20 KiB
RETRO: a Modern, Pragmatic Forth
Welcome to RETRO, my personal take on the Forth language. This is a modern system primarily targetting desktop, mobile, and servers, though it can also be used on some larger (ARM, MIPS32) embedded systems.
The language is Forth. It is untyped, uses a stack to pass data between functions called words, and a dictionary which tracks the word names and data structures.
But it's not a traditional Forth. RETRO draws influences from many sources and takes a unique approach to the language.
RETRO has a large vocabulary of words. Keeping a copy of the Glossary on hand is highly recommended as you learn to use RETRO.
This book will hopefully help you develop a better understanding of RETRO and how it works.
Building RETRO on BSD
Requirements
- c compiler (tested: clang, tcc, gcc)
- make
- standard unix shell
Process
Run make.
This will build the toolchain and then the main retro
executable.
Executables
In the bin/ directory:
retro
retro-unu
retro-muri
retro-extend
retro-embedimage
Building RETRO on Linux
Requirements
- c compiler (tested: clang, tcc, gcc)
- make
- standard unix shell
Process
Run make -f Makefile.linux.
This will build the toolchain and then the main retro
executable.
Executables
In the bin/ directory:
retro
retro-unu
retro-muri
retro-extend
retro-embedimage
Building RETRO on macOS
Requirements
- c compiler (tested: clang, tcc, gcc)
- make
- standard unix shell
Process
Run make.
This will build the toolchain and then the main retro
executable.
Executables
In the bin/ directory:
retro
retro-unu
retro-muri
retro-extend
retro-embedimage
Building RETRO on Windows
C#: retro.cs
This is an implementation of retro-repl in C#. As with
retro-repl it requires the ngaImage in the current
directory when starting.
Building:
csc retro.cs
You'll need to make sure your path has the CSC.EXE in it, or provide a full path to it. Something like this should reveal the path to use:
dir /s %WINDIR%\CSC.EXE
I've only tested building this using Microsoft's .NET tools. It should also build and run under Mono.
Starting RETRO
RETRO can be run for scripting or interactive use. To start it
interactively, run: retro -i or retro -c.
For a summary of the full command line arguments available:
Scripting Usage:
retro filename [script arguments...]
Interactive Usage:
retro [-h] [-i] [-c] [-s] [-f filename] [-t]
-h Display this help text
-i Interactive mode (line buffered)
-c Interactive mode (character buffered)
-s Suppress the 'ok' prompt and keyboard
echo in interactive mode
-f filename Run the contents of the specified file
-t Run tests (in ``` blocks) in any loaded files
Starting RETRO
RETRO can be run for scripting or interactive use. To start it
interactively, run: retro -i or retro -c.
For a summary of the full command line arguments available:
Scripting Usage:
retro filename [script arguments...]
Interactive Usage:
retro [-h] [-i] [-c] [-s] [-f filename] [-t]
-h Display this help text
-i Interactive mode (line buffered)
-c Interactive mode (character buffered)
-s Suppress the 'ok' prompt and keyboard
echo in interactive mode
-f filename Run the contents of the specified file
-t Run tests (in ``` blocks) in any loaded files
Starting RETRO
RETRO can be run for scripting or interactive use. To start it
interactively, run: retro -i or retro -c.
For a summary of the full command line arguments available:
Scripting Usage:
retro filename [script arguments...]
Interactive Usage:
retro [-h] [-i] [-c] [-s] [-f filename] [-t]
-h Display this help text
-i Interactive mode (line buffered)
-c Interactive mode (character buffered)
-s Suppress the 'ok' prompt and keyboard
echo in interactive mode
-f filename Run the contents of the specified file
-t Run tests (in ``` blocks) in any loaded files
Basic Interactions
Start RETRO in interactive mode:
retro -i
You should see something similar to this:
RETRO 12 (rx-2019.6)
8388608 MAX, TIB @ 1025, Heap @ 9374
Ok
At this point you are at the listener, which reads and processes your input. You are now set to begin exploring RETRO.
To exit, run bye:
bye
Using The Glossary
The Glossary is a valuable resource. It provides information on the RETRO words.
Example Entry
f:+
Data: -
Addr: -
Float: FF-F
Add two floating point numbers, returning the result.
Class: class:word | Namespace: f | Interface Layer: rre
Example #1:
.3.1 .22 f:+
Reading The Entry
An entry starts with the word name.
This is followed by the stack effect for each stack. All RETRO systems have Data and Address stacks, some also include a floating point stack).
The stack effect diagrams are followed by a short description of the word.
After the description is a line providing some useful data. This includes the class handler, namespace prefix, and the interface layer that provides the word.
Words in all systems will be listed as all. Some words (like
the pb: words) are only on specific systems like iOS. These
can be identified by looking at the interface layer field.
At the end of the entry may be an example or two.
Access Online
The latest Glossary can be browsed at http://forthworks.com:9999 or gopher://forthworks.com:9999
Programming Techniques
The upcoming chapters provide helpful information on using RETRO with different types of data and hints on how to solve problems in a way consistent with the RETRO system.
Naming Conventions
Word names in RETRO generally follow the following conventions.
Case
Word names are lowercase, with a dash (-) for compound names. Variables use TitleCase, with no dash between compound names. Constants are UPPERCASE, with a dash (-) for compound names.
Namespaces
Words are grouped into broad namespaces by attaching a short prefix string to the start of a name.
The common namespaces are:
| Prefix | Contains |
|---|---|
| array: | Words operating on simple arrays |
| ASCII: | ASCII character constants for control characters |
| buffer: | Words for operating on a simple linear LIFO buffer |
| c: | Words for operating on ASCII character data |
| class: | Contains class handlers for words |
| d: | Words operating on the Dictionary |
| err: | Words for handling errors |
| io: | General I/O words |
| n: | Words operating on numeric data |
| prefix: | Contains prefix handlers |
| s: | Words operating on string data |
| v: | Words operating on variables |
| file: | File I/O words |
| f: | Floating Point words |
| gopher: | Gopher protocol words |
| unix: | Unix system call words |
Stack Diagrams
Most words in RETRO have a stack comment. These look like:
(-)
(nn-n)
As with all comments, a stack comment begins with ( and
should end with a ). There are two parts to the comment.
On the left side of the - is what the word consumes. On
the right is what it leaves.
RETRO uses a short notation, with one character per value taken or left. In general, the following symbols represent certain types of values.
b, n, m, o, x, y, z are generic numeric values
s represents a string
v represents a variable
p, a represent pointers
q represents a quotation
d represents a dictionary header
f represents a `TRUE` or `FALSE` flag.
In the case of something like (xyz-m), RETRO expects z to be
on the top of the stack, with y below it and x below the y
value. And after execution, a single value (m) will be left on
the stack.
Words with no stack effect have a comment of (-)
Working With a Buffer
RETRO provides words for operating on a linear memory area. This can be useful in building strings or custom data structures.
Namespace
Words operating on the buffer are kept in the buffer
namespace.
Working With Characters
RETRO provides words for working with ASCII characters.
Prefix
Character constants are returned using the $ prefix.
Working With The Dictionary
The Dictionary is a linked list containing the dictionary headers.
Namespace
Words operating on the dictionary are in the d: namespace.
Variables
Dictionary is a variable holding a pointer to the most recent
header.
Header Structure
Each entry follows the following structure:
Offset Contains
------ ---------------------------
0000 Link to Prior Header
0001 Link to XT
0002 Link to Class Handler
0003+ Word name (null terminated)
RETRO provides words for accessing the fields in a portable manner. It's recommended to use these to allow for future revision of the header structure.
Accessing Fields
Given a pointer to a header, you can use d:xt, d:class,
and d:name to access the address of each specific field.
There is no d:link, as the link will always be the first
field.
Shortcuts For The Latest Header
RETRO provides several words for operating on the most recent header.
d:last returns a pointer to the latest header. d:last<xt>
will give the contents of the d:xt field for the latest
header. There are also d:last<class> and d:last<name>.
Adding Headers
Two words exist for making new headers. The easy one is
d:create. This takes a string for the name and makes a
new header with the class set to class:data and the XT
field pointing to here.
Example:
'Base d:create
The other is d:add-header. This takes a string, a pointer
to the class handler, and a pointer for the XT field and
builds a new header using these.
Example:
'Base &class:data #10000 d:add-header
Searching
RETRO provides two words for searching the dictionary.
d:lookup takes a string and tries to find it in the
dictionary. It will return a pointer to the dictionary header
or a value of zero if the word was not found.
d:lookup-xt takes a pointer and will return the dictionary
header that has this as the d:xt field, or zero if no match
is found.
Iteration
You can use the d:for-each combinator to iterate over all
entries in the dictionary. For instance, to display the names
of all words:
[ d:name s:put sp ] d:for-each
For each entry, this combinator will push a pointer to the entry to the stack and call the quotation.
Listing Words
Most Forth systems provide WORDS for listing the names of all
words in the dictionary. RETRO does as well, but this is named
d:words.
This isn't super useful as looking through several hundred
names is annoying. RETRO also provides d:words-with to help
in filtering the results.
Example:
'class: d:words-with
Working With Floating Point
Some RETRO systems include support for floating point numbers.
When present, this is built over the system libm using the
C double type.
Floating point values are typically 64 bit IEEE 754 double precision (1 bit for the sign, 11 bits for the exponent, and the remaining 52 bits for the value), i.e. 15 decimal digits of precision.
Prefix
Floating point numbers start with a .
Examples:
Token Value .1 1.0 .0.5 0.5 .-.4 -0.4 .1.3 1.3
Working With Numbers
Numbers in RETRO are signed, 32 bit integers with a range of -2,147,483,648 to 2,147,483,647.
Token Prefix
All numbers start with a # prefix.
Namespace
Most words operating on numbers are in the n: namespace.
Working With Pointers
Prefix
Pointers are returned by the & prefix.
Examples
'Base var
&Base fetch
#10 &Base store
#10 &n:inc call
Notes
The use of & to get a pointer to a data structure (with a
word class of class:data) is not required. I like to use it
anyway as it makes my intent a little clearer.
Pointers are useful with combinators. Consider:
:abs dup n:negative? [ n:negate ] if ;
Since the target quote body is a single word, it is more efficient to use a pointer instead:
:abs dup n:negative? &n:negate if ;
The advantages are speed (saves a level of call/return by avoiding the quotation) and size (for the same reason). This may be less readable though, so consider the balance of performance to readability when using this approach.
Working With Strings
Strings in RETRO are NULL terminated sequences of values representing characters. Being NULL terminated, they can't contain a NULL (ASCII 0).
The character words in RETRO are built around ASCII, but
strings can contain UTF8 encoded data if the host platform
allows. Words like s:length will return the number of bytes,
not the number of logical characters in this case.
Prefix
Strings begin with a single '.
'Hello
'This_is_a_string
'This_is_a_much_longer_string_12345_67890_!!!
RETRO will replace spaces with underscores. If you need both
spaces and underscores in a string, escape the underscores and
use s:format:
'This_has_spaces_and_under\_scored_words. s:format
Lifetime
At the interpreter, strings get allocated in a rotating buffer.
This is used by the words operating on strings, so if you need
to keep them around, use s:keep or s:copy to move them to
more permanent storage.
In a definition, the string is compiled inline and so is in permanent memory.
You can manually manage the string lifetime by using s:keep
to place it into permanent memory or s:temp to copy it to
the rotating buffer.
Mutability
Strings are mutable. If you need to ensure that a string is not altered, make a copy before operating on it or see the individual glossary entries for notes on words that may do this automatically.
Searching
RETRO provides two words for searching within a string.
s:contains-char?
s:contains-string?
s:index-of
s:index-of-string
Comparisons
s:eq?
s:case
Extraction
s:left
s:right
s:substr
Joining
s:append
s:prepend
Tokenization
s:tokenize
s:tokenize-on-string
s:split
s:split-on-string
Conversions
s:to-lower
s:to-upper
s:to-number
Cleanup
s:chop
s:trim
s:trim-left
s:trim-right
Combinators
s:for-each
s:filter
s:map
Other
s:evaluate
s:copy
s:reverse
s:hash
s:length
s:replace
s:format
s:empty
The Return Stack
RETRO has two stacks. The primary one is used to pass data beween words. The second one primarily holds return addresses.
Each time a word is called, the next address is pushed to the return stack.
Internals
The next few chapters dive into RETRO's architecture. If you seek to implement a port to a new platform or to extend the I/O functionality you'll find helpful information here.
Internals: Interface Layers
Nga provides a virtual processor and an extensible way of adding I/O devices, but does not provide any I/O itself. Adding I/O is the responsability of the interface layer.
An interface layer will wrap Nga, providing at least one I/O device (a generic output target), and a means of interacting with the retro image.
It's expected that this layer will be host specific, adding any system interactions that are needed via the I/O instructions. The image will typically be extended with words to use these.
Internals: Nga Virtual Machine
Overview
At the heart of RETRO is a simple MISC (minimal instruction set computer) processor for a dual stack architecture.
This is a very simple and straightforward system. There are 30 instructions. The memory is a linear array of signed 32 bit values. And there are two stacks: one for data and one for return addresses.
Instrution Table
Column:
0 - opcode value 1 - Muri assembly name 2 - Full name 3 - Data Stack Usage 4 - Address Stack Usage
+--------------------------------------------------+
| 0 .. nop - - |
| 1 li lit -n - |
| 2 du dup n-nn - |
| 3 dr drop n- - |
| 4 sw swap xy-yx - |
| 5 pu push n- -n |
| 6 po pop -n n- |
| 7 ju jump a- - |
| 8 ca call a- -A |
| 9 cc conditional call af- -A |
| 10 re return - A- |
| 11 eq equality xy-f - |
| 12 ne inequality xy-f - |
| 13 lt less than xy-f - |
| 14 gt greater than xy-f - |
| 15 fe fetch a-n - |
| 16 st store na- - |
| 17 ad addition xy-n - |
| 18 su subtraction xy-n - |
| 19 mu multiplication xy-n - |
| 20 di divide & remainder xy-rq - |
| 21 an bitwise and xy-n - |
| 22 or bitwise or xy-n - |
| 23 xo bitwise xor xy-n - |
| 24 sh shift xy-n - |
| 25 zr zero return n-n | n- - |
| 26 en end - - |
| 27 ie i/o enumerate -n - |
| 28 iq i/o query n-xy - |
| 29 ii i/o invoke ...n- - |
| |
| Each `li` will push the value in the following |
| cell to the data stack. |
+--------------------------------------------------+
| li du mu .. |
| i lidumu.. 00000001:00000010:00010011:00000000 |
| data for li |
| d 2 00000000:00000000:00000000:00000010 |
| |
| Assembler Directives Instruction Bundles |
| ======================== ==================== |
| : label Combine instruction |
| i bundle names in groups of 4 |
| d numeric-data |
| r ref-to-address-by-name Use only .. after |
| s null-terminated string ju, ca, cc, re, zr |
+--------------------------------------------------+
Misc
There are 810,000 possible combinations of instructions. Only 73 are used in the implementation of RETRO.
Internals: The Retro Image
The actual RETRO language is stored as a memory image for Nga.
Layout
Assuming an Nga built with 524287 cells of memory:
| RANGE | CONTAINS |
|---|---|
| 0 - 1024 | rx kernel |
| 1025 - 1535 | token input buffer |
| 1536 + | start of heap space |
| ............... | free memory for your use |
| 506879 | buffer for string evaluate |
| 507904 | temporary strings (32 * 512) |
| 524287 | end of memory |
The buffers at the end of memory will resize when specific variables related to them are altered.