A panoramic tour of Factor

Factor is a mature, dynamically typed language based on the concatenative paradigm. Getting started with Factor can be daunting since the concatenative paradigm is different from most mainstream languages. This tutorial will guide you through the basics of Factor so you can appreciate its simplicity and power. I assume you are an experienced programmer familiar with a functional language, and I'll assume you understand concepts like folding, higher-order functions, and currying.

Even though Factor is a niche language, it is mature and has a comprehensive standard library covering tasks from JSON serialization to socket programming and HTML templating. It runs in its own optimized VM with very high performance for a dynamically typed language. It also has a flexible object system, a FFI to C, and asynchronous I/O that works a bit like Node.js, but with a much simpler model for cooperative multithreading.

You may wonder why you should care enough about Factor to read this tutorial. Factor has a few significant advantages over other languages, most arising from the fact that it has essentially no syntax:

• refactoring is very easy, leading to short and meaningful function definitions;
• it is extremely succinct, letting the programmer concentrate on what is important instead of boilerplate;
• it has powerful metaprogramming capabilities, exceeding even those of LISPs;
• it is ideal to create DSLs;
• it integrates easily with powerful tools.

Before you start this tutorial, download a copy of Factor so you can follow along with the examples in the listener (the Factor REPL).

I assume you are using Mac OS X or some distribution of Linux, but everything should work the same on other systems, provided you adjust the file paths in the examples.

The first section gives some motivation for the rather peculiar model of computation of concatenative languages, but feel free to skip it if you want to get your feet wet and return to it after some hands on practice with Factor.

Concatenative languages

Factor is a concatenative programming language in the spirit of Forth. What does this even mean?

To understand concatenative programming, imagine a world where every value is a function, and the only operation allowed is function composition. Since function composition is so pervasive, it is implicit, and functions can be literally juxtaposed in order to compose them. So if f and g are two functions, their composition is just f g (unlike in mathematical notation, functions are read from left to right, so this means first execute f, then execute g).

This requires some explanation, since we know functions often have multiple inputs and outputs, and it is not always the case that the output of f matches the input of g. For instance, g may need access to values computed by earlier functions. But the only thing that g can see is the output of f, so the output of f is the whole state of the world as far as g is concerned. To make this work, functions have to thread the global state, passing it to each other.

There are various ways this global state can be encoded. The most naive would use a hashmap that maps variable names to their values. This turns out to be too flexible: if every function can access any piece of global state, there is little control on what functions can do, little encapsulation, and ultimately programs become an unstructured mess of routines mutating global variables.

It works well in practice to represent the state of the world as a stack. Functions can only refer to the topmost element of the stack, so that elements below it are effectively out of scope. If a few primitives are given to manipulate a few elements on the stack (e.g., swap, that exchanges the top two elements on the stack), then it becomes possible to refer to values down the stack, but the farther the value is down the stack, the harder it becomes to refer to it.

So, functions are encouraged to stay small and only refer to the top two or three elements on the stack. In a sense, there is no distinction between local and global variables, but values can be more or less local depending on their distance from the top of the stack.

Notice that if every function takes the state of the whole world and returns the next state, its input is never used anymore. So, even though it is convenient to think of pure functions as receiving a stack as input and outputting a stack, the semantics of the language can be implemented more efficiently by mutating a single stack.

This leaves concatenative languages like Factor in a strange position, they are both extremely functional - only allowing composition of simpler functions into more complex ones - and largely imperative - describing operations on a mutable stack.

Playing with the stack

Let us start looking what Factor actually feels like. Our first words will be literals, like 3, 12.58 or "Chuck Norris". Literals can be thought as functions that push themselves on the stack. Try writing 5 in the listener and then press enter to confirm. You will see that the stack, initially empty, now looks like

5

You can enter more that one number, separated by spaces, like 7 3 1, and get

5
7
3
1

(the interface shows the top of the stack on the bottom). What about operations? If you write , you will run the function, which pops the two topmost elements and pushes their sum, leaving us with

5
7
4

You can put additional inputs in a single line, so for instance - * will leave the single number 15 on the stack (do you see why?).

The function . (a period or a dot) prints the item at the top of the stack, while popping it out of the stack, leaving the stack empty.

If we write everything on one line, our program so far looks like

5 7 3 1   - * .

which shows Factor's peculiar way of doing arithmetic by putting the arguments first and the operator last - a convention which is called Reverse Polish Notation (RPN). Notice that RPN requires no parenthesis, unlike the polish notation of Lisps where the operator comes first, and RPN requires no precedence rules, unlike the infix notation used in most programming languages and in everyday arithmetic. For instance in any Lisp, the same computation would be written

(* 5 (- 7 (  3 1)))

and in familiar infix notation

(7 - (3   1)) * 5

Also notice that we have been able to split our computation onto many lines or combine it onto fewer lines rather arbitrarily, and that each line made sense in itself.

Defining our first word

We will now define our first function. Factor has slightly odd naming of functions: since functions are read from left to right, they are simply called words, and this is what we'll call them from now on. Modules in Factor define words in terms of previous words and these sets of words are then called vocabularies.

Suppose we want to compute the factorial. To start with a concrete example, we'll compute the factorial of 10, so we start by writing 10 on the stack. Now, the factorial is the product of the numbers from 1 to 10, so we should produce such a list of numbers first.

The word to produce a range is called [a,b] (tokenization is trivial in Factor because words are always separated by spaces, so this allows you to use any combination of non-whitespace characters as the name of a word; there are no semantics to the [, the , and the ] in [a,b] since it is just a token like foo or bar).

The range we want starts with 1, so we can use the simpler word [1,b] that assumes the range starts at 1 and only expects the value at the top of the range to be on the stack. If you write [1,b] in the listener, Factor will prompt you with a choice, because the word [1,b] is not imported by default. Factor is able to suggest you import the math.ranges vocabulary, so choose that option and proceed.

You should now have on your stack a rather opaque structure which looks like

T{ range f 1 10 1 }

This is because our range functions are lazy and only create the range when we attempt to use it. To confirm that we actually created the list of numbers from 1 to 10, we convert the lazy response on the stack into an array using the word >array. Enter that word and your stack should now look like

{ 1 2 3 4 5 6 7 8 9 10 }

which is promising!

Next, we want to take the product of those numbers. In many functional languages, this could be done with a function called reduce or fold. Let's look for one. Pressing F1 in the listener will open a contextual help system, where you can search for reduce. It turns out that reduce is actually the word we are looking for, but at this point it may not be obvious how to use it.

Try writing 1 [ * ] reduce and look at the output: it is indeed the factorial of 10. Now, reduce usually takes three arguments: a sequence (and we had one on the stack), a starting value (this is the 1 we put on the stack next) and a binary operation. This must certainly be the *, but what about those square brackets around the *?

If we had written just *, Factor would have tried to apply multiplication to the topmost two elements on the stack, which is not what we wanted. What we need is a way to get a word onto the stack without applying it. Keeping to our textual metaphor, this mechanism is called quotation. To quote one or more words, you just surround them by [ and ] (leaving spaces!). What you get is akin to an anonymous function in other languages.

Let's type the word drop into the listener to empty the stack, and try writing what we have done so far in a single line: 10 [1,b] 1 [ * ] reduce. This will leave 3628800 on the stack as expected.

We now want to define a word for factorial that can be used whenever we want a factorial. We will call our word fact (although ! is customarily used as the symbol for factorial, in Factor ! is the word used for comments). To define it, we first need to use the word :. Then we put the name of the word being defined, then the stack effects and finally the body, ending with the ; word:

: fact ( n -- n! ) [1,b] 1 [ * ] reduce ;

What are stack effects? In our case it is the ( n -- n! ). Stack effects are how you document the inputs from the stack and outputs to the stack for your word. You can use any identifier to name the stack elements, here we use n. Factor will perform a consistency check that the number of inputs and outputs you specify agrees with what the body does.

If you try to write

: fact ( m n -- n! ) [1,b] 1 [ * ] reduce ;

Factor will signal an error that the 2 inputs (m and n) are not consistent with the body of the word. To restore the previous correct definition press Ctrl P two times to get back to the previous input and then enter it.

We can think at the stack effects in definitions both as a documentation tool and as a very simple type system, which nevertheless does catch a few errors.

In any case, you have succesfully defined your first word: if you write 10 fact in the listener you can prove it.

Notice that the 1 [ * ] reduce part of the definition sort of makes sense on its own, being the product of a sequence. The nice thing about a concatenative language is that we can just factor this part out and write

: prod ( {x1,...,xn} -- x1*...*xn ) 1 [ * ] reduce ;
: fact ( n -- n! ) [1,b] prod ;

Our definitions have become simpler and there was no need to pass parameters, rename local variables, or do anything else that would have been necessary to refactor our function in most languages.

Of course, Factor already has a word for the factorial (actually there is a whole math.factorials vocabulary, including many variants of the usual factorial) and a word for the product (product in the sequences vocabulary), but as it often happens introductory examples overlap with the standard library.

Parsing words

If you've been paying close attention so far, you realize I've lied to you. I said each word acts on the stack in order, but there a few words like [, ], : and ; that don't seem to follow this rule.

These are parsing words and they behave differently from simpler words like 5, [1,b] or drop. We will cover these in more detail when we talk about metaprogramming, but for now it is enough to know that parsing words are special.

They are not defined using the : word, but with the word SYNTAX: instead. When a parsing words is encountered, it can interact with the parser using a well-defined API to influence how successive words are parsed. For instance : asks for the next tokens from the parsers until ; is found and tries to compile that stream of tokens into a word definition.

A common use of parsing words is to define literals. For instance { is a parsing word that starts an array definition and is terminated by }. Everything in-between is part of the array. An example of array that we have seen before is { 1 2 3 4 5 6 7 8 9 10 }.

There are also literals for hashmaps, H{ { "Perl" "Larry Wall" } { "Factor" "Slava Pestov" } { "Scala" "Martin Odersky" } }, and byte arrays, B{ 1 14 18 23 }.

Other uses of parsing word include the module system, the object-oriented features of Factor, enums, memoized functions, privacy modifiers and more. In theory, even SYNTAX: can be defined in terms of itself, although of course the system has to be bootstrapped somehow.

Stack shuffling

Now that you know the basics of Factor, you may want to start assembling more complex words. This may sometimes require you to use variables that are not on top of the stack, or to use variables more than once. There are a few words that can be used to help with this. I mention them now since you need to be aware of them, but I warn you that using too many of these words to manipulate the stack will cause your code to quickly become harder to read and write. Stack shuffling requires mentally simulating moving values on a stack, which is not a natural way to program. In the next section we'll see a much more effective way to handle most needs.

Here is a list of the most common shuffling words together with their effect on the stack. Try them in the listener to get a feel for how they manipulate the stack, and explore the online help to find out more.

dup ( x -- x x )
drop ( x -- )
swap ( x y -- y x )
over ( x y -- x y x )
dupd ( x y -- x x y )
swapd ( x y z -- y x z )
nip ( x y -- y )
rot ( x y z -- y z x )
-rot ( x y z -- z x y )
2dup ( x y -- x y x y )

Combinators

Although the words mentioned in the previous paragraph are occasionally useful (especially the simpler dup, drop and swap), you should write code that does as little stack shuffling as possible. This requires practice getting the function arguments in the right order. Nevertheless, there are certain common patterns of needed stack manipulation that are better abstracted away into their own words.

Suppose we want to define a word to determine whether a given number n is prime. A simple algorithm is to test each number from 2 to the square root of n and see whether it is a divisor of n. In this case, n is used in two places: as an upper bound for the sequence, and as the number to test for divisibility.

The word bi applies two different quotations to the single element on the stack above them, and this is precisely what we need. For instance 5 [ 2 * ] [ 3 ] bi yields

10
8

bi applies the quotation [ 2 * ] to the value 5 and then the quotation [ 3 ] to the value 5 leaving us with 10 and then 8 on the stack. Without bi, we would have to first dup 5, then multiply, and then swap the result of the multiplication with the second 5, so we could do the addition

5 dup 2 * swap 3  

You can see that bi replaces a common pattern of dup, then calculate, then swap and calculate again.

To continue our prime example, we need a way to make a range starting from 2. We can define our own word for this [2,b], using the [a,b] range word we discussed earlier

: [2,b] ( n -- {2,...,n} ) 2 swap [a,b] ; inline

What's up with that inline word? This is one of the modifiers we can use after defining a word, another one being recursive. This will allow us to have the definition of a short word inlined wherever it is used, rather than incurring a function call.

Try our new [2,b] word and see that it works

6 [2,b] >array .

Using [2,b] to produce the range of numbers from 2 to the square root of an n that is already on the stack is easy: sqrt floor [2,b] (technically floor isn't necessary here, as [a,b] works for non-integer bounds). Let's try that out

16 sqrt [2,b] >array .

Now, we need a word to test for divisibility. A quick search in the online help shows that divisor? is the word we want. It will help to have the arguments for testing divisibility in the other direction, so we define multiple?

: multiple? ( a b -- ? ) swap divisor? ; inline

Both of these return t

9 3 divisor? .
3 9 multiple? .

If we're going to use bi in our prime definition, as we implied above, we need a second quotation. Our second quotation needs to test for a value in the range being a divisor of n - in other words we need to partially apply the word multiple?. This can be done with the word curry, like this: [ multiple? ] curry.

Finally, once we have the range of potential divisors and the test function on the stack, we can test whether any element satisfied divisibility with any? and then negate that answer with not. Our full definition of prime looks like

: prime? ( n -- ? ) [ sqrt [2,b] ] [ [ multiple? ] curry ] bi any? not ;

Altough the definition of prime is complicated, the stack shuffling is minimal and is only used in the small helper functions, which are simpler to reason about than prime?.

Notice that prime? uses two levels of quotation nesting since bi operates on two quotations, and our second quotation contains the word curry, which also operates on a quotation. In general, Factor words tend to be rather shallow, using one level of nesting for each higher-order function, unlike Lisps or more generally languages based on the lambda calculus, which use one level of nesting for each function, higher-order or not.

Many more combinators exists other than bi (and its relative tri), and you should become acquainted at least with bi, tri, bi* and bi@ by reading about them in the online help and trying them out in the listener.

Vocabularies

It is now time to start writing your functions in files and learn how to import them in the listener. Factor organizes words into nested namespaces called vocabularies. You can import all names from a vocabulary with the word USE:. In fact, you may have seen something like

USE: math.ranges

when you asked the listener to import the word [1,b] for you. You can also use more than one vocabulary at a time with the word USING:, which is followed by a list of vocabularies and terminated by ;, like

USING: math.ranges sequences.deep ;

Finally, you define the vocabulary where your definitions are stored with the word IN:. If you search the online help for a word you have defined so far, like prime?, you will see that your definitions have been grouped under the default scratchpad vocabulary. By the way, this shows that the online help automatically collects information about your own words, which is a very useful feature.

There are a few more words, like QUALIFIED:, FROM:, EXCLUDE: and RENAME:, that allow more fine-grained control over the imports, but USING: is the most common.

On disk, vocabularies are stored under a few root directories, much like with the classpath in JVM languages. By default, the system starts looking up into the directories basis, core, extra, work under the Factor home. You can add more, both at runtime with the word add-vocab-root, and by creating a configuration file .factor-rc, but for now we will store our vocabularies under the work directory, which is reserved for the user.

Generate a template for a vocabulary writing

USE: tools.scaffold
"github.tutorial" scaffold-work

You will find a file work/github/tutorial/tutorial.factor containing an empty vocabulary. Factor integrates with many editors, so you can try "github.tutorial" edit: this will prompt you to choose your favourite editor, and use that editor to open the newly created vocabulary.

You can add the definitions of the previous paragraph, so that it looks like

! Copyright (C) 2014 Andrea Ferretti.
! See http://factorcode.org/license.txt for BSD license.
USING: ;
IN: github.tutorial

: [2,b] ( n -- {2,...,n} ) 2 swap [a,b] ; inline

: multiple? ( a b -- ? ) swap divisor? ; inline

: prime? ( n -- ? ) [ sqrt [2,b] ] [ [ multiple? ] curry ] bi any? not ;

Since the vocabulary was already loaded when you scaffolded it, we need a way to refresh it from disk. You can do this with "github.tutorial" refresh. There is also a refresh-all word, with a shortcut F2.

You will be prompted a few times to use vocabularies, since your USING: statement is empty. After having accepted all of them, Factor suggests you a new header with all the needed imports:

USING: kernel math.functions math.ranges sequences ;
IN: github.tutorial

Now that you have some words in your vocabulary, you can edit, say, the multiple? word with \ multiple? edit. You will find your editor open on the relevant line of the right file. This also works for words in the Factor distribution, although it may be a bad idea to modify them.

This \ word requires a little explanation. It works like a sort of escape, allowing us to put a reference to the next word on the stack, without executing it. This is exactly what we need, because edit is a word that takes words themselves as arguments. This mechanism is similar to quotations, but while a quotation creates a new anonymous function, here we are directly refering to the word multiple?.

Back to our task, you may notice that the words [2,b] and multiple? are just helper functions that you may not want to expose directly. To hide them from view, you can wrap them in a private block like this

<PRIVATE

: [2,b] ( n -- {2,...,n} ) 2 swap [a,b] ; inline

: multiple? ( a b -- ? ) swap divisor? ; inline

PRIVATE>

After making this change and refreshed the vocabulary, you will see that the listener is not able to refer to words like [2,b] anymore. The <PRIVATE word works by putting all definitions in the private block under a different vocabulary, in our case github.tutorial.private.

It is still possible to refer to words in private vocabularies, as you can confirm by searching for [2,b] in the online help, but of course this is discouraged, since people do not guarantee any API stability for private words. Words under github.tutorial can refer to words in github.tutorial.private directly, like prime? does.

Tests and documentation

This is a good time to start writing some unit tests. You can create a skeleton with

"github.tutorial" scaffold-tests

You fill find a generated file under work/github/tutorial/tutorial-tests.factor, that you can open with "github.tutorial" edit-tests. Notice the line

USING: tools.test github.tutorial ;

that imports the unit testing module as well as your own. We will only test the public prime? function.

Tests are written using the unit-test word, which expects two quotations: the first one containing the expected outputs and the second one containing the words to run in order to get that output. Add these lines to github.tutorial-tests:

[ t ] [ 2 prime? ] unit-test
[ t ] [ 13 prime? ] unit-test
[ t ] [ 29 prime? ] unit-test
[ f ] [ 15 prime? ] unit-test
[ f ] [ 377 prime? ] unit-test
[ f ] [ 1 prime? ] unit-test
[ t ] [ 20750750228539 prime? ] unit-test

You can now run the tests with "github.tutorial" test. You will see that we have actually made a mistake, and pressing F3 will show more details. It seems that our assertions fails for 2.

In fact, if you manually try to run our functions for 2, you will see that our defition of [2,b] returns { 2 } for 2 sqrt, due to the fact that the square root of two is less than two, so we get a descending interval. Try making a fix so that the tests now pass.

There are a few more words to test errors and inference of stack effects. unit-test suffices for now, but later on you may want to check must-fail and must-infer.

We can also add some documentation to our vocabulary. Autogenerated documentation is always available for user-defined words (even in the listener), but we can write some useful comments manually, or even add custom articles that will appear in the online help. Predictably, we start with "github.tutorial" scaffold-docs and then "github.tutorial" edit-docs.

The generated file work/github/tutorial-docs.factor imports help.markup and help.syntax. These two vocabularies define words to generate documentation. The actual help page is generated by the HELP: parsing word.

The arguments to HELP: are nested array of the form { $directive content... }. In particular, you see here the directives $values and$description, but a few more exist, such as $errors, $examples and $see-also.

Notice that the type of the output ? has been inferred to be boolean. Change the first lines to look like

USING: help.markup help.syntax kernel math ;
IN: github.tutorial

HELP: prime?
{ $values
    { "n" fixnum }
    { "?" boolean }
}
{ $description "Tests if n is prime. n is assumed to be a positive integer." } ;

and refresh the github.tutorial vocabulary. If you now look at the help for prime?, for instance with \ prime? help, you will see the updated documentation.

You can also render the directives in the listener for quicker feedback. For instance, try writing

{ $values
    { "n" integer }
    { "?" boolean }
} print-content

The help markup contains a lot of possible directives, and you can use them to write stand-alone articles in the help system. Have a look at some more with "element-types" help.

The object system and protocols

Although it is not apparent from what we have said so far, Factor has object-oriented features, and many core words are actually method invocations. To better understand how objects behave in Factor, a quote is in order:

I invented the term Object-Oriented and I can tell you I did not have C in mind.

Alan Kay

The term object-oriented has as many different meanings as people using it. One point of view - which was actually central to the work of Alan Kay - is that it is about late binding of function names. In Smalltalk, the language where this concept was born, people do not talk about calling a method, but rather sending a message to an object. It is up to the object to decide how to respond to this message, and the caller should not know about the implementation. For instance, one can send the message map both to an array and a linked list, but internally the iteration will be handled differently.

The binding of the message name to the method implementation is dynamic, and this is regarded as the core strenght of objects. As a result, fairly complex systems can evolve from the cooperation of independent objects who do not mess with each other internals.

To be fair, Factor is very different from Smalltalk, but still there is the concept of classes, and generic words can defined having different implementations on different classes.

Some classes are builtin in Factor, such as string, boolean, fixnum or word. Next, the most common way to define a class is as a tuple. Tuples are defined with the TUPLE: parsing word, followed by the tuple name and the fields of the class that we want to define, which are called slots in Factor parlance.

Let us define a class for movies:

TUPLE: movie title director actors ;

This also generates setters >>title, >>director and >>actors and getters title>>, director>> and actors>>. For instance, we can create a new movie with

movie new "The prestige" >>title
  "Christopher Nolan" >>director
  { "Hugh Jackman" "Christian Bale" "Scarlett Johansson" } >>actors

We can also shorten this to

"The prestige" "Christopher Nolan"
{ "Hugh Jackman" "Christian Bale" "Scarlett Johansson" }
movie boa

The word boa stands for 'by-order-of-arguments' and is a constructor that fills the slots of the tuple with the items on the stack in order. movie boa is called a boa constructor, a pun on the Boa Constrictor. It is customary to define a most common constructor called <movie>, which in our case could be simply

: <movie> ( title director actors -- movie ) movie boa ;

In fact, boa constructor are some common, that the above line can be shortened to

C: <movie> movie

In other cases, you may want to use some defaults, or compute some fields.

The functional minded will be worried about the mutability of tuples. Actually, slots can be declared to be read-only with { slot-name read-only }. In this case, the field setter will not be generated, and the value must be set a the beginning with a boa constructor. Other valid slot modifiers are initial: - to declare a default value - and a class word, such as integer, to restrict the values that can be inserted.

As an example, we define another tuple class for rock bands

TUPLE: band
  { keyboards string read-only }
  { guitar string read-only }
  { bass string read-only }
  { drums string read-only } ;
: <band> ( keyboards guitar bass drums -- band ) band boa ;

together with one instance

"Richard Wright" "David Gilmour" "Roger Waters" "Nick Mason" <band>

Now, of course everyone knows that the star in a movie is the first actor, while in a rock band it is the bass player. To encode this, we first define a generic word

GENERIC: star ( item -- star )

As you can see, it is declared with the parsing word GENERIC: and declares its stack effects but it has no implementation right now, hence no need for the closing ;. Generic words are used to perform dynamic dispatch. We can define implementations for various classes using the word M:

M: movie star actors>> first ;
M: band star bass>> ;

If you write star . two times, you can see the different effect of calling a generic word on instances of different classes.

Builtin and tuple classes are not all that there is to the object system: more classes can be defined with set operations like UNION: and INTERSECTION:. Another way to define a class is as a mixin.

Mixins are defined with the MIXIN: word, and existing classes can be added to the mixin writing

INSTANCE: class mixin

Methods defined on the mixin will then be available on all classes that belong to the mixin. If you are familiar with Haskell typeclasses, you will recognize a resemblance, although Haskell enforces at compile time that instance of typeclasses implent certain functions, while in Factor this is informally specified in documentation.

Two important examples of mixins are sequence and assoc. The former defines a protocol that is available to all concrete sequences, such as strings, linked lists or arrays, while the latter defines a protocol for associative arrays, such as hashtables or association lists.

This enables all sequences in Factor to be acted upon with a common set of words, while differing in implementation and minimizing code repetition (because only few primitives are needed, and other operations are defined for the sequence class). The most common operations you will use on sequences are map, filter and reduce, but there are many more - as you can see with "sequences" help.

Learning the tools

A big part of the productivity of Factor comes from the deep integration of the language and libraries with the tools around them, which are embodied in the listener. Many functions of the listener can be used programmatically, and vice versa. You have seen some examples of this:

• the help is navigable online, but you can also invoke it with help and print help items with print-content;
• the F2 shortcut or the words refresh and refresh-all can be used to refresh vocabularies from disk while continuing working in the listener;
• the edit word gives you editor integration, but you can also click on file names in the help pages for vocabularies to open them.

The refresh is actually quite smart. Whenever a word is redefined, words that depend on it are recompiled against the new defition. You can check by yourself doing

: inc ( x -- y ) 1   ;
: inc-print ( x -- ) inc . ;
5 inc-print

and then

: inc ( x -- y ) 2   ;
5 inc-print

This allows you to always keep a listener open, improving your definitions, periodically saving your definitions to file and refreshing, without ever having to reload Factor.

You can also save the whole state of Factor with the word save-image and later restore it by starting Factor with

./factor -i=path-to-image

In fact, Factor is image-based and only uses files when loading and refreshing vocabularies.

The power of the listener does not end here. Elements of the stack can be inspected by clicking on them, or by calling the word inspector. For instance try writing

TUPLE: trilogy first second third ;
: <trilogy> ( first second third -- trilogy ) trilogy boa ;
"A new hope" "The Empire strikes back" "Return of the Jedi" <trilogy>
"George Lucas" 2array

You will get an item that looks like

{ ~trilogy~ "George Lucas" }

on the stack. Try clicking on it: you will be able to see the slots of the array and focus on the trilogy or on the string by double-clicking on them. This is extremely useful for interactive prototyping. Special objects can customize the inspector by implementing the content-gadget method.

There is another inspector for errors. Whenever an error arises, it can be inspected with F3. This allows you to investigate exceptions, bad stack effects declarations and so on. The debugger allows you to step into code, both forwards and backwards, and you should take a moment to get some familiarity with it. You can also trigger the debugger manually, by entering some code in the listener and pressing Ctrl w.

Another feature of the listener allows you to benchmark code. As an example, we write an intentionally inefficient Fibonacci:

DEFER: fib-rec
: fib ( n -- f(n) ) dup 2 < [ ] [ fib-rec ] if ;
: fib-rec ( n -- f(n) ) [ 1 - fib ] [ 2 - fib ] bi   ;

(notice the use of DEFER: to define two mutually recursive words). You can benchmark the running time writing 40 fib and then pressing Ctrl t instead of Enter. You will get timing information, as well as other statistics. Programmatically, you can use the time word on a quotation to do the same.

You can also add watches on words, to print inputs and outputs on entry and exit. Try writing

\ fib watch

and then run 10 fib to see what happens. You can then remove the watch with \ fib reset.

Another very useful tool is the lint vocabulary. This scans word definitions to find duplicated code that can be factored out. As an example, let us define a word to check if a string starts with another one. Create a test vocabulary

"lintme" scaffold-work

and add the following definition

USING: kernel sequences ;
IN: lintme

: startswith? ( str sub -- ? ) dup length swapd head = ;

Load the lint tool with USE: lint and write "lintme" lint-vocab. You will get a report mentioning that the word sequence length swapd is already used in the word (split) of splitting.private, hence it could be factored out.

Now, you would not certainly want to modify the source of a word in the standard library - let alone a private one - but in more complex cases the lint tool is able to find actual repetitions. It is a good idea to lint your vocabularies from time to time, to avoid code duplication and as a good way to discover library words that you may have accidentally redefined.

Finally, there are a few utilities to inspect words. You can see the definition of a word in the help tool, but a quicker way can be see. Or, vice versa, you may use usage. to inspect the callers of a given word. Try \ reverse see and \ reverse usage..

Metaprogramming

We now venture into the metaprogramming world, and write our first parsing word. By now, you have seen a lot of parsing words, such as [. {, H{, USE:, IN:, <PRIVATE, GENERIC: and so on. Each of those is defined with the parsing word SYNTAX: and interacts with Factor's parser.

The parser accumulates tokens onto an accumulator vector, unless it finds a parsing word, which is executed immediately. Since parsing words execute at compile time, they cannot interact with the stack, but they have access to the accumulator vector. Their stack effect must be ( accum -- accum ). Usually what they do is ask the parser for some more tokens, do something with them, and finally push a result on the accumulator vector with the word suffix!.

As an example, we will define a literal for DNA sequences. A DNA sequence is a sequence of one of the bases cytosine, guanine, adenine and thymine, which we will denote by the letters c, g, a, t. Since there are four possible bases, we can encode each with two bits. Let use define a word

: dna>bits ( token -- bits ) {
  { "a" [ { f f } ] }
  { "c" [ { t t } ] }
  { "g" [ { f t } ] }
  { "t" [ { t f } ] }
} case ;

where the first bit represents whether the basis is a purine or a pyrimidine, and the second one identifies bases that pair together.

Our aim is to read a sequence of letters a, c, g, t - possibly with spaces - and convert them to a bit array. Factor supports bit arrays, and literal bit arrays look like ?{ f f t }.

Our syntax for DNA will start with DNA{ and get all tokens until the closing token } is found. The intermediate tokens will be put into a string, and using our function dna>bits we will map this string into a bit array. To read tokens, we will use the word parse-tokens. There are a few higher-level words to interact with the parser, such as parse-until and parse-literal, but we cannot apply them in our case, since the tokens we will find are just sequences of a c g t, instead of valid Factor words. Let us start with a simple approximation that just reads tokens between our delimiters and outputs the string obtained by concatenation

SYNTAX: DNA{ "}" parse-tokens concat suffix!

You can test the effect by doing DNA{ a ccg t a g }, which should output "accgtag". As a second approximation, we transform each letter into a boolean pair:

SYNTAX: DNA{ "}" parse-tokens concat
  [ 1string dna>bits ] { } map-as suffix! ;

Notice the use of map-as instead of map. Since the target collection is not a string, we did not use map, which preserves the type, but map-as, which take as an additional argument an examplar of the target collection - here { }. Our final version flattens the array of pairs with concat and finally makes into a bit array:

SYNTAX: DNA{ "}" parse-tokens concat
  [ 1string dna>bits ] { } map-as
  concat >bit-array suffix! ;

If you try it with DNA{ a ccg t a g } you should get

`?{ f f t t t t f t t f f f f t }`

Let us make another very simple example, stolen from John Benediktsson, which is about infix syntax for ranges. Until now, we have used [a,b] to create a range. We can make a syntax that is friendlier to people coming from other languages using ... as an infix word.

We can use scan-object to ask the parser for the next parsed object, and unclip-last to get the top element from the accumulator vector. This way, we can define ... simply with

SYNTAX: ... unclip-last scan-object [a,b] suffix! ;

You can try it with 12 ... 18 >array.

We only scratched the surface of parsing words; in general, they allow you to perform arbitrary computations at compile time, enabling powerful forms of metaprogramming.

In a sense, Factor syntax is completely flat, and parsing words allow you to introduce syntaxes more complex than a stream of tokens to be used locally. This permits to increase the Factor language by adding many new features as libraries. In principle, it would even be possible to have an external language compile to Factor - say JavaScript - and embed it as a Factor DSL inside the boundaries of a <JS ... JS> parsing word. Some taste is needed not to abuse too much of this to introduce styles that are much too alien in the concatenative world.

When the stack is not enough

Until now I have cheated a bit, and tried to avoid writing examples that would have been too complex to write in concatenative style. Truth is, you will find occasions where this is too restrictive. Fortunately, parsing words allow you to break these restrictions, and Factor comes with a few to handle the most common annoyances.

One thing you may want to do is to actually name local variables. The :: word works like :, but allows you to actually bind the name of stack parameters to variables, so that you can use them multiple times, in the order you want. For instance, let us define a word to solve quadratic equations. I will spare you the purely stack-based version, and present you a version with locals (this will require the locals vocabulary):

:: solveq ( a b c -- x )
  b neg
  b b * 4 a c * * - sqrt
   
  2 a * / ;

In this case we have chosen the sign, but we can do better and output both solutions:

:: solveq ( a b c -- x1 x2 )
  b neg
  b b * 4 a c * * - sqrt
  [   ] [ - ] 2bi
  [ 2 a * / ] bi@ ;

You can check that this definition works with something like 2 -16 30 solveq, which should output both 3.0 and 5.0. Apart from being written in RPN style, our first version of solveq looks exactly the same it would in a language with local variables. For the second definition, we apply both the and - operations to -b and delta, using the combinator 2bi, and then divide both results by 2a using bi@.

There is also support for locals in quotations - using [| - and methods - using M:: - and one can also create a scope where to bind local variables outside definitions using [let. Of course, all of these are actually compiled to concatenative code with some stack shuffling. I encourage you to browse examples for these words, but bear in mind that their usage in practice is actually much less prominent than one would expect - about 1% of Factor's own codebase.

Another more common case happens when you need to specialize a quotation to some values, but these do not appear in the right place. Remember that you can partially apply a quotation using curry. But this assumes that the value you are applying should appear leftmost in the quotation; in the other cases you need some stack shuffling. The word with is a sort of partial application with a hole. It also curries a quotation, but uses the third element on the stack instead of the second. Also, the resulting curried quotation will be applied to an element inserting it in the second position.

The example from the documentation probably tells more than the above sentence: try writing 1 { 1 2 3 } [ / ] with map.

Let me take again prime?, but this time write it without using helper words:

: prime? ( n -- ? ) [ sqrt 2 swap [a,b] ] [ [ swap divisor? ] curry ] bi any? not ;

Using with instead of curry, this simplifies to

: prime? ( n -- ? ) 2 over sqrt [a,b] [ divisor? ] with any? not ;

If you do not visualize what is happening, you may want to consider the fry vocabulary. It defines fried quotations; these are quotations that have holes in them - marked by _ - that are filled with values from the stack.

The first quotation is rewritten more simply as

[ '[ 2 _ sqrt [a,b] ] call ]

Here we use a fried quotation - starting with '[ - to inject the element on the top of the stack in the second position, and then use call to evaluate the resulting quotation. The second quotation becomes simply

[ '[ _ swap divisor? ] ]

so an alternative defition of prime? is

: prime? ( n -- ? ) [ '[ 2 _ sqrt [a,b] ] call ] [ '[ _ swap divisor? ] ] bi any? not ;

Depending on your taste, you may find this version more readable. In this case, the added clarity is probably lost due to the fact that the fried quotations are themselves inside quotations, but occasionally their use can do a lot to simplify the flow.

Finally, there are times where one just wants to give names to variables that are available inside some scope, and use them where necessary. These variables can hold values that are global, or at least not local to a single word. A typical example could be the input and output streams, or database connections.

Factor allows you to create dynamic variables and bind them in scopes. The first thing is to create a symbol for a variable, say

SYMBOL: favorite-language

Then one can use the word set to bind the variable and get to retrieve its values, like

"Factor" favorite-language set
favorite-language get

Scopes are nested, and new scopes can be created with the word with-scope. Try for instance

: on-the-jvm ( -- ) [
  "Scala" favorite-language set
  favorite-language get .
] with-scope ;

If you run on-the-jvm you will get "Scala" printed, but still after execution favorite-language get return "Factor".

All the tools that we have seen in this section should be used when necessary, as they break concatenativity and make words less easy to factor, but they can greatly increase clarity when needed. Factor has a very practical approach and does not shy from offering features that are less pure but nevertheless often useful.

Input/Output

We now leave the tour of the language, and start investigating how to interact with the outside world. I will begin in this section with some examples of input/output, but inevitably this will lead into a discussion of asynchrony. The rest of the tutorial will then go in more detail about parallel and distributed computing.

Factor implements efficient asynchronous input/output facilities, similar to NIO on the JVM or the Node.js I/O system. This means that input and output operations are performed in the background, leaving the foreground task free to perform work while the disk is spinning or the network is buffering packets. Factor is currently single threaded, but asynchrony allows it to be rather performant for applications that are I/O-bound.

All of Factor input/output words are centered on streams. Streams are lazy sequences which can be read or written to, typical examples being files, network ports or the standard input and output. Factor holds a couple of dynamic variables called input-stream and output-stream, which are used by most I/O words. These variables can be rebound locally using with-input-stream, with-output-stream and with-streams. When you are in the listener, the default streams write and read in the listener, but once you deploy your application as an executable, they are usually bound to the standard input and output of your console.

The words <file-reader> and <file-writer> (or <file-appender>) can be used to create a read or write stream to a file, given its path and encoding. Putting everything together, we make a simple example of a word that reads each line of a file encoded in UTF8, and writes the first letter of the line to the listener.

First, we want a safe-head word, that works like head, but returns its input if the sequence is too short. To do so, we will use the word recover, which allows us to declare a try-catch block. It requires two quotations: the first one is executed, and on failure, the second one is executed with the error as input. Hence we can define

: safe-head ( seq n -- seq' ) [ head ] [ 2drop ] recover ;

This is mostly an occasion to show an example of exceptions, as Factor defines the short word, which takes a sequence and a number, and returns the minimum between the length of the sequence and the number. This allows us to write simply

: safe-head ( seq n -- seq' ) short head ;

With this definition, we can make a word to read the first character of the first line:

: read-first-letters ( path -- )
  utf8 <file-reader> [
    readln 1 safe-head write nl
  ] with-input-stream ;

Using the helper word with-file-reader, we can also shorten this to

: read-first-letters ( path -- )
  utf8 [
    readln 1 safe-head write nl
  ] with-file-reader ;

Unfortunately, we are limited to one line. To read more lines, we should chain calls to readln until one returns f. Factor helps us with the word file-lines, which lazily iterates over lines. Our final definition becomes

: read-first-letters ( path -- )
  utf8 file-lines [ 1 safe-head write nl ] each ;

When the file is small, one can also use file-contents to read the whole contents of a file in a single string. Factor defines many more words for input/output, which cover many more cases, such as binary files or sockets.

We end this section investigating some words to walk the filesystem. Our aim is a very minimal implementation of the ls command.

The word directory-entries lists the contents of a directory, giving a list of tuple elements, each one having the slots name and type. You can see this by trying "/home" directory-entries [ name>> ] map. If you inspect the directory entries, you will see that the type is either directory or regular-file (well, there are symlinks as well, but we will ignore them for simplicity). Hence we can define a word that lists files and directories with

: list-files-and-dirs ( path -- files dirs )
    directory-entries [ type>>  regular-file  = ] partition ;

With this, we can define a word ls that will print directory contents as follows:

: ls ( path -- )
  list-files-and-dirs
  "DIRECTORIES:" write nl
  "------------" write nl
  [ name>> write nl ] each
  "FILES:" write nl
  "------" write nl
  [ name>> write nl ] each ;

Try the word on your home directory to see the effects. In the next section, we shall look at how to create an executable for our simple program.

Deploying programs

There are two ways to run Factor programs outside the listener: as scripts, which are interpreted by Factor, or as standalone executable compiled for your platform. Both require you to define a vocabulary with an entry point (altough there is an even simpler way for scripts), so let's do that first.

Start by creating our ls vocabulary with "ls" scaffold-work and make it look like this:

! Copyright (C) 2014 Andrea Ferretti.
! See http://factorcode.org/license.txt for BSD license.
USING: accessors command-line io io.directories io.files.types
  kernel namespaces sequences ;
IN: ls

<PRIVATE

: list-files-and-dirs ( path -- files dirs )
    directory-entries [ type>>  regular-file  = ] partition ;

PRIVATE>

: ls ( path -- )
  list-files-and-dirs
  "DIRECTORIES:" write nl
  "------------" write nl
  [ name>> write nl ] each
  "FILES:" write nl
  "------" write nl
  [ name>> write nl ] each ;

When we run our vocabulary, we will need to read arguments from the command line. Command-line arguments are stored under the command-line dynamic variable, which holds an array of strings. Hence - forgetting any error checking - we can define a word which runs ls on the first command-line argument with

: ls-run ( -- ) command-line get first ls ;

Finally, we use the word MAIN: to declare the main word of our vocabulary:

MAIN: ls-run

Having added those two lines to your vocabulary, you are now ready to run it. The simplest way is to run the vocabulary as a script with the -run flag passed to Factor. For instance to list the contents of my home I can do

./factor -run=ls /home/andrea

In order to produce an executable, we must set some options and call the deploy word. The simplest way to do this graphically is to invoke the deploy-tool word. If you write "ls" deploy-tool, you will be presented with a window to choose deployment options. For our simple case, we will leave the default options and choose Deploy.

After a little while, you should be presented with an executable that you can run like

cd ls
./ls /home/andrea

Try making the ls program more robust by handling missing command-line arguments and non-existent or non-directory arguments.

Multithreading

As we have said, the Factor runtime is single-threaded, like Node. Still, one can emulate concurrency in a single-threaded setting by making use of coroutines. These are essentially cooperative threads, which periodically release control with the yield word, so that the scheduler can decide which coroutine to run next.

Although cooperative threads do not allow to make use of multiple cores, they still have some benefits:

• input/output operations can avoid blocking the entire runtime, so that one can implement quite performant applications if I/O is the bottleneck;
• user interfaces are naturally a multithreaded construct, and they can be implemented in this model, as the listener itself shows;
• finally, some problems may just naturally be easier to write making use of the multithreaded constructs.

For the cases where one wants to make use of multiple cores, Factor offers the possibility of spawning other processes and communicating between them with the use of channels, as we will see in a later section.

Threads in Factors are created out of a quotation and a name, with the spawn word. Let us use this to print the first few lines of Star Wars, one per second, each line being printed inside its own thread. First, let us write those lines inside a dynamic variable:

SYMBOL: star-wars

"A long time ago, in a galaxy far, far away....

It is a period of civil war. Rebel
spaceships, striking from a hidden
base, have won their first victory
against the evil Galactic Empire.

During the battle, rebel spies managed
to steal secret plans to the Empire's
ultimate weapon, the DEATH STAR, an
armored space station with enough
power to destroy an entire planet.

Pursued by the Empire's sinister agents,
Princess Leia races home aboard her
starship, custodian of the stolen plans
that can save her people and restore
freedom to the galaxy...."
"\n" split star-wars set

We will spawn 18 threads, each one printing a line. The operation that a thread must run amounts to

star-wars get ?nth print

Note that dynamic variables are shared between threads, so each one has access to star-wars. This is fine, since it is read-only, but the usual caveats about shared memory in a multithreaded settings apply.

Let us define a word for the thread workload

: print-a-line ( i -- ) star-wars get ?nth print ;

If we give the i-th thread the name "i", our example amounts to

18 [0,b) [
  [ [ print-a-line ] curry ]
  [ number>string ]
  bi spawn
] each

Note the use of curry to send i to the quotation that prints the i-th line. This is almost what we want, but it runs too fast. We need to put the thread to sleep for a while. So we clear the stack that now contains a lot of thread objects and look for the sleep word in the help.

It turns out that sleep does exactly what we need, but it takes a duration object as input. We can create a duration of i seconds with... well i seconds. So we define

: wait-and-print ( i -- ) dup seconds sleep print-a-line ;

Let us try

18 [0,b) [
  [ [ wait-and-print ] curry ]
  [ number>string ]
  bi spawn
] each

Instead of spawn, we can also use in-thread which uses a dummy thread name and discards the returned thread, simplifying the above to

18 [0,b) [
  [ wait-and-print ] curry in-thread
] each

This is good enough for our simple purpose. In serious applications theads will be long-running. In order to make them cooperate, one can use the yield word to signal that the thread has done a unit of work, and other threads can gain control. You also may want to have a look at other words to stop, suspend or resume threads.

Servers and Furnace

A very common case for using more than one thread is when writing server applications. When writing network applications, it is common to start a thread for each incoming connection (remember that these are green threads, so they are much more lightweight than OS threads).

To simplify this, Factor has the word spawn-server, which works like spawn, but in addition repeatedly spawns the quotation until it returns f. This is still a very low-level word: in reality one has to do much more: listen for TCP connections on a given port, handle connection limits and so on.

The vocabulary io.servers allows to write and configure TCP servers. A server is created with the word <threaded-server>, which requires an encoding as a parameter. Its slots can then be set to configure logging, connection limits, ports and so on. The most important slot to fill is handler, which contains a quotation that is executed for each incoming connection. You can see a very simple example of server with

"resource:extra/time-server/time-server.factor" edit-file

We will raise the level of abstraction even more and show how to run a simple HTTP server. First, USE: http.server.

An HTTP application is built out of a responder. A responder is essentially a function from a path and an HTTP request to an HTTP response, but more concretely it is anything that implements the method call-responder*. Responses are instances of the tuple response, so are usually generated calling <response> and customizing a few slots. Let us write a simple echo responder:

TUPLE: echo-responder ;

: <echo-responder> ( -- responder ) echo-responder new ;

M: echo-responder call-responder*
  drop
  <response>
    200 >>code
    "Document follows" >>message
    "text/plain" >>content-type
    swap concat >>body ;

Responders are usually combined to form more complex responders in order to implement routing and other features. In our simplistic example, we will use just this one responder, and set it globally with

<echo-responder> main-responder set-global

Once you have done this, you can start the server with 8080 httpd. You can then visit http://localhost:8080/hello/ /from/ /factor in your browser to see your first responder in action. You can then stop the server with stop-server.

Now, if this was all that Factor offers to write web applications, it would still be rather low level. In reality, web applications are usually written using a web framework called Furnace.

Furnace allows us - among other things - to write more complex actions using a template language. Actually, there are two template languages shipped by default, and we will use Chloe. Furnace allows us to use create page actions from Chloe templates, and in order to create a responder we will need to add routing.

Let use first investigate a simple example of routing. To do this, we create a special type of responder called a dispatcher, that dispatches requests based on path parameters. Let us create a simple dispatcher that will choose between our echo responder and a default responder used to serve static files.

dispatcher new-dispatcher
  <echo-responder> "echo" add-responder
  "/home/andrea" <static> "home" add-responder
  main-responder set-global

Of course, substitute the path /home/andrea with any folder you like. If you start again the server with 8080 httpd, you should be able to see both our simple echo responder (under /echo) and the contents of your files (under /home). Notice that directory listing is disabled by default, you can only access the content of files.

Now that you know how to do routing, we can write page actions in Chloe. Things are starting to become complicated, so we scaffold a vocabulary with "hello-furnace" scaffold-work. Make it look like this:

! Copyright (C) 2014 Andrea Ferretti.
! See http://factorcode.org/license.txt for BSD license.
USING: accessors furnace.actions http http.server
  http.server.dispatchers http.server.static kernel sequences ;
IN: hello-furnace


TUPLE: echo-responder ;

: <echo-responder> ( -- responder ) echo-responder new ;

M: echo-responder call-responder*
  drop
  <response>
    200 >>code
    "Document follows" >>message
    "text/plain" >>content-type
    swap concat >>body ;

TUPLE: hello-dispatcher < dispatcher ;

: <example-responder> ( -- responder )
  hello-dispatcher new-dispatcher
    <echo-responder> "echo" add-responder
    "/home/andrea" <static> "home" add-responder
    <page-action>
      { hello-dispatcher "greetings" } >>template
    "chloe" add-responder ;

Most things are the same as we have done in the listener. The only difference is that we have added a third responder in our dispatcher, under chloe. This responder is created with a page action. The page action has many slots - say, to declare the behaviour of receiving the result of a form - but we only set its template. This is the pair with the dispatcher class and the relative path of the template file.

In order for all this to work, create a file work/hello-furnace/greetings.xml with a content like

<?xml version='1.0' ?>

<t:chloe xmlns:t="http://factorcode.org/chloe/1.0">
  <p>Hello from Chloe</p>
</t:chloe>

Reload the hello-furnace vocabulary and <example-responder> main-responder set-global. You should be able to see the results of your efforts under http://localhost:8080/chloe. Notice that there was no need to restart the server, we can change the main responder dynamically.

This ends our very brief tour of Furnace. It actually does much more than this: form validation and handling, authentication, logging and more. But this section is already getting too long, and you will have to find out more in the documentation.

Processes and channels

As I said, Factor is single-threaded from the point of view of the OS. If we want to make use of multiple cores, we need a way to spawn Factor processes and communicate between them. Factor implements two different models of message-passing concurrency: the actor model, which is based on the idea of sending messages asynchronously between threads, and the CSP model, based on the use of channels.

As a warm-up, we will make a simple example of communication between threads in the same process.

FROM: concurrency.messaging => send receive ;

We can start a thread that will receive a message and print it repeatedly:

: print-repeatedly ( -- ) receive . print-repeatedly ;
[ print-repeatedly ] "printer" spawn

A thread whose quotation starts with receive and calls itself recursively behaves like an actor in Erlang or Akka. We can then use send to send messages to it. Try "hello" over send and then "threading" over send.

Channels are slightly different abstractions, used for instance in Go and in Clojure core.async. They decouple the sender and the receiver, and are usually used synchronously. For instance, one side can receive from a channel before some other party sends something to it. This just means that the receiving end yields control to the scheduler, which waits for a message to be sent before giving control to the receiver again. This feature sometimes makes it easier to synchronize multithreaded applications.

Again, we first use a channel to communicate between threads in the same process. As expected, USE: channels. You can create a channel with <channel>, write to it with to and read from it with from. Note that both operations are blocking: to will block until the value is read in a different thread, and from will block until a value is available.

We create a channel and give it a name with

SYMBOL: ch
<channel> ch set

Then we write to it in a separate thread, in order not to block the UI

[ "hello" ch get to ] in-thread

We can then read the value in the UI with

ch get from

We can also invert the order:

[ ch get from . ] in-thread
"hello" ch get to

This works fine, since we had set the reader first.

Now, for the interesting part: we will start a second Factor instance and communicate via message sending. Factor transparently supports sending messages over the network, serializing values with the serialize vocabulary.

Start another instance of Factor, and run a node server on it. We will use the word <inet4>, that creates an IPv4 address from a host and a port, and the <node-server> constructor

USE: concurrency.distributed
f 9500 <inet4> <node-server> start-server

Here we have used f as host, which just stands for localhost. We will also start a thread that keeps a running count of the numbers it has received.

FROM: concurrency.messaging => send receive ;
: add ( x -- y ) receive   dup . add ;
[ 0 add ] "adder" spawn

Once we have started the server, we can make a thread available with register-remote-thread:

dup name>> register-remote-thread

Now we switch to the other instance of Factor. Here we will receive a reference to the remote thread and start sending numbers to it. The address of a thread is just the address of its server and the name we have registered the thread with, so we obtain a reference to our adder thread with

f 9500 <inet4> "adder" <remote-thread>

Now, we reimport send just to be sure (there is an overlap with a word having the same name in io.sockets, that we have imported)

FROM: concurrency.messaging => send receive ;

and we can start sending numbers to it. Try 3 over send, and then 8 over send - you should see the running total printed in the other Factor instance.

What about channels? We go back to our server, and start a channel there, just as above. This time, though, we publish it to make it available remotely:

USING: channels channels.remote ;
<channel> dup publish

What you get in return is an id you can use remotely to communicate. For instance, I just got 72581372615274718877979307388951222312843084896785643440879198703359628058956 (yes, they really want to be sure it is unique!).

We will wait on this channel, thereby blocking the UI:

swap from .

In the other Factor instance we use the id to get a reference to the remote channel and write to it

f 9500 <inet4> 72581372615274718877979307388951222312843084896785643440879198703359628058956 <remote-channel>
"Hello, channels" over to

In the server instance, the message should be printed.

Remote channels and threads are both useful to implement distributed applications and make good use of multicore servers. Of course, it remains the question how to start worker nodes in the first place. Here we have done it manually - if the set of nodes is fixed, this is actually an option.

Otherwise, one could use the io.launcher vocabulary to start other Factor instances programmatically.

Where to go from here?

We have covered a lot of ground, and I hope that by now you have a feeling whether Factor clicks for you. You can now work your way through the documentation, and hopefully contribute to Factor yourself.

Let me end with a few tips:

• when starting to write Factor, it is very easy to deal a lot with stack shuffling. Learn the combinators well, and do not fear to throw away your first examples;
• no definition is too short: aim for one line;
• the help system and the inspector are your best friends.

To be fair, we also have to mention some drawbacks of Factor:

• first, the community is really small. What they have done is impressive, but do not hope to find a lot of information on the internet;
• the concatenative model is very powerful, but also very hard to get right;
• Factor lacks threads: although the distributed processes make up for it, they incur some cost in serialization;
• finally, Factor does not currently have a package manager, and this probably hinders contribution.

We have to balance the last observation with the convenience of having the whole source tree of Factor available in the image, which certainly makes it easier to learn about libraries. Let me suggest a few vocabularies that you may want to have a look at:

• first, I have not talked a lot about errors and exceptions. Learn more with "errors" help;
• the macros vocabulary implements a form of compile time metaprogramming less general than parsing words, but still quite convenient;
• the models vocabulary lets you implement a form of dataflow programming using objects with observable slots;
• the match vocabulary implements a form of pattern matching;
• the monads vocabulary implements Haskell-style monads.

I think these vocabularies are a testament to the power and expressivity of Factor. Happy hacking!

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