Forth in 7 easy steps Mar 2016

Forth is a simple language, with simple rules and a simple execution model. It’s also self-hosted and interactive - you can type this at the prompt:

1 2 + .<cr>

What you’ll get as response is this (details may differ, everything below is for Mecrisp Forth):

1 2 + . 3  ok.<lf>
       ^^^^^^^^^^^ added by Forth

All code is parsed into a sequence of “words”, which is anything between whitespace. The above code consist of 4 words: 1, 2, +, and . - these words operate on a data stack. Numbers “push themselves” onto the stack, i.e. first 1, then 2. The word + is predefined, it replaces the two stack entries by a single one, their sum. The word . pops the top value off the stack and prints it as number followed by a space.

Since there are no more words left, Mecrisp then prints ok. followed by a linefeed, and waits for a new line of text to parse, so it can process more words.

That’s it. The essence of Forth. A few more steps like this, and you’ll have the complete story.

To define this as a new demo word, type this (let’s omit the obvious closing <cr> from now on):

: demo  1 2 + . ;

The : word starts a definition and ; ends it. Nothing interesting happens, but now this works:

demo 3 ok.
    ^^^^^^ added by Forth

New words must be defined in terms of existing ones. Here’s another definition:

: JC's-triple-demo  demo demo demo ;

Word names can be anything other than whitespace (including UTF-8, and even 1 or 2 - but that could lead to major confusion).

That’s step 2, now you know how to extend the language.

The definition of words is an important mechanism. You’ve already seen the data stack, but there is also a stack for words and their compiled code, which is called the “dictionary” in Forth. New words are added to the end, and words are looked up in reverse order, so that the last one will be used when words are re-defined.

The : word is quite special: it will parse the next word in the input stream and add it as a fresh definition to the dictionary. It also sets a “state” flag to compile mode. And then it returns to the main loop in Forth to process all remaining words.

And here’s The Big Trick, part 1: the main loop will parse more words, but since the state flag is set, it will append calls to these words to the dictionary instead of executing them.

At some point, Forth will need to finish the definition and return to run mode.

The Big Trick, part 2: a word can be marked as “immediate”. When this is the case, it overrides the state logic in the main loop, and gets executed right away, even in compile mode. So there’s an immediate ; word in Forth, which does two things: add a “return” statement to the end of the dictionary, and reset the state flag to zero.

Immediate words enable magical behaviour in Forth, because they’ll switch back to run mode during compilation. They can do anything (or to be more precise: they are in fact the compiler).

That’s step 3. This is how Forth unifies “run mode” and “compile mode”.

Forth has conditionals and loops. Here’s a rewritten version of the above:

: looping-demo  3 0 do demo loop ;

This should be readable by now: 3 and 0 just get added to the stack, do pops them as loop limits (in funky reverse order), then comes the loop body, followed by loop which presumably knows how to count and repeat, and the closing semicolon to finish the definition of looping-demo.

It should come as no surprise that do and loop are immediate words. They append code to the dictionary to implement the do loop and use the data stack (while in compile mode) to track branch offsets. There’s also a word called i to push the current loop value on the stack. As you can see, common words in Forth tend to have very short names.

This example loops until a key is pressed (key? pushes a flag on the stack, which until uses):

: boring-demo  begin demo key? until ;

Since if, do, begin, etc generate code, they can only be used inside word definitions. You can’t use them interactively, i.e. in run mode, but you can enter a definition and call it, all on one line.

Immediate words are also used to implement if, else, then, and several other jump-based words. The funky order of else and then takes a little getting used to, but it’s trivial stuff:

: even-demo  123 2 mod 0 = if ." EVEN!" then ;
: even-odd-demo  123 2 mod 0 = if ." EVEN!" else ." ODD!" then ;

Several new words have been used here. The mod word is used to calculate modulo-two, = will compare two values, and ." ..." prints a string. Note the space after the opening quote: in Forth, everything is a word, so the print-string word is called ." and must be enclosed in spaces. Note also that the closing quote does not need a space in front, because the ." word plays special tricks with parsing.

Such unconventional syntax details come from the fact that Forth uses stacks and treats everything as words - the price of a simple uniform data + parsing model.

Congratulations, this was step 4, with a little peek inside the compiler!

Words can call other words, and do loops can be nested (it might help to view do loops as a special way to “call” their body repeatedly).

This nesting is actually what all other languages also have, using a “return” stack. In Forth, the return stack is separate from the data stack. This is what gives the language its concatenative properties (a term coined decades after Forth was invented).

When a defined word is executed, it pushes the current instruction pointer on the return stack, and starts executing its own code. When it returns, it pops the instruction pointer back off the return stack and resumes where it left off. Do loops also use the return stack to store some state.

The return stack is fully exposed in Forth, which allows some amazing tricks, such as a one-line coroutine implementation, as well as a simple collaborative multi-tasker implementation.

In day-to-day use, the data and return stacks are the only ones that matter. The dictionary (i.e. code stack) and a stack to allocate RAM variables from can safely be ignored most of the time.

What about other data? Here is a constant and a variable definition:

123456789 constant MY-CONST
987654321 variable my-var

Constants are just that, they can be used wherever a value is needed, and push their value onto the data stack when executed. The convention is to write them in uppercase, but Forth is case insensitive (for ASCII characters, not for UTF-8!), so it won’t matter during use.

A variable pushes its address. To fetch (and then print) the value, you need to use the @ word:

my-var @ . 987654321  ok.
          ^^^^^^^^^^^^^^^ added by Forth

To store a value, there’s the ! word, which expects a value and an address on the stack:

123 my-var !  my-var @ . 123  ok.
                        ^^^^^^^^^ added by Forth

For allocating larger memory areas in RAM, there’s the buffer: word:

200 buffer: my-buffer

This sets aside a 200-byte word-aligned area in RAM. It remains available as long as my-buffer is in the dictionary. Executing my-buffer will push its buffer address on the (data) stack.

You’ve made it through step 5, now you know all about stacks and memory.

As you can imagine, there are a large number of words in Forth, each with their own behaviour and stack effect. Inline comments between words called ( and ) are normally used to document a word, followed by a \ comment about what it does. If ! were defined in Forth (it isn’t, it’s a primitive), it could have been documented as follows:

: ! ( u|n a-addr -- ) \ Stores single number in memory
  ... ;

Where u|n means: an unsigned or signed integer, and a-addr means aligned address. Everything before the -- is what is expected as stack input, everything after is the stack result (nothing in this example). These are just comments and conventions, Forth will skip all that.

Likewise, + might have been defined as:

: + ( u1|n1 u2|n2 -- u3|n3 ) ... ; \ Addition

Many words affect only the data stack, but a few mess with the return stack. Like so:

: >r ( x -- ) ( R: -- x ) ... ;

What this says is: the value on the data stack before calling >r will end up on the return stack afterwards: so >r moves an item from the data stack to the return stack (better get it off again with r> or rdrop before the current word returns, else the code will probably crash!).

These stack effect comments are a critical part of the documentation of each word, since there are usually no local variables in Forth.

Here is the glossary of the pre-defined words in Mecrisp Forth. There are a few hundred of them, but no worries: you can explore and gradually expand your vocabulary - only a fraction of these are needed to start programming in Forth.

Yeay, step 6 - you’re all set to build up your Forth vocabulary!

The last step that remains, is to try things out and look for examples and more documentation on the web. See the “Dive into Forth” series, part one, two, and three for a recent exploration here at JeeLabs. The PWM module (documented here) shows one example of how to implement a hardware feature in Forth.

For old, but still useful material, see the Thinking Forth and Starting Forth books by Leo Brodie, Julian Noble’s introduction, documents on the forth.org site, and links mentioned on the forum.

A last point to make, is that Forth lives extremely “close to the metal”. Any suggestion of high-level coding is purely smoke-and-mirrors. It has just enough machinery to be reasonably useful, and to let you compile and extend it with more definitions to do whatever you need.

And there you have it: Forth in sixteen hundred, ehm… words. Hopefully this intro can help you wrap your mind around an intriguingly powerful and concise programming language.

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