An introduction to the Poplog language subsystems.
Select headings to return to index
There are four languages provided as standard components of Poplog:
Pop-11 (HELP * POP11) Prolog (HELP * PROLOG) Common Lisp (HELP * CLISP) Standard ML (HELP * PML)
Pop-11 is the core language of Poplog and is always available; the other languages are defined by the libraries
LIB * PROLOG LIB * CLISP LIB * PML
Each language is implemented as a separate subsystem. The main purpose of a subsystem is to define a compiler for the language it implements: the compiler may be used both to load program files and as a top-level loop, evaluating interactive input. A subsystem will also typically make available a set of documentation relevant to the language, and may redefine certain system procedures (such as Ved commands) to tailor them to the language.
Individual subsystems are identified by name: the names of the four standard subsystems are
Language Subsystem Name -------- -------------- Pop-11 pop11 Prolog prolog Common Lisp lisp Standard ML ml
Strictly speaking, there are two subsystems defined for Prolog: "prolog" and "top". This reflects the fact that Prolog supports two distinct modes of compilation: a "(re)consult" mode, in which terms are read as clauses to be added to the database, and a "top-level" mode, in which input terms are read as goals to be executed. For most circumstances however, the "prolog" subsystem is the one to use.
For full details of the subsystem mechanism, see REF * SUBSYSTEM. This will be of interest mainly to experienced users who want to tailor the existing subsystems or build new ones.
Multiple subsystems can coexist in the same Poplog process: you can, for example, load all three language libraries without conflict. To find out whether a particular subsystem has been compiled, use the procedure
is_subsystem_loaded(ss_name) -> bool
One subsystem has always to be selected as the "primary" (or "default") subsystem: its name is contained in the variable subsystem. Try loading the following line to find out the primary subsystem for your process:
subsystem =>
The primary subsystem determines the interactive compiler used outside Ved, and the default documentation directories to search in response to HELP, TEACH and REF commands.
The standard Shell and DCL commands for running the Poplog languages:
prolog clisp pml
have the necessary subsystem libraries pre-compiled, and the corresponding language subsystem chosen as the primary subsystem. You can make your own saved images containing any mixture of languages and with your choice of primary subsystem with the library * MKIMAGE.
The main purpose of a subsystem is to define a compiler for the language it implements. A compiler is simply a procedure which takes an input source (typically a file name or a device) as argument. The standard subsystem compiler procedures are:
Subsystem Compiler File Extension --------- -------- -------------- pop11 pop11_compile .p prolog prolog_compile .pl (or .pro) lisp lisp_compile .lsp (or .lisp, .l, .cl) ml ml_compile .ml (or .sig)
You can call these compilers directly from Pop-11, but should rarely need to do so: each language has its own particular method of compiling files.
The above table also indicates an implicit association of subsystems and their compilers with program files, based on the extension (or "type") of the file name. The procedure loadcompiler is a "generic" compiler which uses this association to determine automatically the specialised compiler to use on any particular file. loadcompiler is invoked by the Pop-11 syntax construct load, so that
load afile.p
compiles "afile.p" using pop11_compile, but
load afile.pl
calls the Prolog compiler instead. This is true also of the Ved command
<ENTER> load file
A similar strategy is employed in regard to library files. Each subsystem has a different set of library files, and while again each language will provide special syntax for loading or examining these files, the generic procedure loadlib uses the file extension on its argument (if there is one) to determine the appropriate library directories to search as well as the compiler to use. loadlib is called both by the Pop-11 syntax construct lib and by the Ved lib command; the Ved showlib command also uses the file extension to help locate library files.
See REF * loadcompiler, * loadlib for more information.
You can change from one subsystem compiler to another with the procedure
ss_name -> sys_compiler_subsystem(`c`);
(where the argument `c` means change the `current' subsystem.)
"Switching subsystem" means abandoning the currently executing subsystem compiler and replacing it with an invocation of the selected compiler. In an interactive session, you'll see the prompt change, and find yourself talking a different language; used within a file, it means only that the rest of the file will be compiled with the new compiler, leaving the outer subsystem unchanged.
The macros
prolog clisp pml
defined by the language libraries are useful abbreviations for
"top" -> sys_compiler_subsystem(`c`); "lisp" -> sys_compiler_subsystem(`c`); "ml" -> sys_compiler_subsystem(`c`);
respectively.
sys_compiler_subsystem is a Pop-11 procedure, so can be called directly only from the "pop11" subsystem. However, in every language the word
pop11
typed to the prompt switches into the "pop11" subsystem from which any other subsystem can be reached. Also, some languages provide a more direct interface: in Prolog, for example, the directive
:- prolog_language(ss_name).
behaves identically to
ss_name -> sys_compiler_subsystem(`c`);
A subsystem also provides documentation particular to the language concerned. This documentation will normally be kept in a directory (or directory tree) separate from the main Poplog documentation. When you use a command such as
help topic
the appropriate help file is sought first amongst the documentation relevant to the primary subsystem, before being sought in the general pool (determined by * vedhelplist). This is also true of the other documentation commands, such as TEACH, REF, DOC, ??, etc.
From inside Ved, you can always get hold of documentation relevant to a particular subsystem by prefixing the appropriate command with the subsystem name. For example, you can view documentation on the Prolog error handler by doing
<ENTER> prolog help error
or on the Common Lisp flet special-form with
<ENTER> lisp ref flet
These will work even when the Prolog and Lisp compilers aren't loaded.
Using the subsystem name as a command prefix is a special case of a more general facility described below.
Ved uses the same association of file extensions and subsystems discussed above to select a subsystem appropriate to each edit buffer. The association is in fact based on the * vedfiletypes mechanism, so you can tailor it to your own taste.
The command
<ENTER> subsystem
reports the subsystem associated with the current buffer. You can assign a different subsystem by adding an argument to the command: e.g.
<ENTER> subsystem prolog
makes "prolog" the subsystem for the current buffer.
Certain Ved commands will behave differently depending on the subsystem selected, particularly commands which invoke a compiler: the command
<ENTER> lmr
(see HELP * LMR) will load the marked range using the compiler for the selected subsystem; the immediate-mode command
<ENTER> im
(see HELP * IM) uses the buffer subsystem to determine which compiler to run in the interactive window. You can run different immediate-mode compilers simply by using files with different extensions: the commands
<ENTER> im output.p <ENTER> im output.pl
will bring up two interactive windows, one for Pop-11 and one for Prolog.
Further details of the way Ved is adapted for different languages can be found in
HELP * PLOGVED HELP * LISPVED HELP * PMLVED
Regardless of the subsystem currently associated with a file, you can choose to run any Ved command relative to a particular subsystem by prefixing the command with the subsystem name.
This has already been demonstrated in connection with getting hold of documentation for particular subsystems, so that the command
<ENTER> prolog help topic
will search for any Prolog-specific documentation on the given topic.
The facility has a more general applicability however. For example, the command
<ENTER> prolog lmr
will compile the marked range as a Prolog program, while
<ENTER> lisp f insert
will locate the definition of the Lisp function insert. Such commands are useful for mixed-language programming, where a single file may contain fragments of code written in various languages. However, unlike the documentation examples, these do require that the corresponding subsystem compiler (Prolog or CLisp in this case) has been loaded.
Entering a subsystem name as a command with no arguments takes you out of Ved and runs the top-level for that subsystem, making it the primary subsystem: so
<ENTER> prolog
takes you out of Ved and into the Prolog top-level.
REF * SUBSYSTEM
Detailed information about subsystems.
HELP * MKIMAGE
Making a saved image including subsystems.
--- C.all/help/subsystems --- Copyright University of Sussex 1996. All rights reserved.