FAQ

About Pony

How’d Pony come to be?

Check out “An Early History of Pony”.

Why’s it called “Pony”?

Interesting question. We hear that a lot. If you read enough of “An Early History of Pony”, you’ll get your answer.

What makes Pony different?

See the “What makes Pony different” section of “Discover”.

Why would I use Pony instead of language X?

That’s a hard question to answer. Language X is probably very compelling for some problems. It’s probably less compelling for others. Such is computers. In the end, the best we can do is tell you what Pony is good at and you can make the decision for yourself. To learn more about Pony, we suggest checking out the “Discover” section of the website. There’s a portion of that section called “Why Pony” that might answer your question.

Where can I find the Pony roadmap?

There is no official roadmap. Pony is a volunteer driven project. Unlike many programming languages, we don’t have corporate backing. Our users add features and fix issues based on their needs. Pony users solve the problems that matter to them, and we all benefit.

Many of us who are regular contributors share some general goals as we move towards an official 1.0 release. We are working towards making Pony a stable, rock-solid platform for writing high-performance, concurrent applications.

We invite you to join our small but growing community and help push Pony forward. We’re still at an early stage, and new community members can have a huge influence on the language. Join us!

Does Pony really prevent data races?

So, this question usually comes in many different forms. And the question usually arises from a misunderstanding of the difference between a “data race” and a “race condition”.

Pony prevents data races. It can’t stop you from writing race conditions into your program.

To learn more about the differences between race conditions and data races, check out “Race Condition vs. Data Race” by John Regehr.

Are there any examples of something complex built in Pony?

Yes! Here’s a few projects we regularly point people towards:

  • Jylis - A distributed in-memory database for Conflict-free Replicated Data Types
  • Novitiate - A procedurally generated RPG inspired by Rogue and written in Pony.
  • Wallaroo - Distributed Stream Processor written in Pony

Does Pony have a formatter?

At this time, no. There is no formatter for Pony code. The lack of a formatter doesn’t indicate that the Pony core team is opposed to code formatters. There is strong interest in having a code formatter for Pony.

An easy to maintain code formatter would share parsing code with the compiler. At this time, sharing said code is difficult. We are working towards making maintaining a code formatter and other parsing related tooling easier. We plan to introduce a code formatter in the future.

Code

How can I turn a ref into a val?

Generally, you can’t.

A ref may have any number of untracked mutable aliases, so it can’t be declared immutable. If you start with iso or trn instead, you can guarantee that by consuming it you are destroying the only mutable reference, hence allowing it to become immutable. See the capabilities section of the tutorial for more information.

Similarly, a ref may sometimes be “lifted” to a val capability if it was created in the isolated region formed by a recover block, from which the compiler guarantees that no mutable references can leak. See the recover section of the tutorial for more information.

What does Foo() do if my Foo class has both create() and apply() methods? Does it call both?

Yes, it calls both. However perhaps not in the way you imagine.

  • Foo creates a new object instance using create()
  • () is the apply call on the newly created Foo instance

This example program demonstrates what is going on:

use "debug"

class Foo
  new create() =>
    Debug("create called")

  fun apply() =>
    Debug("apply called")

actor Main
  new create(env: Env) =>
    Debug("call Foo()")
    Foo()

    Debug("call Foo")
    let foo = Foo

    Debug("call foo()")
    foo()

Check out the example program in the Pony Playground.

For more information see the Sugar section of the the tutorial.

How can I write code that works for every kind of Number in Pony?

If you want to support all kinds of numeric types for which Pony supports there are three ways to approach this problem:

  • Via Inheritance:

You pick the trait or interface that satisfies the operations you need from those numbers (e.g. support for sub is defined in trait val Real[A: Real[A] val]) and use this as a type or type parameter in Generics. This works fine for a lot of cases. But e.g. literal inference will not work here, as this example shows:

// does not compile
primitive OneForAllInheritance
  fun do_generic_stuff[T: Real[T] val](t: T): T ? =>
    if t == T.min_value() then
      error
    else
      t.divmod(1) // can be fixed by: t.sub(T.from[U8](1))
    end

. Why is that? Literal inference only works for the built-in primitive types, but Integer[T] val could be extended by further not yet known classes.

  • Via Type Expressions:

There are plenty of predefined union types for all classes of numbers in Pony. E.g. type Signed is (I8 | I16 | I32 | I64 | I128 | ILong | ISize). There is also Unsigned, Int, Float and Number. Just pick what you need as your type.

// does not compile
primitive OneForAllTypeExpressions
  fun do_generic_stuff[T: Int](t: T): T ? =>
    if t == T.min_value() then
      error
    else
      t.sub(1)
    end

But there is a problem with this approach. Not only do we state that T could be any element of Int. It could even be a combination of them. e.g. ( U8 | I16 ) and how you use t and T here must work for all possible instantiations of Int, that is every possible combination.

  • Via Type Expressions AND Inheritance:

To get rid of the literal inference problem and to make sure only single numeric types are accepted, not unions of them, in order to use the methods on the different integer types for our generic problem, we need to combine both the type unions and inheritance to get to a safe solution: (Int & Integer[T] val). The reason why this works is that Integer[T] val can only be instantiated by a single concrete Int element, so we get rid of the combinations and at the same time make the literal inference work as we lock our generic T down to single concrete types being part of Int:

primitive OneForAllCombined
  fun do_generic_stuff[T: (Int & Integer[T] val)](t: T): T ? =>
    if t == T.min_value() then
      error
    else
      t.sub(1)
    end

Nice!

Comparisons to other languages

How is Pony different than Erlang/Elixir?

The answer is deep and complicated. Fortunately, Scott Fritchie went to a great deal of trouble answering it in his talk The wide world of almost-actors: comparing the Pony to BEAM languages.

Are pony actors lightweight like Elixir/Erlang’s actors, or Go’s goroutines?

Yes! In Pony, the overhead of an empty actor on a 64-bit system is roughly 240 bytes – depending on your system’s size_t and alignment. Complete actor overhead includes a message queue, per-actor heap, and GC bookkeeping; therefore, memory use increases as an actor accumulates messages and grows its heap. Actors do not have individual stacks, rather they use the stack of the OS thread they are scheduled on.

Relatively, Elixir/Erlang actors use ~5x more memory and goroutines use ~8x more memory, but critically Elixir/Erlang and Go handle memory far differently than Pony. The memory management approach that is “best” is project-dependent – Pony offers one more option you can consider for your particular needs.

Compiling

What are Pony’s supported CPU platforms?

As of ponyc release 0.26.0, we provide the following support:

  • Full support for 64-bit platforms
    • x86 and ARM CPUs only
  • Experimental, incomplete, and/or buggy support for 32-bit platforms
    • x86 and ARM CPUs only
    • See GitHub issues #2836 and #1576 for more information.

I get a “requires dynamic” error when compiling, how do I solve it?

/usr/bin/ld.gold: error: ./fb.o: requires dynamic R_X86_64_32
  reloc against 'Array_String_val_Trace' which may
  overflow at runtime; recompile with -fPIC

try running ponyc with the --pic flag.

For example:

ponyc --pic examples/helloworld

As of Pony 0.17.0, if you are building ponyc from source, you can have --pic automatically set for you. When building ponyc, run the following make command and your generated ponyc binary will always supply --pic without you having to set it.

make default_pic=true

On Windows I get fatal error LNK1112: module machine type 'x86' conflicts with target machine type 'x64'

Only 64-bit Windows is supported.

Make sure you’re running a cmd.exe/powershell.exe that does not include 32-bit VS environment variables.

This error occurs when ponyc is compiled in a 32-bit Visual Studio Developer Command Prompt.

Ecosystem

Does Pony have a package manager?

That would be yes and no. Package manager means different things to different people. What we have is a simple dependency manager called corral that we are working on growing into a full featured tool. Whether that is a more full featured “dependency manager” or more full featured “package manager” depends on how you define the two terms.

Is there SSL support?

Yes! There used to be SSL support in the Pony standard library, but it’s been moved out into its own library.

Linking

How can I supply custom linker parameters?

So, you need to link your program to a custom library or otherwise pass a particular linker option? You can accomplish your goal using the ponyc --linker option.

You’ll need to know what your current linker is. To get it, compile a pony program and pass --verbose 3.

On Linux, MacOS or other Unix-like

Then examine the output. You should see something like:

ld -execute -no_pie -dead_strip -arch x86_64 -macosx_version_min 10.8
  -o ./timer ./timer.o -L"/usr/local/lib/pony/0.9.0-e64bff88c/bin/"
  -L"/usr/local/lib/pony/0.9.0-e64bff88c/bin/../lib"
  -L"/usr/local/lib/pony/0.9.0-e64bff88c/bin/../packages"
  -L"/usr/local/lib" -lponyrt -lSystem

The ld is the linker command that is usually used on MacOS or Linux. If I wanted to link in the library Foo and needed to pass -lFoo then I would compile my program as:

ponyc --linker="ld -lFoo"

That would change the linker command that ponyc runs to:

ld -lFoo -execute -no_pie -dead_strip -arch x86_64 -macosx_version_min 10.8
  -o ./timer ./timer.o -L"/usr/local/lib/pony/0.9.0-e64bff88c/bin/"
  -L"/usr/local/lib/pony/0.9.0-e64bff88c/bin/../lib"
  -L"/usr/local/lib/pony/0.9.0-e64bff88c/bin/../packages"
  -L"/usr/local/lib" -lponyrt -lSystem

On Windows

Compiling a pony program with --verbose 3 will produce something like:

cmd /C ""C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.11.25503\bin\HostX64\x64\link.exe"
  /DEBUG /NOLOGO /MACHINE:X64 /OUT:.\helloworld.exe .\helloworld.obj
  /LIBPATH:"C:\Program Files (x86)\Windows Kits\10\Lib\10.0.15063.0\ucrt\x64"
  /LIBPATH:"C:\Program Files (x86)\Windows Kits\10\Lib\10.0.15063.0\um\x64"
  /LIBPATH:"C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.11.25503\lib\x64"
  /LIBPATH:"C:\pony\ponyc\build\release-llvm-3.9.1"
  /LIBPATH:"C:\pony\ponyc\build\release-llvm-3.9.1\..\..\packages"
  kernel32.lib msvcrt.lib Ws2_32.lib advapi32.lib vcruntime.lib legacy_stdio_definitions.lib dbghelp.lib ucrt.lib libponyrt.lib "

The path ending in link.exe is the linker that the pony compiler is currently using.

To add options to the link command, I would compile my program as something like:

ponyc --linker="C:\OtherPath\link.exe /LIBPATH:C:\Foo"

Runtime

Does Pony have green threads?

The short answer is no. Pony doesn’t have green threads. By default, the Pony scheduler starts 1 “actor thread” per available CPU. These threads are used to schedule actors. Each of these threads is a kernel thread.

The longer answer is “it depends”. Actors are Pony’s unit of concurrency and many people when asking if Pony has green threads really are asking about how concurrency is modeled. You, as a Pony programmer, never interact with scheduler threads directly, you never interact with any sort of thread. You worry about actors and sending messages between them.

What is causal messaging?

When we say that Pony has causal messaging, we mean that the Pony runtime provides certain guarantees about message delivery order. Given two actors, actor A and actor B. All messages sent from actor A will arrive at actor B in the order they were sent by Actor A and will be processed by Actor B in the same order. The causal ordering between messages will be preserved as an invariant by the runtime.

Causal messaging extends to a chain of actors. For example, if Actor B sends messages to Actor C in response to messages from Actor A, then there is a causal messaging chain from Actor A to Actor C. The cause of a message is a message from another actor and the ordering will be preserved.

It is essential to understand that the causal ordering of messages only exists between the two actors who are the sender and recipient of those messages. So for example, if actors X and Y send to actor Z, there is no guarantee what order the messages will be processed by Z except that we can say, all messages sent by X will be processed in the order they were sent, as will all messages sent by Y. However, messages from X and Y can arrive at Z interleaved in any order that maintains the causal ordering of X to Z and Y to Z.

When do programs exit?

Programs exit when they reach quiescence.

Quiescence is perhaps a term you are unfamiliar with unless you have worked with actor systems previously. In short, quiescence is the state of being calm or otherwise inactive. In Pony, individual actors as well as the program can be quiescent. Because of this there is no need to explicitly exit your program, once no more work is being performed and no more work can be produced then the program will exit.

An individual actor is quiescent when:

  • it has an empty message queue
  • it has no aliases remaining to it so cannot be sent new messages
  • it is not registered for events from the runtime via the ASIO subsystem

Once all actors are quiescent, the program will terminate.