A Quick Introduction to Pd-Lua

Albert Gräf <aggraef@gmail.com>
Computer Music Dept., Institute of Art History and Musicology
Johannes Gutenberg University (JGU) Mainz, Germany
February 2023

This document is licensed under CC BY-SA 4.0. Other formats: Markdown source, PDF
Permanent link: https://agraef.github.io/pd-lua/tutorial/pd-lua-intro.html

Why Pd-Lua?

Pd's facilities for data structures, iteration, and recursion are somewhat limited, thus sooner or later you'll probably run into a problem that can't be easily solved by a Pd abstraction any more. At this point you'll have to consider writing an external object (or just external, for short) in a "real" programming language instead. Pd externals are usually programmed using C, the same programming language that Pd itself is written in. But novices may find C difficult to learn, and the arcana of Pd's C interface may also be hard to master.

Enter Pd-Lua, the Pd programmer's secret weapon, which lets you develop your externals in the Lua scripting language. Pd-Lua was originally written by Claude Heiland-Allen and has since been maintained by a number of other people in the Pd community. Lua, from PUC Rio, is open-source (under the MIT license), mature, very popular, and supported by a large developer community. It is a small programming language, but very capable, and is generally considered to be relatively easy to learn. For programming Pd externals, you'll also need to learn a few bits and pieces which let you interface your Lua functions to Pd, as explained in this tutorial, but programming externals in Lua is still quite easy and a lot of fun.

Using Pd-Lua, you can program your own externals ranging from little helper objects to full-blown sequencers and algorithmic composition tools. Pd-Lua only allows you to program control objects at this time (for doing dsp, you might consider using Faust instead), but it gives you access to Pd arrays and tables, as well as a number of other useful facilities such as clocks and receivers, which we'll explain in some detail. Pd-Lua also ships with a large collection of instructive examples which you'll find helpful when exploring its possibilities.

Note that we can't possibly cover Pd or the Lua language themselves here, so you'll have to refer to other online resources to learn about those. In particular, check out the Lua website, which has extensive documentation available, and maybe have a look at Derek Banas' video tutorial for a quick overview of Lua. For Pd, we recommend the Pd FLOSS Manual at https://flossmanuals.net/ to get started.

Installation

Pd-Lua works inside any reasonably modern Pd flavor. This encompasses vanilla Pd, of course, but also Purr Data which includes an up-to-date version of Pd-Lua for Lua 5.4 and has it enabled by default, so you should be ready to go immediately; no need to install anything else. The same is true for plugdata (version 0.6.3 or later), a Pd flavor which can also run as a plug-in inside a DAW.

With vanilla Pd, you can install the pdlua package from Deken (not recommended, because at the time of this writing that's a really old version based on Lua 5.1). The official Debian package, maintained by IOhannes Zmölnig, is based on Lua 5.2. If you want to use a reasonably up-to-date Lua version, your best bet is to get Pd-Lua from the author's Github repository, which has been updated to work with Lua 5.3 and later. Compilation instructions are in the README, and you'll also find some Mac and Windows binaries there. In either case, after installing Pd-Lua you also have to add pdlua to Pd's startup libraries.

If all is well, you should see a message like the following in the Pd console (note that for vanilla Pd you'll have to switch the log level to 2 or more to see that message):

This will also tell you the Lua version that Pd-Lua is using, so that you can install a matching version of the stand-alone Lua interpreter if needed. Lua should be readily available from your package repositories on Linux, and for Mac and Windows you can find binaries on the Lua website. In the following we generally assume that you're using Lua 5.3 or later.

If all is not well and you do not see that message, then most likely Pd-Lua refused to load because the Lua library is missing. This shouldn't happen if you installed Pd-Lua from a binary package, but if it does then you'll have to manually install the right version of the Lua library to make Pd-Lua work (5.1 for the Deken package, 5.2 for the Debian package, and usually 5.4 otherwise). Make sure that you install the package with the Lua library in it; on Debian, Ubuntu and their derivatives this will be something like liblua5.4-0.

A basic example

With that out of the way, let's have a look at the most essential parts of a Lua external. To make an external, say foo, loadable by Pd-Lua, you need to put it into a Lua script, which is simply a text file with the right name (which must be the same as the object name, foo in this case) and extension (which needs to be .pd_lua), so the file name will be foo.pd_lua in this example.

Any implementation of an object must always include:

Here is a prototypical example (this is the contents of the foo.pd_lua file):

Note that in the first line we called pd.Class:new():register with the name of the object class as a string, which must be the same as the basename of the script, otherwise Pd's loader will get very confused, create the wrong object class, print a (rather cryptic) error message, and won't be able to create the object.

We also assigned the created class (which is represented as a Lua table) to a variable foo (which we made local to the script file here, as explained below). We need that variable as a qualifier for the methods of the object class, including initialize. You can actually name that variable whatever you want, as long as you use that name consistently throughout the script. This can be useful at times, if the actual class name you chose, as it is known to Pd and set with pd.Class:new():register (as well as being the basename of your .pd_lua script), is a jumble of special characters such as fo:o#?!, which isn't a valid Lua identifier.

Next comes the initialize method, which is implemented as a Lua function, prefixing the method name with the name of the class variable we created above and a colon, i.e., foo:initialize. (This colon syntax is used for all functions that represent methods, which receive the called object as an implicit self parameter; please check the section on function definitions in the Lua manual for details.) As a bare minimum, as is shown here, this method must return true, otherwise the loader will assume that the object creation has failed, and will complain that it couldn't create the object with an error message.

We mention in passing here that Pd-Lua also provides a parameter-less postinitialize method which can be used to execute code after the object has been created, but before the object starts processing messages. We'll see an example of this method later.

NOTE: Pd-Lua runs all Lua objects in the same instance of the Lua interpreter. Therefore, as a general guideline, we want to keep the global name space tidy and clean. That's why we made foo a local variable, which means that its scope is confined to this single script. Note that this isn't needed for the member variables and methods, as these are securely stowed away inside the object and not accessible from the outside anyway, if the class variable is local. But the same caveat applies to all variables and functions in the script file that might be needed to implement the object, so normally you want to mark these as local, too (or turn them into member variables and methods, if that seems more appropriate).

We mention in passing that global variables and functions may also have their uses if you need to share a certain amount of global state between different Lua objects. But even then it's usually safer to have the objects communicate with each other behind the scenes using receivers, which we'll explain later.

To actually use the object class we just created, Pd needs be able to find our foo.pd_lua file. We'll discuss different approaches in the following section, but the easiest way to achieve this is to just drop foo.pd_lua into the directory that your patch is in (say, pd-lua in your home directory). Now we can just create our first foo object (hit Ctrl+1, then type the object name foo), and we should see something like this:

First light

Hooray, it works! :)) Well, this object doesn't do anything right now, so let's equip it with a single inlet/outlet pair. This is what the initialize method is for, so we have to edit that method accordingly.

NB: If you closed the editor already and don't remember where the file is, you can just right-click the object and choose Open, which will open the .pd_lua file in your favorite text editor, as configured in your desktop and/or shell environment.

Note that, as we already mentioned, the self variable here is an implicit parameter of any Lua method, which refers to the object itself. Every Pd-Lua object has two member variables inlets and outlets which let us specify the number of inlets and outlets our object should have. This needs to be done when the object is initialized; afterwards, the number of inlets and outlets is set in stone and can't be changed any more.

Next, we have to make sure that Pd picks up our edited class definition. Since the Pd-Lua loader will never reload the .pd_lua file for any given object class during a Pd session, we will have to save the patch, quit Pd, relaunch it and reopen the patch:

Inlet and outlet

So there, we got our single inlet/outlet pair now. To do anything with these, we finally have to add some message handlers to our object. Say, for instance, we want to handle a bang message by incrementing a counter and outputting its current value to the outlet. We first have to initialize the counter value in the initialize method. As we want each foo object to have its own local counter value, we create the counter as a member variable:

It's not necessary to declare the self.counter variable in any way, just give it an initial value and be done with it. Finally, we have to add a method for the bang message, which looks as follows:

We'll dive into the naming conventions for message handlers later, but note that in_1 specifies the first (and only) inlet and bang the kind of message we expect. In the body of the method we increment the self.counter value and output its new value on the first (and only) outlet. This is done by the predefined self:outlet method which takes three arguments: the outlet number, the (Pd) data type to output, and the output value itself. (In general, it's possible to have multiple values there, e.g., when outputting a list value. Therefore the output value is always specified as a Lua table, hence the curly braces around the float output value.)

Throwing everything together, our Lua external now looks as follows:

So let's relaunch Pd, reload the patch again, and add some GUI elements to test it out:

Basic example

Note that this is still a very basic example. While the example is complete and fully functional, we have barely scratched the surface here. Pd-Lua also allows you to process an object's creation arguments (employing the atoms parameter of the initialize method, which we didn't use above), log messages and errors in the Pd console, create handlers for different types of input messages, output data to different outlets, work with Pd arrays, clocks, and receivers, and even do some live coding. We will dive into each of these topics in the following sections.

Where your Lua files go

As already mentioned, the externals (.pd_lua files) themselves can go either into the directory of the patch using the external, or into any other directory on Pd's search path (on Linux, this generally includes, ~/.pd-externals, or ~/.pd-l2ork-externals when running pd-l2ork or purr-data).

The Lua loader temporarily sets up Lua's package.path so that it includes the directory with the external, so you can put any Lua modules (.lua files) required by the external into that directory.

If you need/want to use Lua libraries from other locations (which aren't on the standard Lua package.path), then you'll have to set up the LUA_PATH environment variable accordingly. When using LuaRocks, Lua's most popular package manager, it usually takes care of this for you when set up properly. Otherwise you can set LUA_PATH manually in your system startup files, such as ~/.bashrc or ~/.xprofile on Linux. E.g.:

Note that ? is a placeholder for the module name, the semicolon ; can be used to separate different locations, and a double semicolon ;; adds Lua's standard search path (make sure that you quote those special characters so that the shell doesn't try to interpret them). You should always include the double semicolon somewhere, otherwise the Lua interpreter won't be able to find its standard library modules any more. Also note that you may want to place the ;; in front of the path instead, if the standard locations are to be searched before your custom ones.

Creation arguments

Besides the implicit self argument, the initialize method has two additional parameters:

For instance, let's say that we want to equip our foo object with an optional creation argument, a number, to set the initial counter value. This can be done as follows:

Here we check that the first creation argument is a number. In that case we use it to initialize the counter member variable, otherwise a default value of 0 is set. Note that if there is no creation argument, atoms[1] will be nil which is of type "nil", in which case the zero default value will be used.

Note that currently our bang handler outputs the value after incrementing it, which seems a bit awkward now that we can actually specify the counter's start value. Let's rework that method so that it spits out the current value before incrementing it:

Note that it's perfectly fine to invoke self:outlet at any point in the method.

While we're at it, we might as well add an optional second creation argument to specify the step value of the counter. Try doing that on your own, before peeking at the solution below!

Got it? Good. Here is our final script:

That was easy enough. If you've been following along, you also know by now how to reload the patch and add a few bits to test the new features. For instance:

Creating arguments

Log messages and errors

As soon as your objects get more complicated, you'll probably want to add some messages indicating to the user (or yourself) what's going on inside the object's methods. To these ends, Pd-Lua provides the following two facilities which let you output text messages to the Pd console:

For instance, let's use these facilities to have our foo object post the initial counter value in the initialize method, as well as report an error if any of the given creation arguments is of the wrong type. Here is the suitably modified initialize method:

And here's how the console log looks like after loading our test patch, and creating an erroneous foo bad object:

Log messages

Note that the foo bad object was still created with the appropriate defaults after the error message, so the initialize method ran through to the end alright. If you want the object creation to fail after printing the error message, you only have to add a return false statement in the elseif branch, after the call to self:error. Try it! (Of course, you won't be able to locate the object using the printed error message in this case, since the object wasn't actually created. But "Find Last Error" will still work, since Pd itself will also print a "couldn't create" error message.)

Here's another fun exercise: Let's have foo print a welcome message when it first gets invoked. This can be done by adding a variable init to the foo class itself, which is shared between different object instances, as follows:

You should put this after the definition of foo (i.e., after the line with the pd.Class:new() call), but before any code that uses this variable. Note that we could also just have used an ordinary local variable at script level instead, but this illustrates how you create static class members in Lua.

You still have to add the code which outputs the welcome message. An obvious place for this is somewhere in initialize, but here we use the postinitialize method for illustration purposes:

This will print the message just once, right after the first foo object is created. There's another finalize method which can be used to perform any kind of cleanup when an object gets destroyed. For instance, let's rework our example so that it keeps track of the actual number of foo objects, and prints an additional message when the last foo object is deleted. To these ends, we turn foo.init into a counter which keeps track of the number of foo objects:

Here are the messages logged in the console if we now load our test patch and then go on to delete all foo objects in it:

Lua errors

We all make mistakes. It's inevitable that you'll run into errors in the Lua code you wrote, so let's finally discuss how those mishaps are handled. Pd-Lua simply reports errors from the Lua interpreter in the Pd console. For instance, suppose that we mistyped pd.post as pd_post in the code for the one-time welcome message above. You'll see an error message like this in the console:

In this case the error happened in the initialize method, so the object couldn't actually be created, and you will have to correct the typo before going on. Fortunately, the message tells us exactly where the error occurred, so we can fix it easily. Syntax errors anywhere in the script file will be caught and handled in a similar fashion.

Runtime errors in inlet methods, on the other hand, will allow your objects to be created and to start executing; they just won't behave as expected and cause somewhat cryptic errors to be printed in the console. For instance, let's suppose that you forgot the curly braces around the float value in self:outlet (a fairly common error), so that the method reads:

Lua is a dynamically-typed language, so this little glitch becomes apparent only when you actually send a bang message to the object, which causes the following errors to be logged in the console:

Ok, so the first message tells us that somewhere Pd-Lua expected a table but got a non-table value. The second message actually comes from the C routine deep down in the bowls of Pd-Lua which does the actual output to an outlet. If you see that message, it's a telltale sign that you tried to output an atom not properly wrapped in a Lua table, but it gives no indication of where that happened either, other than that you can use "Find Last Error" to locate the object which caused the problem.

It goes without saying that the Pd-Lua developers could have chosen a better error message there. Well, at least we now have an idea what happened and in which object, but we may then still have to start peppering our code with pd.post calls in order to find (and fix) the issue.

Inlets and outlets

As we've already seen, the number of inlets and outlets is set with the inlets and outlets member variables in the initialize method of an object. You can set these to any numbers you want, including zero. (In the current implementation, fractional numbers will be truncated to integers, and negative numbers will be treated as zero. If the variables aren't set at all, they also default to zero.)

Inlets

Let's have a look at the inlets first. Pd-Lua supports a number of different forms of inlet methods which enable us to process any kind of Pd message. In the following list, "1" stands for any literal inlet number (counting the inlets from left to right, starting at 1), and "sym" for any symbol denoting either one of the predefined Pd message types (bang, float, symbol, pointer, and list), or any other (selector) symbol at the beginning of a Pd meta message. Note that, as usual, in your code these methods are always prefixed with the class name, using Lua colon syntax.

These alternatives are tried in the indicated order, i.e., from most specific to most general. In addition, Pd-Lua understands the following specific sym type selectors and adjusts the number and type of the extra ... arguments accordingly:

Note that there can only be zero or one additional arguments in this case (besides the inlet number for in_n_sym). In contrast, the two most generic kinds of methods, in_1 and in_n, always have the type/selector symbol sel (a string) and the remaining message arguments atoms (a Lua table) as arguments.

Among these, the methods for bang, float, and list are probably the most frequently used, along with in_1 or in_n as a catch-all method for processing any other kind of input message. We've already employed the in_1_bang method in our basic example above. Here are some (rather contrived) examples for the other methods; we'll see some real examples of some of these later on.

(Note that we omitted the pointer type in the above examples, as it is rarely used in Lua externals. But if you want, you can also receive such values, which will be represented as "userdata" a.k.a. C pointers on the Lua side. In Lua 5.4 it is possible to print such values using the %p format specifier of string.format, while in older Lua versions you will have to use the Lua tostring() function for this purpose.)

Outlets

Luckily, things are much simpler on the output side. As we've already seen, to output a message to an outlet, you simply call self:outlet(n, sel, atoms) with the following arguments:

Here are some common examples:

Usually, self:outlet will be called in the inlet methods of an object, but you'll also see it in clocks and receivers, which we'll discuss later.

Note that, by convention, most Pd objects handle inlets and outlets in a certain order, namely:

You'll also see this guideline being used in the Fibonacci number example in the next section. Let us emphasize again that this is merely a convention and thus you're not obliged to follow it, but most built-in and external Pd objects do. Thus if your Lua object works differently for no good reason, then seasoned Pd users will think that it is malfunctioning. There are some rare cases, however, where it's legitimate to deviate from these rules. Consider, for instance, the built-in timer object whose right inlet is the "hot" one.

Fibonacci number example

Nobody in their right mind would actually bother to implement counters in Lua, since they're very easy to do directly in Pd. So let's now take a look at a slightly more interesting example, the Fibonacci numbers. It is also instructive to see how surprisingly difficult it is to write this as a Pd abstraction (you should actually give it a try), whereas it is really dead-easy in Lua.

If you know some math or have studied the Golden ratio, then you've probably heard about these. Starting from the pair 0, 1, the next number is always the sum of the two preceding ones: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, etc. It goes without saying that these numbers grow pretty quickly (with the ratio of successive numbers approaching the Golden ratio). Thus we may want to limit their range, which is also useful if we want to use these numbers in a musical context, e.g., employing them as the basis of MIDI note numbers. One idea which produces both mathematically and musically interesting results is to take the numbers modulo m, i.e., just retain their remainders when divided by the given modulus. As these sequences all have a finite range, they must repeat eventually, but they have a surprisingly large period (also known as the Pisano period in number theory) even for small values of m.

So, without any further ado, here is a Pd-Lua object which calculates the Fibonacci numbers for a given modulus (10 by default, which, as Wikipedia will tell you, has a Pisano period of 60). We actually compute (and output) the numbers in pairs, since we have to keep track of the pairs anyway in order to compute them efficiently.

And here you can see the object running in a little test patch which outputs the two streams of Fibonacci notes to two different MIDI channels. The two streams can be enabled and disabled individually with the corresponding spigots, and you can also change the modulus on the fly.

Fibonacci example

Using arrays and tables

Pd's arrays provide an efficient means to store possibly large vectors of float values. These are often used for sample data (waveforms) of a given size (the number of samples), but can also be employed to store copious amounts of numerical control data. Arrays are usually associated with a graphical display (called a graph), and Pd's table object lets you create an array along with a graph as a special kind of subpatch.

While Pd-Lua cannot currently process audio data in real-time, it does provide a Table class to represent array and table data, and a few functions to query and manipulate that data. This comes in handy, e.g., if you want to fill an array with a computed waveform. While Pd has its own corresponding facilities, complicated waveforms are often much easier to create in Lua, which offers a fairly complete set of basic mathematical functions in its standard library, and a whole lot more through 3rd party libraries such as Numeric Lua.

Here are the array/table functions provided by Pd-Lua. Note that like in Pd arrays, indices are zero-based and thus range from 0 to tab:length()-1.

One important point worth mentioning here is that arrays and tables are subject to change at any time in Pd, as they may have their properties changed, be deleted, and recreated with new parameters. This means that you will have to call pd.Table:new():sync(name), assign it to a local variable, and check that value every time you want to access the Pd array in a method.

Here is a simple example of a luatab object which takes the array name as a creation argument, and generates a waveform of the given frequency whenever a float value is received on the single inlet. After finishing generating the waveform, a bang message is output on the single outlet.

And here is a sample patch running the luatab object:

Table example

In the same vein, the Pd-Lua distribution includes a much more comprehensive example ltabfill.pd_lua, which leverages Lua's load function to create a waveform from a user-specified Lua function created dynamically at runtime (instead of being hard-coded into the Lua code, which is what we did above).

Using clocks

Clocks are used internally in Pd to implement objects which "do things" when a timeout occurs, such as delays, pipes, and metronomes. Pd-Lua exposes this functionality so that objects written in Lua can do the same. The following functions are provided:

We mention in passing that you can call self.clock:delay(time) as soon as the clock has been created, even in the initialize method of an object. Furthermore, you can have as many clocks as you want in the same object, carrying out different actions, as long as you assign each clock to a different method.

Presumably, the self.clock:destruct() method should also be invoked automatically when setting self.clock to nil, but the available documentation isn't terribly clear on this. So we recommend explicitly calling self.clock:destruct() in self:finalize to be on the safe side, as the documentation advises us to do, because otherwise "weird things will happen."

Also note that self.clock:set() isn't terribly useful right now, because it refers to Pd's internal "systime" clock which isn't readily available in Pd-Lua.

With these caveats in mind, here is a little tictoc object we came up with for illustration purposes, along with the usual test patch.

Clock example

More comprehensive examples using clocks can be found in the Pd-Lua distribution; have a look, e.g., at ldelay.pd_lua and luametro.pd_lua. Also, lpipe.pd_lua demonstrates how to dynamically create an entire collection of clocks in order to implement a delay line for a stream of messages.

Using receivers

As every seasoned Pd user knows, Pd also enables you to transmit messages in a wireless fashion, using receiver symbols, or just receivers for short, as destination addresses. In Pd, this is done through the built-in send and receive objects, as well as the "send" and "receive symbol" properties of GUI objects.

Sending messages to a receiver in Pd-Lua is straightforward:

This works pretty much like the outlet method, but outputs messages to the given receiver instead. For instance, let's say you have a toggle with receiver symbol onoff in your patch, then you can turn on that toggle with a call like pd.send("onoff", "float", {1}). (Recall that the atoms argument always needs to be a table, even if it is a singleton, lest you'll get that dreaded "no atoms??" error that we discussed earlier).

One complication are receiver symbols using a $0- patch id prefix, which are commonly used to differentiate receiver symbols in different toplevel patches or abstractions, in order to prevent name clashes. A Pd-Lua object doesn't have any idea of what toplevel patch it is located in, and what the numeric id of that patch is, so you'll have to expand the $0- prefix on the Pd side and pass it, e.g., as a creation argument. For instance, suppose that the toggle receiver is in fact named $0-onoff, then something like the following Pd-Lua object will do the trick, if you invoke it as luasend $0-onoff:

Of course, this also handles ordinary receive symbols just fine if you pass them as a creation argument. Here is a little test patch showing luasend in action:

Send example

It is worth noting here that the same technique applies whenever you need to pass on "$" arguments to a Pd-Lua object in a Pd abstraction.

So let's have a look at receivers now. These work pretty much like clocks in that you create them registering a method, and destroy them when they are no longer needed:

Note that the same caveat applies to receivers as in the case of clocks. That is, you should use the destruct method to destroy receivers in the finalize routine of the receiving object, so that they don't hang around when their object is long dead. Otherwise, you guessed it, "weird things will happen."

Here is a little example which receives any kind of message, stores it, and outputs the last stored message when it gets a bang on its inlet.

The obligatory test patch:

Receive example

Live coding

I've been telling you all along that in order to make Pd-Lua pick up changes you made to your .pd_lua files, you have to relaunch Pd and reload your patches. Well, in this section we are going to discuss Pd-Lua's live coding features, which let you modify your sources and have Pd-Lua reload them on the fly, without ever exiting the Pd environment. This rapid incremental style of development is one of the hallmark features of dynamic programming environments like Pd and Lua. Musicians also like to employ it to modify their algorithmic composition programs live on stage, which is where the term "live coding" comes from. You'll probably be using live coding a lot while developing your Pd-Lua externals, but I've kept this topic for the final section of this guide, because it requires a good understanding of Pd-Lua's basic features. So without any further ado, let's dive right into it now.

First, we need to describe the predefined Pd-Lua object classes pdlua and pdluax, so that you know what's readily available. But we'll also discuss how to add a reload message to your existing object definitions. This is quite easy to do by directly employing Pd-Lua's dofile method, which is also what both pdlua and pdluax use internally.

pdlua

The pdlua object accepts a single kind of message of the form load filename on its single inlet, which causes the given Lua file to be loaded and executed. Since pdlua has no outlets, its uses are rather limited. However, it does enable you to load global Lua definitions and execute an arbitrary number of statements, e.g., to post some text to the console or transmit messages to Pd receivers using the corresponding Pd-Lua functions. For instance, here's a little Lua script loadtest.lua which simply increments a global counter variable (first initializing it to zero if the variable doesn't exist yet) and posts its current value in the console:

To run this Lua code in Pd, you just need to connect the message load loadtest.lua to pdlua's inlet (note that you really need to specify the full filename here, there's no default suffix):

pdlua example

Now, each time you click on the load loadtest.lua message, the file is reloaded and executed, resulting in some output in the console, e.g.:

Also, you can edit the script between invocations and the new code will be loaded and used immediately. E.g., if you change counter + 1 to counter - 1, you'll get:

That's about all there is to say about pdlua; it's a very simple object.

pdluax

pdluax is a bit more elaborate and allows you to create real Pd-Lua objects with an arbitrary number of inlets, outlets, and methods. To these ends, it takes a single creation argument, the basename of a .pd_luax file. This file is a Lua script returning a function to be executed in pdluax's own initialize method, which contains all the usual definitions, including the object's method definitions, in its body. This function receives the object's self as well as all the extra arguments pdluax was invoked with, and should return true if creation of the object succeeded.

For instance, here's a simplified version of our foo counter object, rewritten as a .pd_luax file, to be named foo.pd_luax:

Note the colon syntax self:in_1_bang(). This adds the bang method directly to the self object rather than its class, which is pdluax. (We obviously don't want to modify the class itself, which may be used to create any number of different kinds of objects, each with their own collection of methods.) Also note that the outer function is "anonymous" (nameless) here; you can name it, but there's usually no need to do that, because this function will be executed just once, when the corresponding pdluax object is created. Another interesting point to mention here is that this approach of including all the object's method definitions in its initialization method works with regular .pd_lua objects, too; try it!

In the patch, we invoke a .pd_luax object by specifying the basename of its script file as pdluax's first argument, adding any additional creation arguments that the object itself might need:

pdluax example

These pdluax foo objects work just the same as their regular foo counterparts, but there's an important difference: The code in foo.pd_luax is loaded every time you create a new pdluax foo object. Thus you can easily modify that file and just add a new pdluax foo object to have it run the latest version of your code. For instance, in foo.pd_luax take the line that reads:

Now change that + operator to -:

Don't forget to save your edits, then go back to the patch and recreate the pdluax foo object on the left. The quickest way to do that is to just delete the object, then use Pd's "Undo" operation, Ctrl+Z. Et voilà: the new object now decrements the counter rather than incrementing it. Also note that the other object on the right still runs the old code which increments the counter; thus you will have to give that object the same treatment if you want to update it, too.

While pdluax is considered Pd-Lua's main workhorse for live coding, it also has its quirks. Most notably, the syntax is different from regular object definitions, so you have to change the code if you want to turn it into a .pd_lua file. Also, having to recreate an object to reload the script file is quite disruptive (it resets the internal state of the object), and may leave objects in an inconsistent state (different objects may use various different versions of the same script file). Sometimes this may be what you want, but it makes pdluax somewhat difficult to use. It's not really tailored for interactive development, but it shines if you need a specialized tool for changing your objects on a whim in a live situation.

Fortunately, if you're not content with Pd-Lua's built-in facilities for live coding, it's easy to roll your own using the internal dofile method, which is discussed in the next subsection.

dofile and dofilex

So let's discuss how to use dofile in a direct fashion. Actually, we're going to use its companion dofilex here, which works the same as dofile, but loads Lua code relative to the "externdir" of the class (the directory of the .pd_lua file) rather than the directory of the Pd canvas with the pdlua object, which is what dofile does. Normally, you don't have to worry about these intricacies, but they will matter if Lua class names are specified using relative pathnames, such as ../foo or bar/baz. Since we're reloading class definitions here, it's better to use dofilex so that our code doesn't break if we later change the directory structure.

The method we sketch out below is really simple and doesn't have any of the drawbacks of the pdluax object, but you still have to add a small amount of boilerplate code to your existing object definition. Here is how dofilex is invoked:

The return values of dofilex are those of the Lua script, along with the path under which the script was found. If the script itself returns no value, then only the path will be returned. (We don't use any of this information in what follows, but it may be useful in more elaborate schemes. For instance, pdluax uses the returned function to initialize the object, and the path in setting up the object's actual script name.)

Of course, dofilex needs the name of the script file to be loaded. We could hardcode this as a string literal, but it's easier to just ask the object itself for this information. Each Pd-Lua object has a number of private member variables, among them _name (which is the name under which the object class was registered) and _scriptname (which is the name of the corresponding script file, usually this is just _name with the .pd_lua extension tacked onto it). The latter is what we need. Pd-Lua also offers a whoami() method for this purpose, but that method just returns _scriptname if it is set, or _name otherwise. Regular Pd-Lua objects always have _scriptname set, so it will do for our purposes.

Finally, we need to decide how to invoke dofilex in our object. The easiest way to do this is to just add a message handler (i.e., an inlet method) to the object. For instance, say that the object is named foo which is defined in the foo.pd_lua script. Then all you have to do is add something like the following definition to the script:

As we already discussed, this code uses the object's internal _scriptname variable, and so is completely generic. You can just copy this over to any .pd_lua file, if you replace the foo prefix with whatever the name of your actual class variable is. With that definition added, you can now just send the object a reload message whenever you want to have its script file reloaded.

NOTE: This works because the pd.Class:new():register("foo") call of the object only registers a new class if that object class doesn't exist yet; otherwise it just returns the existing class.

By reloading the script file, all of the object's method definitions will be overwritten, not only for the object receiving the reload message, but for all objects of the same class, so it's sufficient to send the message to any (rather than every) object of the class. Also, existing object state (as embodied by the internal member variables of each object) will be preserved.

In general all this works pretty well, but there are some caveats, too. Note that if you delete one of the object's methods, or change its name, the old method will still hang around in the runtime class definition until you relaunch Pd. That's because reloading the script does not erase any old method definitions, it merely replaces existing and adds new ones.

Finally, keep in mind that reloading the script file does not re-execute the initialize method. This method is only invoked when an object is instantiated. Thus, in particular, reloading the file won't change the number of inlets and outlets of an existing object. Newly created objects will pick up the changes in initialize, though, and have the proper number of inlets and outlets if those member variables were changed.

Let's give this a try, using luatab.pd_lua from the "Using arrays and tables" section as an example. In fact, that's a perfect showcase for live coding, since we want to be able to change the definition of the waveform function f in luatab:in_1_float on the fly. Just add the following code to the end of luatab.pd_lua:

Now launch the luatab.pd patch and connect a reload message to the luatab wave object, like so:

Livecoding example 1

Next change the wavetable function to whatever you want, e.g.:

Return to the patch, click the reload message, and finally reenter the frequency value, so that the waveform gets updated:

Livecoding example 2

Remote control

The method sketched out in the preceding subsection works well enough for simple patches. However, having to manually wire up the reload message to one object of each class that you're editing is still quite cumbersome. In a big patch, which is being changed all the time, this quickly becomes unwieldy. Wouldn't it be nice if we could equip each object with a special receiver, so that we can just click a message somewhere in the patch to reload a given class, or even all Pd-Lua objects at once? And maybe even do that remotely from the editor, using the pdsend program?

Well, all this is in fact possible, but the implementation is a bit too involved to fully present it here. So we have provided this in a separate pdx.lua module, which you can find in the sources accompanying this tutorial.

Setting up an object for this kind of remote control is easy, though. First, you need to import the pdx module into your script, using Lua's require:

Then just call pdx.reload(self) somewhere in the initialize method. This will set up the whole receiver/dofilex machinery in a fully automatic manner. Finally, add a message like this to your patch, which goes to the special pdluax receiver (note that this is completely unrelated to the pdluax object discussed previously, it just incidentally uses the same name):

When clicked, this just reloads all Pd-Lua objects in the patch, provided they have been set up with pdx.reload. You can also specify the class to be reloaded (the receiver matches this against each object's class name):

Or maybe name several classes, like so:

You get the idea. Getting set up for remote control via pdsend isn't much harder. E.g., let's say that we use UDP port 4711 on localhost for communicating with Pd, then you just need to connect netreceive 4711 1 to the ; pdluax reload message in a suitable way, e.g.:

Remote control 1

You can then use pdsend 4711 localhost udp to transmit the reload message to Pd when needed. You probably don't want to run those commands yourself, but a decent code editor will let you bind a keyboard command which does this for you. Myself, I'm a die-hard Emacs fan, so I've included a little elisp module pd-remote.el in the accompanying examples which shows how to do this. Once you've added this to your .emacs, you can just type Ctrl+C Ctrl+K in Emacs to make Pd reload your Lua script after saving it. It doesn't get much easier than that. Moreover, for your convenience I've added a little abstraction named pd-remote.pd which takes care of the netreceive and messaging bits and will look much tidier in your patches.

NOTE: To use these facilities in your own patches, you'll have to copy pdx.lua and pd-remote.pd to your project directory or some other place where Pd finds them. The pd-remote.el file can be installed in your Emacs site-lisp directory if needed. Please also check the pd-remote repository on GitHub for the latest pd-remote version and further details. This also includes a pointer to a Visual Studio Code extension written by Baris Altun which can be used as a replacement for pd-remote.el if you're not familiar with Emacs, or just prefer to use VS Code as an editor.

So here's the full source code of our reworked luatab example (now with the in_1_reload handler removed and the pdx.reload call added to the initialize method):

And here's a little gif showing the above patch in action. You may want to watch this in Typora or your favorite web browser to make the animation work.

Remote control 2

So there you have it: three (or rather four) different ways to live-code with Pd-Lua. Choose whatever best fits your purpose and is the most convenient for you.

Conclusion

Congratulations! If you made it this far, you should have learned more than enough to start using Pd-Lua successfully for your own projects. You should also be able to read and understand the many examples in the Pd-Lua distribution, which illustrate all the various features in much more detail than we could muster in this introduction. You can find these in the examples folder, both in the Pd-Lua sources and the pdlua folder of your Pd installation.

The examples accompanying this tutorial (including the pdx.lua, pdlua-remote.el and pdlua-remote.pd files mentioned at the end of the previous section) are also available for your perusal in the examples subdirectory of the folder where you found this document.

Kudos to Claude Heiland-Allen for creating such an amazing tool, it makes programming Pd externals really easy and fun. Thanks are also due to Roberto Ierusalimschy for Lua, which for me is one of the best-designed, easiest to learn, and most capable multi-paradigm scripting languages there are today, while also being fast, simple, and light-weight.