pre-linked, implements zero-terminated strings and backslashstrings.
DSTRINGS-EXT
  a work kindly provided by David N. Williams, implements dynamic strings over a string stack, and allows to use strings as freely as many other integral types.
FLOATING-EXT
  usually an external module now, a case of a standard ANS Forth wordset pushed to be external.
EDIT-EXT
  implements a block editor for pfe, just three forth words are exported but it contains the full functionality in the C-compiled binary module.
HELP-EXT
  the user doesn't need to load this module explicitly, the pfe core contains a word "HELP" that will implicitly load the HELP-EXT if it has not been loaded yet, it has actually a deferred ENVIRONMENT?-query in its definition. When the HELP-EXT module has been activated successfully, the word following "HELP" will be handed over, and the HELP-EXT module will serach the "$prefix/include/pfe/*.h" files for a description of the word. No need to leave pfe to view a short description of the word.
TOOLBELT-EXT
  the PFE main core does already contain most of the toolbelt words but some are missing, and the word "NOT" has a different defintion. To help to run toolbelt-based forth applications, the TOOLBELT-EXT provides optimized routines, and it can be activated just like any other wordset extension.
GFORTH-EXT
  similar for some gforth extensions, here we add some extension that are sought to be compatible with their resp. gforth definitions to help application writers to run their gforth-based programs on top of the PFE. Words are added on request, just contact the PFE maintainer, it does not implement a complete series of gforth primitives.
STRUCT-EXT
  implements one of the many variants of STRUCT extensions for forth - this one follows the MPE model at great lenghts, it provides FSL structs and a few compatibility words with gforth structs. There is a STRUCTS extension that builds on top of the STRUCT extension to help even more to port applications to the PFE.
SMART-GO-EXT
  A syntax extension to provide the PFE with a variant of extended literals. You can hook the outer interpreter via DEFERs and have it automatically call other words for a given prefix. For the curious.
MODULE-EXT
  provides a module system with hidden words and export words. Again, just one model in a large set of other variants in the forth world, and this one is rather simple. It could be implemented in highlevel forth as well. For the curious.

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