Developing concurrent programs with Pony
Subtyping
Listing 5 shows yet a further incompatibility in capabilities: The history
declaration stipulates box
(line 3), whereas in line 10, res
attempts to insert an iso
reference using push()
.
Of course consume
deletes res
; however, without the capability references, you could not write a meaningful program in Pony. Figure 5 shows the possible replacements that work in the direction of the arrow. For example, an iso
can replace any other capability, but trn
can only be used for val
or ref
, because trn
permits additional read references of the box
capability, which would contradict the exclusive character of iso
. However, val
can be replaced by trn
after a consume
removes the last writing reference of trn
type.

If you replace ref
, you can allow other writing references without problems. The box
and tag
capabilities are not critical for val
and ref
because the latter cannot write. A blog post by developer John Mumm provides a more detailed explanation [9].
Blue Horses
The main.pony
file shown in Listing 6 is the entry point in the example program; in the tradition of C, Pony uses main
as the identifier for the program start point.
Like the familiar main()
function in C or the main()
method in Java, Pony defines an actor. In line 4, the actor's constructor accepts a reference to the executing environment of the program as an object of type Env
. The next lines save the reference in the field with the name env
in the Main
instance.
Line 6 creates the actor of type Painter
, as defined by Listing 5. The constructor uses this
to receive simultaneously a reflexive reference to the Main
actor. The for
loop in lines 7 to 10 iterates over the field with the primitives [Amber, Blue, Crimson, Other]
or, preferably, its iterator object.
On the basis of field values, line 8 generates an object of type Resource
, as defined in Listing 3, and stores the reference in the run-time variable x
. The expression recover Resource(x) end
introduces a mechanism for arbitrarily setting a capability. In the present form, recover
generates a reference of type iso
.
Line 9 transfers the resource
to the Painter
actor. A further deletion process, courtesy of consume res
, occurs before sending. As mentioned earlier, Painter
finally calls the notify
behavior in line 12. The message is a String
type and ends up in the shell thanks to the env
object.
Conclusions
Thanks to Pony, programmers can write what looks to be secure, high-performance code for concurrent programs. It is free of data races and deadlocks and follows the style of object-oriented languages. The actor model should not cause any trouble, but the difficulty of understanding capabilities could deter many users. The matter is even more complex when developers need to consider the resulting effects of a collection of several capabilities.
If the complexity does not deter you, you can improve the quality of your multithreaded programs; moreover, Pony can be docked on existing projects by integrating C code via the Foreign Function Interface (FFI).
Infos
- Pony: http://ponylang.org
- Actor model: http://www.brianstorti.com/the-actor-model/
- Capabilities: http://man7.org/linux/man-pages/man7/capabilities.7.html
- Rust: http://rust-lang.org
- Java 9 plans: http://openjdk.java.net/projects/jdk9/spec/
- Dynamic classes (citation 7): https://en.wikipedia.org/wiki/Comparison_of_programming_languages_by_type_system#cite_note-7
- Pony on GitHub: https://github.com/ponylang/ponyc
- The concept of ownership in Rust: https://doc.rust-lang.org/book/ownership.html
- Sharing reference capabilities: http://jtfmumm.com/blog/2016/03/06/safely-sharing-data-pony-reference-capabilities/
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