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About Forth
Forth is a stack-based programming language created by Charles H. Moore in the early 1970s. It was designed with simplicity, directness, and interactive exploration in mind. Forth has been used for many years to do scientific work and program embedded systems: it was used to control telescopes and was running on some devices used in space missions among other things. Forth quickly evolved into multiple implementations targetting various computer architectures. None of them really took off and became popular. Nonetheless, the ideas behind Forth continue to garner the interest of many different people in very different (often unrelated) fields. Nowadays, Forth languages are used by hackers and artists for their peculiarity. Forth is simple, direct and beautiful to implement. Forth is an elegant and minimal language to learn. It is easy to understand, to extend and to apply to a specific task. The Forth we use in Cagire is specialized in making live music. We think of it as a DSL: a Domain Specific Language.
Why Forth?
Most programming languages nowadays use a complex syntax made of variables, expressions and statements like x = 3 + 4. Forth works differently. It is way more simple than that, has almost no syntax and performs computations in a quite unique way. You push values onto a stack and apply words that transform them:
3 4 +
This program leaves the number 7 on the stack. There are no variables, no parentheses, no syntax to remember. You just end up with words and numbers separated by spaces. For live coding music, this directness is quite exciting. All you do is think in terms of transformations and add things to the stack: take a note, shift it up, add reverb, play it.
The Stack
The stack is where values live. When you type a number, it goes on the stack. When you type a word, it usually takes values off and puts new ones back.
3 ( stack: 3 )
4 ( stack: 3 4 )
+ ( stack: 7 )
The stack is last-in, first-out. The most recent value is always on top. This means that its often better to read Forth programs from the end to the beginning: from right to left, from the bottom to the top.
Words
Everything in Forth is either a number or a word. Words are like functions but conceptually simpler. They have no arguments or return values in the traditional sense. They just manipulate the stack directly.
dup ( duplicate the top value )
drop ( discard the top value )
swap ( swap the top two values )
Words compose naturally on the stack. To double a number:
3 dup + ( 3 3 +)
There are a lot of words in a Forth and thus, Cagire has a Dictionary embedded directly into the application. You can also create your own words. They will work just like the already existing words. There are good reasons to create new words on-the-fly:
- To make synth definitions.
- To abstract some piece of code that you use frequently.
- To share data and processes between different steps.
Values
Four types of values can live on the stack:
- Integers:
42,-7,0 - Floats:
0.5,3.14,-1.0 - Strings:
"kick","hello" - Quotations:
{ dup + }(code as data)
Quotations are special. They let you pass code around as a value. This is how conditionals and loops work. Think nothing of it for now, you will learn more about how to use it later on.
Stack Notation
Documentation uses a notation to show what words do:
( before -- after )
For example, + has the signature ( a b -- sum ). It takes two values and leaves one.
The Command Register
Traditional Forth programs print text to a terminal. Cagire's Forth builds sound commands instead. This happens through an invisible accumulator called the command register. The command register has two parts:
- a sound name (what instrument to play)
- a list of parameters (how to play it)
Three types of words interact with it:
kick sound ;; sets the sound name
0.5 gain ;; adds a parameter
. ;; emits the command and clears the register
The word sound (or its shorthand s) sets what sound to play. Parameter words like gain, freq, decay, or verb add key-value pairs to the register. Nothing happens until you emit with . (dot). At that moment, the register is packaged into a command and sent to the audio engine.
This design lets you build sounds incrementally:
"sine" sound
c4 note
0.5 gain
0.3 decay
0.4 verb
.
Each line adds something to the register. The final . triggers the sound. You can also write it all on one line:
"sine" s c4 note 0.5 gain 0.3 decay 0.4 verb .
The order of parameters does not matter. You can even emit multiple times in a single step. If you need to discard the register without emitting, use clear:
"kick" s 0.5 gain clear ;; nothing plays, register is emptied
"hat" s . ;; only the hat plays
This is useful when conditionals might cancel a sound before it emits.
More details
- Each step has its own stack and independant runtime.
- Word definitions and variable definitions are shared by all steps.