Bluish Coder

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


2017-07-31

Reference Capabilities, Consume and Recover in Pony

I've written about reference capabilities in the Pony programming language spread across some of my other posts but haven't written about them directly. This post is my attempt to provide an intuitive understanding of reference capabilities and when to use consume and recover. Hopefully this reduces the confusion when faced with reference capability compilation errors and the need to memorize capability tables.

Reference capabilities are used to control aliasing. Controlling aliasing is important when sharing data to avoid data races in the presence of multiple threads. An example of aliasing is:

let a: Object = create_an_object()
let b: Object = a

In that snippet, 'b' is an alias to the object referenced by 'a'. If I pass 'b' to a parallel thread then the same object can be mutated and viewed by two different threads at the same time resulting in data races.

Reference capabilities allow annotating types to say how they can be aliased. Only objects with a particular set of reference capabilities can be shared across actors.

tag - opaque reference

A tag reference capability is an opaque reference to an object. You cannot read or write fields of the object referenced through a tag alias. You can store tag objects, compare object identity and share them with other actors. The sharing is safe since no reading or writing of state is allowed other than object identity.

let a: Object tag = create_a_tag()
let b: Object tag = a
if a is b then ...we're the same object... end

There is also the digestof operator that returns a unique unsigned 64 bit integer value of an object. This is safe to use on tag:

let a: Object tag = create_a_tag()
env.out.print("Id is: " + (digestof a).string())

The tag reference capability is most often used for actors and passing references to actors around. Actor behaviours (asynchronous method calls) can be made via tag references.

val - immutable and sharable

A val reference capability on a variable means that the object is immutable. It cannot be written via that variable. Only read-only fields and methods can be used. Aliases can be shared with other actors because it cannot be changed - there is no issue of data races when accessed from multiple threads. There can be any number of aliases to the object but they all must be val.

let a: Object val = create_an_object()
let b: Object val = a
let c: Object val = a
call_some_function(b)
send_to_an_actor(c)

All the above are valid uses of val. Multiple aliases can exist within the same actor or shared with other actors.

ref - readable, writable but not sharable

The ref reference capability means the object is readable and writable but only within a single actor. There can exist multiple aliases to it within an actor but it cannot be shared with other actors. If it were sharable with other actors then this would allow data races as multiple threads of control read or write to it in a non-deterministic manner.

let a: Object ref = create_a_ref_object()
let b: Object ref = a
call_some_function(b)

The above are valid uses of ref if they are occuring within a single actor. The following are invalid uses - they will result in a compile error:

let a: Object ref = create_a_ref_object()
send_to_an_actor(a)
let b: Object val = a

The send_to_an_actor call would result in an alias of a being accessible from another thread. This would cause data races so is disallowed and results in a compilation error. The assignment to an Object val is also a compilation error. The reasoning for this is access via b would assume that the object is immutable but it could be changed through the underlying alias a. If b were passed to another actor then changes made via a will cause data races.

iso - readable, writable uniquely

The iso reference capability means the object is readable and writable but only within a single actor - much like ref. But unlike ref it cannot be aliased. There can only be one variable holding a reference to an iso object at a time. It is said to be 'unique' because it can only be written or read via that single unique reference.

let a: Object iso = create_an_iso_object()
let b: Object iso = a
call_some_function(a)

The first line above creates an iso object. The other two lines are compilation errors. The assignment to b attempts to alias a. This would enable reading and writing via a and b which breaks the uniqueness rule.

The second line calls a function passing a. This is an implicit alias of a in that the parameter to call_some_function has aliased. It is readable and writable via a and the parameter in call_some_function.

When it comes to fields of objects things get a bit more complicated. Reference capabilities are 'deep'. This means that the capability of an enclosing object affects the capability of the fields as seen by an external user of the object. Here's an example that won't work:

class Foo
  var bar: Object ref = ...

let f: Foo iso = create_a_foo()
let b: Object ref = create_an_object()
f.bar = b

If this were to compile we would have a ref alias alive in b and another alias to the same object alive in the bar field of f. We could then pass our f iso object to another actor and that actor would have a data race when trying to use bar since the original actor also has an alias to it via b.

The uniqueness restriction would seem to make iso not very useful. What makes it useful is the ability to mark aliases as no longer used via the consume keyword.

consume - I don't want this alias

The consume keyword tells Pony that an alias should be destroyed. Not the object itself but the variable holding a reference to it. By removing an alias we can pass iso objects around.

let a: Object iso = create_an_iso_object()
let b: Object iso = consume a
call_some_function(consume b)

This snippet creates an iso object referenced in variable a. The consume in the second line tells Pony that a should no longer alias that object. It's floating around with nothing pointing to it now. Pony refers to this state as being 'ephemeral'. At this point the variable a doesn't exist and it is a compile error to use it further. The object has no aliases and can now be assigned to another variable, in this case b. This meets the requirements of iso because there is still only one reference to the object, via b.

The function call works in the same manner. The consume b makes the object ephemeral and can then be assigned to the parameter for the function and still meet the uniqueness requirements of iso.

iso objects can be sent to other actors. This is safe because there is only a single alias. Once it has been sent to another actor, the alias from the original actor cannot read or be written to because the alias it had was consumed:

let a: Object iso = create_an_iso_object()
send_to_an_actor(consume a)

Converting capabilities

Given an iso reference that is consumed, can that ephemeral object be assigned to other reference capabilities other than iso? The answer to that is yes.

Intuitively this makes sense. If you have no aliases to an object then when you alias that object you can make it whatever capability you want - it is like having created a new object, nothing else references it until you assign it. From that point on what you can do with it is restricted by the reference capability of the variable you assigned it to.

let a: Object iso = create_an_iso()
let b: Object val = consume a

let c: Object iso = create_an_iso()
let d: Object ref = consume c;

The above are examples of valid conversions. You can have an iso, make changes to the object, then consume the alias to assign to some other reference capability. Once you've done that you are restricted by that new alias:

let c: Object iso = create_an_iso()
let d: Object ref = consume c;
send_to_an_actor(d)

That snippet is an error as ref cannot be sent to another actor as explained earlier. This is also invalid:

let c: Object iso = create_an_iso()
let d: Object ref = consume c;
send_to_an_actor(c)

Here we are trying to use c after it is consumed. The c alias no longer exists so it is a compile error.

What if you want to go the other way and convert a val to an iso:

let a: Object val = create_a_val()
let b: Object iso = consume a

This is an error. Consuming the a alias does not allow assigning to another reference capability. Because val allows multiple aliases to exist the Pony compiler doesn't know if a is the only alias to the object. There could be others aliases elsewhere in the program. iso requires uniqueness and the compiler can't guarantee it because of this. The same reasoning is why the following is an error:

let a: Object val = create_a_val()
let b: Object ref = consume a

Intuitively we can reason why this fails. ref allows reading and writing within an actor. val requires immutability and can have multiple aliases. Even though we consume a there may be other aliases around, like the iso example before. Writing to the object via the b alias would break the guarantee of any other aliases to a.

Given this, how do you do this type of conversion? This is what the recover expression is used for.

recover - restricted alias conversion

A recover expression provides a block scope where the variables that can enter the scope are restricted based on their reference capability. The restriction is that only objects that you could send to an actor are allowed to enter the scope of the recover expression. That is iso, val and tag.

Within the recover expression you can create objects and return them from the expression as a different reference capability than what you created them as. This is safe because the compiler knows what entered the block, knows what was created within the block, and can track the aliases such that it knows it's safe to perform a particular conversion.

let a: Object val = recover val
                      let b: Object ref = create_a_ref()
                      ...do something...
                      b
                    end

In this snippet we create a ref object within the recover block. This can be returned as a val because the compiler knows that all aliases to that ref object exist within the recover block. When the block scope exits those aliases don't exist - there are no more aliases to the object and can be returned as the reference type of the recover block.

How does the compiler know that the b object wasn't stored elsewhere within the recover block? There are no global variables in Pony so it can't be stored globally. It could be passed to another object but the only objects accessable inside the block are the restricted ones mentioned before (iso, val and tag). Here's an attempt to store it that fails:

var x: Object ref = create_a_ref()
let a: Object val = recover val
                      let b: Object ref = create_a_ref()
                      x = b
                      b
                    end 

This snippet has a ref object created in the enclosing lexical scope of the recover expression. Inside the recover an attempt is made to assign the object b to that variable x. Intuitively this should not work - allowing it would mean that we have a readable and writeable alias to the object held in x, and an immutable alias in a allowing data races. The compiler prevents this by not allowing a ref object from the enclosing scope to enter a recover expression.

Can we go the other way and convert a val to a ref using recover? Unfortunately the answer here is no.

let a: Object ref = recover ref
                      let b: Object val = create_a_ref()
                      ...do something...
                      b
                    end

This results in an error. The reason is a val can be stored in another val variable in the enclosing scope because val objects are safely shareable. This would make it unsafe to return a writeable alias to the val if it is stored as an immutable alias elsewhere. This code snippet shows how it could be aliased in this way:

let x: Object val = create_a_val()
let a: Object val = recover val
                      let b: Object val = create_a_ref()
                      x = b
                      b
                    end

We are able to assign b to a variable in the enclosing scope as the x variable is a val which is one of the valid reference capabilities that can be accessed from within the recover block. If we were able to recover to a ref then we'd have a writeable and an immutable alias alive at the same time so that particular conversion path is an error.

A common use for recover is to create objects with a reference capability different to that defined by the constructor of the object:

class Foo
  new ref create() => ...

let a: Foo val = recover val Foo end
let b: Foo iso = recover iso Foo end

The reference capability of the recover expression can be left out and then it is inferred by the capability of the variable being assigned to:

let a: Foo val = recover Foo end
let b: Foo iso = recover Foo end

Two more reference capabilities to go. They are box and trn.

box - allows use of val or ref

The box reference capability provides the ability to write code that works for val or ref objects. A box alias only allows readonly operations on the object but can be used on either val or ref:

let a: Object ref = create_a_ref()
let b: Object val = create_a_val()
let c: Object box = a
let d: Object box = b

This is particularly useful when writing methods on a class that should work for a receiver type of val and ref.

class Bar
  var count: U32 = 0

  fun val display(out:OutStream) =>
    out.print(count.string())

actor Main
  new create(env:Env) =>
    let b: Bar val = recover Bar end
    b.display(env.out)

This example creates a val object and calls a method display that expects to be called by a val object (the "fun val" syntax). The this from within the display method is of reference capability val. This compiles and works. The following does not:

let b: Bar ref = recover Bar end
b.display(env.out)

Here the object is a ref but display expects it to be val. We can change display to be ref and it would work:

fun ref display(out:OutStream) =>
  out.print(count.string())

But now we can't call it with a val object as in our first example. This is where box comes in. It allows a ref or a val object to be assigned to it and it only allows read only access. This is safe for val as that is immutable and it is safe for ref as an immutable view to the ref:

fun box display(out:OutStream) =>
  out.print(count.string())

Methods are box by default so can be written as:

fun display(out:OutStream) =>
  out.print(count.string())

As an aside, the default of box is the cause for a common "new to Pony" error message where an attempt to mutate a field in an object fails with an "expected box got ref" error:

fun increment() => count = count + 1

This needs to be the following as the implicit box makes the this immutable within the method:

fun ref increment() => count = count + 1

trn - writeable uniquely, consumable to immutable

A trn reference capability is writeable but can be consumed to an immutable reference capability, val. This is useful for cases where you want to create an object, perform mutable operations on it and then make it immutable to send to an actor.

let a: Array[U32] trn = recover Array[U32] end
a.push(1)
a.push(2)
let b: Array[U32] val = consume a
send_to_actor(b)

box and ref methods can be called on trn objects:

class Bar
  var count: U32 = 0

  fun box display(out:OutStream) =>
    out.print(count.string())

  fun ref increment() => count = count + 1

actor Main
  new create(env:Env) =>
    let a: Bar trn = recover Bar end
    a.increment()
    a.display(env.out)

This provides an alternative to the "How do I convert a ref to a val?" question. Instead of starting with a ref inside a recover expression you can use trn and consume to a val later.

You can use iso in place of trn in these examples. Where trn is useful is passing it to box methods to perform readonly operations on it. This is difficult with iso as you have to consume the alias everytime you pass it around, and the methods you pass it to have to return it again if you want to perform further operations on it. With trn you can pass it directly.

actor Main
  let out: OutStream

  fun display(b: Bar box) =>
    b.display(out)

  new create(env:Env) =>
    out = env.out

    let a: Bar trn = recover Bar end
    display(a)
    let b : Bar val = consume a
    send_to_actor(b)

The equivalent with iso is more verbose and requires knowledge of ephemeral types (the hat, ^, symbol):

actor Main
  let out: OutStream

  fun display(b: Bar iso): Bar iso^ =>
    b.display(out)
    consume b

  new create(env:Env) =>
    out = env.out

    let a: Bar iso = recover Bar end
    let b: Bar iso = display(consume a)
    let c: Bar val = consume b
    send_to_actor(c)

Capability Subtyping

I've tried to use a lot of examples to help gain an intuitive understanding of the capability rules. The Pony Tutorial has a Capability Subtyping page that gives the specific rules. Although technical seeming the rules there encode our implicit understanding. This section is a bit more complex and isn't necessary for basic Pony programming if you have a reasonable grasp of it intuitively. It is however useful for working out tricky capability errors and usage.

The way to read those rules are that "<:" means "is a subtype of" or "can be substituted for". So "ref :< box" means that a ref object can be assigned to a box variable:

let a: Object ref = create_a_ref()
let b: Object box = a

The effects are transitive. So if "iso^ <: iso" and "iso <: trn" and "trn <: ref" then "iso^ <: ref":

let a: Object iso = create_an_iso()
let b: Object ref = consume a

Notice we start with iso^ which is an ephemeral reference capability. We get ephemeral types with consume. So consuming the iso gives an iso^ which can be assigned to a ref due to the transitive subtyping path above.

Why couldn't we assign the iso directly without the consume? This is explained previously using inutition but following the rules on the subtyping page we see that "iso! <: tag". The ! is for an aliased reference capability. When we do "something = a" we are aliasing the iso and the type of that a in that expression is iso!. This can only be assigned to a tag according to that rule:

let a: Object iso = create_an_iso()
let b: Object tag = a

Notice there is no "iso! <: iso" which tells us that an alias to an iso cannot be assigned to an iso which basically states the rule that iso can't be aliased.

In a previous section I used an ephemeral type in a method return type:

fun display(b: Bar iso): Bar iso^ =>
    b.display(out)
    consume b

This was needed because the result of display was assigned to an iso:

let b: Bar iso = display(consume a)

If we used Bar iso as the return type then the compiler expects us to be aliasing the object being returned. This alias is of type iso!. The error message states that iso! is not a subtype of iso which is correct as there is no "iso! :< iso" rule. Thankfully the error message tells us that "this would be possible if the subcap were more ephemeral" which is the clue that we need the return type to be ephemeral.

Viewpoint Adaption

I briefly mentioned in a previous section that reference capabilities are 'deep' and this is important when accessing fields of objects. It is also important when writing generic classes and methods and using collections.

Viewpoint adaption is described in the combining capabilities part of the tutorial. I also have a post, Borrowing in Pony which works through some examples.

The thing to remember is that a containing objects reference capability affects the reference capabilities of fields and methods when accessed via a user of the container.

let a: Array[Bar ref] iso = recover Array[Bar ref] end
a.push(Bar)
try
  let b: Bar ref = a(0)?
end

Here is an iso array of Bar ref objects. The third line to retrieve an element of the array fails to compile, stating that tag is not a subtype of ref. Where does the tag come from? Intuitively we can reason that we shouldn't be able to get a ref alias of an item in an iso array as that would give us two ref aliases of an item in an iso that could be shared across actors. This can give data races.

Viewpoint adaption encodes this in the type rules. We have receiver, a of type iso, attempting to call the apply method of the array. This method is declared as:

fun apply(i: USize): this->A ?

The this->A syntax is the viewpoint adaption. It states that the result of the call is the reference capability of A as seen by this. In our case, this is an iso and A is a ref. The viewpoint adaption for iso->ref is tag and that's where the tag in the error message comes from.

We could get an immutable alias to an item in the array if the array was trn:

let a: Array[Bar ref] trn = recover Array[Bar ref] end
a.push(Bar)
try
  let b: Bar box = a(0)?
end

The viewpoint adaption table shows that trn->ref gives box. To get a ref item we'd need the array to be ref:

let a: Array[Bar ref] ref = recover Array[Bar ref] end
a.push(Bar)
try
  let b: Bar ref = a(0)?
end

For more on viewpoint adaption I recommend my Borrowing in Pony and Bang, Hat and Arrow posts.

Miscellaneous things

In my examples I've used explicit reference capabilities in types to make it a bit clearer of what is happening. They can be left off in places to get reasonable defaults.

When declaring types the default capability is the capability defined in the class:

class ref Foo

let a: Foo = ...

class val Bar
let b: Bar = ...

The type of a is Foo ref and the type of Bar is Bar val due to the annotation in the class definition. By default classes are ref if they aren't annotated.

The type of an object returned from a constructor is based on the type defined in the constructor:

class Foo
  new ref create() => ...
  new val create_as_val() => ...
  new iso create_as_iso() => ...

let a: Foo val = Foo.create_as_val()

Here the Foo class is the default type ref, but there are constructors that explicitly return ref, val and iso objects. As shown previously you can use recover to change the capability returned by a constructor in some instances.

let a: Foo val = recover Foo.create() end // Ok - ref to val
let b: Foo ref = recover Foo.create_as_val() end // Not Ok - val to ref

As can be seen, converting val to ref using recover is problematic as shown in previous examples.

Conclusion

Reference capabilities are complex for people new to Pony. An intuitive grasp can be gained without needing to memorize tables of conversions. This intuitive understanding requires thinking about how objects are being used and whether such use might cause data races or prevent sharing across actors. Based on that understanding you pick the capabilities to match what you want to use. For those times that errors don't match your understanding use the viewpoint adaption and capability subtyping tables to work out where things are going wrong. Over time your intuitive understanding improves for these additional edge cases.

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