Bluish Coder

Programming Languages, Martials Arts and Computers. The Weblog of Chris Double.


2016-03-15

Closures in Pony

Note 2017-08-01: Recent releases of Pony have changed syntax for lambda calls and partial function calls. This post has been updated to work with these changes as of Pony 0.16.1.

The Pony programming language has support for closures but the documentation hasn't caught up with changes to the implementation. This post describes some of the features of Pony closures with examples. These examples require the latest version of Pony from the github repository. I used commit e4f6f91d.

Lambda

Closures are functions combined with an environment mapping names to values. Pony has a {...} syntax that can be used to create them. It's possible to create a function that captures from the lexical scope and to create a function with no capturing at all.

The following example shows using a lambda that does not capture anything (note that this is an artificial example - the List type already has various mapping and fold functions on it):

use "collections"

actor Main
  new create(env:Env) =>
    let l = List[U32]
    l.push(10)
    l.push(20)
    l.push(30)
    l.push(40)
    let r = reduce(l, 0, {(a:U32,b:U32): U32 => a + b})
    env.out.print("Result: " + r.string())

  fun reduce(l: List[U32], acc: U32, f: {(U32, U32): U32} val): U32 =>
    try
      let acc' = f(acc, l.shift()?)
      reduce(l, acc', f)
    else
      acc
    end

The reduce function takes an anonymous function as a parameter, named f. This is is defined as {(U32, U32): U32}. This syntax means that it must take two U32 arguments and returns a U32. A U32 is a 32-bit unsigned integer.

The val is a reference capability annotation. It states that f is an immutable value. Since nothing inside f can be mutated it is safe for f to be shared with other actors. Lambda functions that do not close over other variables default to val as they cannot change anything outside of the function. We need to explicitly annotate the val as {...} declarations default to ref. A compile error would result without it due to passing a val object to a ref parameter.

Compiling and running produces the result 100 as expected:

$ ponyc example
$ ...
$ ./example1
  Result: 100

Lambda with environment

The following example closes over the env variable so that it can be accessed within the lambda to print the String contained in the List:

use "collections"

actor Main
  new create(env:Env) =>
    let l = List[String]
    l.push("hello")
    l.push("world")
    for_each(l, {(s:String)(env) => env.out.print(s)})

  fun for_each(l: List[String], f: {(String)}) =>
    try
      f(l.shift()?)
      for_each(l, f)
    end

Notice that the lambda has an additional set of parameters, the (env). This is an argument list of the variables that are being closed over. Variables listed there can be accessed from the body of the lambda. Pony requires you to be explicit about what gets captured vs many other languages that implicitly capture variables in scope. Variables can be renamed in this parameter list using the following syntax:

for_each(l, {(s:String)(myenv=env) => myenv.out.print(s)})

The lambda here returns None which is the default so it is left off the declaration {(String)}. This declaration requires a function that takes a single String argument and returns None. The return value is also left off the actual lambda expression. No reference capability annotations are required here as {...} defaults to ref and a lambda that closes over variables also defaults to ref.

Lambda modifying closed variables

A modification of the above example might be to keep a count of each time the lambda is called and display the count:

actor Main
  new create(env:Env) =>
    let l = List[String]
    l.push("hello")
    l.push("world")
    var count = U32(0)
    for_each(l, {ref(s:String)(env,count) =>
                    env.out.print(s)
                    count = count + 1})
    // Displays '0' as the count
    env.out.print("Count: " + count.string())

fun for_each(l: List[String], f: {ref(String)}) =>
  try
    f(l.shift()?)
    for_each(l, f)
  end

The main thing to note here is that both the lambda definition and the declaration have a ref prefixed to the argument list. This signifies a lambda that might mutate itself when called. The effect of this is that within the body of the lambda the receiver of method calls and field access (ie. the this of the lambda object) is a ref vs the default of box. A box receiver won't allow ref functions to be called and mutating requires a ref receiver.

There is an issue with this example as noted by the comment. The final count is displayed as zero. The assignment of count within the lambda changes the count field within the lambda object. The count within the lambda increments and could be displayed showing the increasing values. The count outside of this does not. There is no reference or pointer to the closed over variable. Indirection can be used to do this by using a one element array:

var count : Array[U32] = Array[U32].init(0, 1)
for_each(l, {ref(s:String)(env,count) =>
              try
                env.out.print(s)
                count(0)? = count(0)? + 1
              end})
try
  env.out.print("Count: " + count(0)?.string())
end

The array access is wrapped with try as it is a partial function and can fail. Another approach could be to move the act of counting into an Actor:

use "collections"

actor Counter
  var n: U32 = 0

  be increment() =>
    n = n + 1

  be print(env:Env) =>
    env.out.print("Count: " + n.string())

actor Main
  new create(env:Env) =>
    let l = List[String]
    l.push("hello")
    l.push("world")
    let counter = Counter
    for_each(l, {(s:String)(env,counter) =>
                  env.out.print(s)
                  counter.increment()})
    counter.print(env)

  fun for_each(l: List[String], f: {(String)}) =>
    try
      f(l.shift()?)
      for_each(l, f)
    end

Actors have the tag reference capability and are easier to pass around to other objects, including lambda.

The example here is contrived in that it can be done without modifying a captured variable. Sylvan produced the following approach, using the methods in the collections package:

use "collections"

actor Main
  new create(env:Env) =>
    let l = List[String]
    l.push("hello")
    l.push("world")

    let count = l.fold[U32](
      {(x: U32, s: String)(env): U32 =>
        env.out.print(s)
        x + 1}, 0)

    env.out.print("Count: " + count.string())

Object Literals

Object literals are a way to create objects inline without having to name a class or actor. They look like:

object
  let x = 10
  fun foo() =>
    ...do something...
end

Lambda's are actually syntactic sugar on top of object literals. A lambda expansion looks like:

// Lambda
{(s:String)(env) => env.out.print(s)}

// Expands to:
object
  var env:Env = env
  fun apply(s:String) =>
    env.out.print(s)
end

The apply method of an object is called when the () function call syntax is used. Keeping this syntactic transformation in mind can help solve errors that occur when using lambda. The example earlier where the count total didn't account for the increments in the lambda becomes obvious here. The transformation would be:

var count = U32(0)
for_each(l, object
              var env:Env = env
              var count:U32 = count
              fun ref apply(s:String) =>
                env.out.print(s)
                count = count + 1
            end)
env.out.print("Count: " + count.string())

Note the ref annotation in the apply method. This is what the {ref(...)} syntax results in. Without the ref in the {} syntax the syntactic expansion is fun apply(...). The default reference capability for fun in objects is box.

The object literal expansion helps show the difference between {(string)} ref and {ref(String))}. The first requires an object with the given reference capability to match the type. Although the object is a ref, the apply method is the default, a box. This means it cannot modify any fields in the object. In the case of a lambda it won't be able to modify the captured variables:

object ref
  fun apply(...)
end

To match the second type definition it requires an object where the apply method itself has the ref capability. This allows apply to modify fields of the object:

object
  fun ref apply(...)
end

Partial Application

Related to lambda is partial application. It allows supplying some arguments to a function (or constructor or behaviour) and returns something that allows supplying the other arguments later.

In cases where you would use a lambda to return a function that sets the value of some arguments you can use partial application instead. The following example creates a lambda that adds five to another number:

actor Main
  new create(env:Env) =>
    let add5 = {(a:U32): U32 => 5 + a}
    env.out.print(add5(10).string())
    env.out.print(add5(20).string())

With partial application this becomes:

actor Main
  new create(env:Env) =>
    let add5 = U32(5)~add()
    env.out.print(add5(10).string())
    env.out.print(add5(20).string())

The use of + is an alias for the add method on an object. In the example above the ~ operator represents partial application. We are binding the receiver of the add method to be the 32-bit unsigned value 5. The resulting object can be called with the remaining arguments.

Partial application allows binding any arguments, by position or by name. The tutorial goes into a lot of detail.

Accumulator Generator

Paul Graham has an accumulator generator problem with examples in various languages. The problem is defined as:

Write a function foo that takes a number n and returns a function that takes a number i, and returns n incremented by i.

A Pony implementation, using closures, could be:

actor Main
  fun foo(n:U32): {ref(U32): U32} =>
    var s: Array[U32] = Array[U32].init(n, 1)
    {ref(i:U32)(s): U32 =>
      try
        s(0)? = s(0)? + i
        s(0)?
      else
        0
      end}

  new create(env:Env) =>
    var f = foo(5)
    env.out.print(f(10).string())
    env.out.print(f(20).string())

This uses the one element array trick to enable modifying the captured variable due to the requirement that the number n is incremented. It works only for U32 rather than any number which is also part of the problem definition. Pony has generics which would allow solving this but I'll leave that as an exercise or a future post.

Further reading

Some links to documentation and posts about lambda and reference capabilities to dive into more detail:

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