Post4 (Post-Forth)

Copyright 2007, 2024 Anthony Howe. All rights reserved.

Overview

Post4 is a hosted indirect threaded Forth dialect written in C, based on the "Forth 200x Draft 19.1, 2019-09-30". Post4 aims to implement the fewest possible built-in words in C, those that are needed to interact with memory and I/O, leaving the remaining standard words to be implemented in Forth.

    usage: post4 [-TV][-b file][-c file][-h size][-i file][-m size]
                 [script [args ...]]
    
    -b file         open a block file
    -c file         word definition file; default post4.p4 from $POST4_PATH
    -h size         history size in lines; default 16
    -i file         include file; can be repeated; searches $POST4_PATH
    -m size         data space memory in KB; default 128
    -T              enable tracing; see TRACE
    -V              build and version information
    
    If script is "-", read it from standard input.

The environment variable POST4_PATH provides a colon separated search path for the post4.p4 core word definitions file and include files. If POST4_PATH is undefined, then an OS specific default path is used. A specific word definition file can be specified with -c.

By default no block file is opened. Use -b file to open a block file at start-up; otherwise see BLOCK-OPEN and BLOCK-CLOSE words.

Post4 reads input from standard input and writes to standard output, which can be redirected:

Example 1 - pipe input

    echo "123 69 + ." | post4

Example 2 - here document

    post4 - hey babe <<EOF
    include-path dumpargs.p4
    args .s dropall cr
    s\" Hello world (again)!\r\n" type
    EOF

Post4 cell size is equivalent to C's intptr_t, which for most current systems are 64 bits.

Exit Status

• 0 Post4 and application exited successfully.
• 1 The application returned a negative result.
• 2 Command line usage error.
• 3 One or more uncaught exceptions occurred.
• >3 Some application error occurred.

Glossary of Words

• Standard Core
• Block File
• Double-Cell
• File Access
• Floating-Point
• Memory
• Seach-Order
• String
• Tool
• THROW Codes

Building

Post4 is written in ISO C17 using only one extension, Labels As Values, which is supported by gcc and clang compilers. Post4 has been built and tested on NetBSD, Cygwin, Alpine Linux, Rocky Linux, and Ubuntu Linux. Its been reported to build on MacOS too.

    $ ./configure --help
    $ ./configure
    $ make tests
    $ make run              # Manually test in the source tree.
    ok BYE
    $ sudo make install
    $ post4

If gcc is the default compiler, it is possible to override that to test building with a different C compiler suite, most likely Clang for example:

    $ make CC=clang clean build

Building for debug can be done in two ways. The first enables debug options in Post4 amd JNI as needed:

    $ ./configure --enable-debug
    $ make clean build tests

Alternative following an earlier configuration, possible to build a temporary debug build:

    $ ./configure
    $ cd src
    $ make DBG='-g -O0' clean build

The DBG macro can be used to override default CFLAGS to try different compiler optimisations. In the case of -O the last one specified overrides the previous occurences. By default Post4 builds with -Os, because small is beautiful. If speed is more of concerning simply use DBG='-O2' or DBG='-O3'.

Java Native Interface

JNI support is availble. See the JNI documentation about building, API, and examples.

Interactive Usage

When Post4 is invoked without a script file argument or redirected input, then the user is presented with an ok prompt. The user can enter Post4 numbers and words. Pressing the terminal Enter (Return) key ends the input line, which will be interpreted. All Post4 words are treated as case-insensitive.

To leave Post4 either type the EOF terminal character, BYE, or code BYE-CODE (where code is number to return to the user's shell). For those not familiar with Forth, there is a QUIT word, but it only terminates a running program; it does not return to the host OS.

TTY Line Editing

Post4 implements simple TTY style line editing and history for interactive input. GNU readline, BSD libedit, and rlwrap are not supported. See also stty(1).

    up      ^K      Cycle to the previous input line to edit.
    down    ^L      Cycle to the next input line to edit.
    left    right   Cursor left or right within line.
    ERASE   ^H  ^?  Erase character before the cursor.
    WERASE          Erase the previous white space delimited word.
    KILL            Erase current line input.
    EOL     ^M  ^J  Newline submits input line.
    EOF             End of file.

• Assumes ANSI terminal escape sequences.

Numeric I/O

The default numeric input/output base is decimal (base 10). Setting variable BASE will change the default radix used for numeric I/O, which can be between between 2 and 36, eg. 16 BASE ! sets hexadecimal. There are four shorthand words BINARY, OCTAL, DECIMAL, and HEX that set BASE to 2, 8, 10, or 16 repectively.

Regardless of the current value of BASE, it is possible to input numbers in one of the three common bases without having to change the value of BASE. Prefixing a number with %, #, $ can set a binary, decimal, or hex value; an optional minus sign given after the prefix to indicate a negative number. For example:

    %1111111  = #127  = $7f  = $000000000000007f
    %-1111111 = #-127 = $-7f = $ffffffffffffff81

When current BASE is ten (10) its possible to input floating point numbers (provided support is enabled) with a decimal point and/or scientific notation. For example:

     0.0    =  0E0  =  0e0
     1.0    =  1E0  =  1e0  =  +1.E0
     12.3   =  123E-1
    -0.123  = -123e-3

Floating point numbers are placed on the float stack, which is separate from the data and return stacks. Also the float stack is small, though at least six (6) deep. See Floating-Point Words, in particular words f. and fs. to start with.

• Note the input notation, 123E, where there is no value following the exponent is not supported.

It is also possible to input a character constant or backslash escape character. Simple use single-quotes around the character or backslash-escape string (see also CHAR and [CHAR]). For example:

    'A'     ASCII upper case A.             '_'     ASCII underscore.
    'b'     ASCII lower case B.             '\n'    ASCII linefeed.
    '9'     ASCII digit 9.

The following C-style backslash escapes are supported:

    \?      delete                          \n      linefeed
    \\      backslash                       \r      carriage-return
    \a      bell                            \s      space
    \b      backspace                       \t      tab
    \e      escape                          \v      vertical tab
    \f      formfeed                        \z      nul

Because Forth uses whitespace for input delimiters, in particular space (ASCII 32), the only way to input a literal space character is with:

    32      ASCII numeric value.
    '\s'    Backslash escape.
    BL      Forth word, short for blank.

Forth has strings: counted and length prefixed. Counted strings are a carry-over from historical Forth, where the string stored in memory, is prefixed by a length octet, limiting the string length to 255 ASCII octets. The standard Forth character string, when stored in memory, is prefixed with an unsigned numeric length cell. As a Post4 extension, character strings are always NUL terminated as a convenience for interacting with the C library and host OS API.

Some examples of words using strings. Note that the whitespace character (space, tab, newline) immediately after the word, eg. S\", delimits the word from the text that follows and is not part of the text:

    ." some text"                           Print some text at runtime.
    C" counted text"                        Define a counted string.
    ."  <- space printed"                   Print text with leading space (or tab, hard to tell).
    .\" \s<- space printed"                 Print text with leading space escaped for clarity.
    S" some text"                           Store some text.
    S\" escaped\atext\r\n"                  Store some escaped text.
    .( lots of text
       spanning multiple lines)             Show some text (now).
    .( ed(1\) is the standard text editor)  Escape terminating delimiter.
    ABORT" text message"                    What I do wrong now?

Examples

catch_throw.p4

A demonstration of using CATCH and THROW.

    $ post4 -i catch_throw.p4
    Enter TRY-IT or RETRY-IT words to test CATCH / THROW.
    ok 

dumpargs.p4

Simple demonstration on how to access the command line arguments.

    $ post4 dumpargs.p4 hello world, tell us more!

ed.p4

Block Editor; actually three block editor word sets:

1. An ed(1) like block line editor using LIST, PRINT, and CHANGE.
2. An interactive single line editor using EDIT; left & right cursor keys, tab toggles insert or replace mode, delete backspace, ESC quits.
3. A full "screen" block editor using ED; all the commands of the interactive single line editor, plus up & down cursor keys, CTRL+G goto block, CTRL+P & CTRL+N block, and ESC menu.

NOTE: that EDIT and ED are hard coded with ANSI terminal escape sequences.

    $ post4 -b .post4.blk -i ed.p4
    Type ED to start editor.
    ok ED

life.p4

Conway's Game of Life

Rules

The universe of the Game of Life is an infinite two-dimensional orthogonal grid of square cells, each of which is in one of two possible states, live or dead. Every cell interacts with its eight neighbours, which are the cells that are directly horizontally, vertically, or diagonally adjacent. At each step in time, the following transitions occur:

1. Any live cell with fewer than two live neighbours dies, as if caused by under population.
2. Any live cell with more than three live neighbours dies, as if by overcrowding.
3. Any live cell with two or three live neighbours lives on to the next generation.
4. Any dead cell with exactly three live neighbours becomes a live cell.

The initial pattern constitutes the seed of the system. The first generation is created by applying the above rules simultaneously to every cell in the seed - births and deaths happen simultaneously, and the discrete moment at which this happens is sometimes called a tick (in other words, each generation is a pure function of the one before). The rules continue to be applied repeatedly to create further generations.

    $ post4 life.p4

life1d.p4

One Dimensional Cellular Atomata

This example demostrates five Wolfram rules 30, 90, 110, 184, and 104 using the same initial state.

    $ post4 life1d.p4

rand.p4

Pseudo-random number generators. One is the simple code example from the ISO C11 draft April 12, 2011,

    \ Example from the ISO C11 draft April 12, 2011
    
    SRAND ( -- aaddr )
        Random number seed variable.
        
    RAND ( -- u )
        Return random number between 0..32767.
    
    \ Pseudo-Random Sequence Generator for 32-Bit CPUs
    \ Bruce Schneier, Dr. Dobb's Journal, v. 17, n. 2, February 1992, pp. 34-40.
    
    RANDA ( -- aaddr )
    RANDB ( -- aaddr )
    RANDC ( -- aaddr )
        Random number seed variables for three Linear Feedback Shift
        Registers.
        
    RANDOMXOR ( bits -- u )
        Return random number u generated from bits, XORing the bits from the
        three generators.
        
    RANDOMMAJ ( bits -- u )
        Return random number u generated from bits, based on the majority of
        one or zero returned from the three generators.

Example:

    $ post4
    ok INCLUDE rand.p4
    ok 12345 SRAND !
    ok RAND . RAND . RAND . CR
    21468 9988 22117
    ok 12345 RANDA ! 54321 RANDB ! $deadbeef RANDC !
    ok 16 RANDOMXOR U. 16 RANDOMXOR U. 16 RANDOMXOR U. CR
    52917 27383 64651
    ok 16 RANDOMMAJ U. 16 RANDOMMAJ U. 16 RANDOMMAJ U. CR
    7 9885 36906
    ok 

wumpus.p4

Hunt The Wumpus game ported from the original BASIC source written by Gregory Yob.

    $ post4 -i wumpus.p4
    ... instructions ...
    Type PLAY to start.
    ok PLAY
    
    BATS NEARBY
    YOU ARE IN ROOM 8
    TUNNELS LEAD TO    1   7   9
    (S)HOOT OR (M)OVE? M
    WHERE TO? 7
    ZAP--SUPER BAT SNATCH! ELSEWHEREVILLE FOR YOU!
    
    YOU ARE IN ROOM 13
    TUNNELS LEAD TO   12  14  20
    (S)HOOT OR (M)OVE? M
    WHERE TO? 14
    
    I SMELL A WUMPUS!
    YOU ARE IN ROOM 14
    TUNNELS LEAD TO    4  13  15
    (S)HOOT OR (M)OVE? 

References

Forth 200x
http://www.forth200x.org/

Forth Standard
https://forth-standard.org/

Forth Discussions
https://github.com/ForthHub/discussion/discussions

Forth
https://en.wikipedia.org/wiki/Forth_(programming_language)

Starting Forth
https://www.forth.com/starting-forth/

Moving Forth Series
https://www.bradrodriguez.com/papers/