Bluish Coder

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


2016-05-04

Bang, Hat and Arrow in Pony

If you've looked at Pony programming language code you've probably seen use of punctuation symbols in places and wondered what they meant. This post is an attempt to explain three of those - the bang, hat and arrow (!, ^ and -> respectively). Note that this is my understanding based on usage, reading the tutorials and watching videos so there may be errors. I welcome corrections!

Bang

The bang symbol (otherwise known as an exclamation mark) combined with a type name can be thought of as the type of an alias of the given type. Having an alias of an object means having another reference to that object. So an alias to a String iso is of type String iso!. This matters mostly in generic code which will be explained later but it does come up in error messages.

If you see ! in an error message like "iso! is not a subtype of iso" this means you are probably trying to assign an object that cannot be aliased without first consuming it.

If you see ! in a type declaration in code like "let foo: A!" then you can read this as "replace A! with a type that can safely hold an alias to A". If A is a String iso then A! would be a String tag for example (following the rules for aliased substitution.

Bang in errors

The following code demonstrates something that is often encountered by first time Pony users:

class Something

actor Main
  new create(env: Env) =>
    let a = recover iso Something end
    Foo(a)

actor Foo
  new create(s: Something iso) =>
    None

Here we have a class called Something. A new instance of it is created in the Main actor with reference capability iso. A new Foo actor is created passing this instance to it. This will fail to compile as we are aliasing the Something object held in a. a holds a reference to it and the variable s holding the argument to the Foo constructor is holding a reference to it at the same time. Objects with a reference capability of iso cannot have more than one reference to it. The error from the compiler will look like:

Error:
e1/main.pony:6:9: argument not a subtype of parameter
    Foo(a)
        ^
    Info:
    e1/main.pony:9:14: parameter type: Something iso
      new create(s: Something iso) =>
                 ^
    e1/main.pony:6:9: argument type: Something iso!
        Foo(a)
            ^
    e1/main.pony:1:1: Something iso! is not a subtype of Something iso: iso! is not a subtype of iso

This error states that the expected type of the parameter for the Foo constructor is of type Something iso but the type that we passed is a Something iso!. It further explains things by noting that Something iso! is not a subtype of Something iso because iso! is not a subtype of iso.

Armed with the knowledge that the bang symbol means the type for an alias this can be read as the argument passed was an alias to a Something iso. This is an error as iso cannot be aliased - this is what iso! is not a subtype of iso means. The subtyping relationship for aliases is outlined in the Capability Subtyping section of the tutorial.

The code can be fixed by consuming the a so it is no longer aliased:

let a = recover iso Something end
Foo(consume a)

Bang in generics

The other place where you'll see the alias type is in generic code. The following non-generic code compiles fine:

class Something
  let a: U8

  new create(x: U8) =>
    a = x

actor Main
  new create(env: Env) =>
    let aint = Something(42)

U8 defaults to val reference capability which can be aliased. This allows the assignment to the field a in Something which is aliasing the x object. If we make this a generic so that any type can be used then it fails to compile:

class Something[A]
  let a: A

  new create(x: A) =>
    a = x

actor Main
  new create(env: Env) =>
    let aint = Something[U8](42)

The error is:

Error:
e3/main.pony:5:7: right side must be a subtype of left side
    a = x
      ^
    Info:
    e3/main.pony:4:17: right side type: A #any !
      new create(x: A) =>
                    ^
    e3/main.pony:5:5: left side type: A #any
        a = x
        ^
    e3/main.pony:4:17: A #any ! is not a subtype of A #any: the subtype has no constraint
      new create(x: A) =>
                ^

For now, ignore the #any in the error message. I'll expand on this later but it's informing us that the type A is unconstrained and can have any reference capability.

The error states that x is an A! but a is an A and A! is not a subtype of A so the assignment cannot happen.

This occurs Because A is unconstrained. It can be any reference capability. Therefore the code must be able to be compiled under the assumption that the most restrictive reference capability can be used. It works fine with val, which can be aliased, but not with iso which cannot. Therefore the generic code cannot be compiled. You can see how iso would fail by expanding a version using String iso:

class Something
  let a: String  iso

  new create(x: String iso) =>
    a = x

actor Main
  new create(env: Env) =>
    let aint = Something(recover iso String end)

The error is:

Error:
e5/main.pony:5:7: right side must be a subtype of left side
    a = x
      ^
    Info:
    e5/main.pony:4:17: right side type: String iso!
      new create(x: String iso) =>
                    ^
    e5/main.pony:2:10: left side type: String iso
      let a: String  iso
             ^
    e5/main.pony:4:17: String iso! is not a subtype of String iso: iso! is not a subtype of iso
      new create(x: String iso) =>
                    ^

This is the same error that the generic code is giving us. The generic code can be fixed in a few ways. The first is to constrain the type so that it is a specific reference capability that works. Here it is changed to val:

class Something[A: Any val]
  let a: A

  new create(x: A) =>
    a = x

actor Main
  new create(env: Env) =>
    let aint = Something[U8](42)

The A: Any val syntax constrains the type parameter to be a subtype of the type after the :. In this case, any type with a reference capability of val. This won't work if you want to be able to use any aliasable type (eg ref as well as val):

class Something[A: Any val]
  let a: A

  new create(x: A) =>
    a = x

actor Main
  new create(env: Env) =>
    let aint = Something[U8](42)
    let bint = Something[String ref](recover ref String end)

The error here is obvious in that we are trying to pass a ref parameter to a function expecting a val. Pony generics solves this by allowing code to be polymorphic over the reference capability. There are specific annotations for classes of reference capabilities. They are:

#read  = { ref, val, box }                = Anything you can read from
#send  = { iso, val, tag }                = Anything you can send to an actor
#share = { val, tag }                     = Anything you can send to more than one actor
#any   = { iso, trn, ref, val, box, tag } = Default of a constraint
#alias = {ref,val, box, tag}              = Set of capabilities that alias as themselves (used by compiler)

A version that will work for ref, val and box becomes:

class Something[A: Any #read]
  let a: A

  new create(x: A) =>
    a = x

actor Main
  new create(env: Env) =>
    let aint = Something[U8](42)
    let bint = Something[String ref](recover ref String end)

But what if you want it to work with non-aliasable types like iso? A solution is to consume the parameter:

class Something[A]
  let a: A

  new create(x: A) =>
    a = consume x

actor Main
  new create(env: Env) =>
    let aint = Something[U8](42)
    let bint = Something[String ref](recover ref String end)
    let cint = Something[String iso](recover iso String end)

Another solution is to declare the field type to be A! instead of A. In the String iso case using A means String iso which cannot hold an alias. Using A! means String iso! which should be read as "a type that can safely alias a String iso". Looking at the Aliased substitution table this is a tag:

class Something[A]
  let a: A!

  new create(x: A) =>
    a = x

actor Main
  new create(env: Env) =>
    let aint = Something[U8](42)
    let bint = Something[String ref](recover ref String end)
    let cint = Something[String iso](recover iso String end)

In this case we are using ! to tell the compiler to use a reference capability that works for whatever the type of A is. An iso becomes a tag, a trn becomes a box, a ref stays a ref, etc.

Hat

The hat symbol (or ^) is an ephemeral type. It's the type of an object that is not assigned to a variable. consume x is used to prevent aliasing of x but at the point of being consumed and before it is assigned to anything else, what type is it? If x is type A then the type of consume x is A^. Constructors always return an ephemeral type as they create objects and return them but they aren't yet assigned to anything.

The following example creates a Box type that acts like single instance array of String iso objects. A value can be stored and updated. A utility function Foo.doit takes a String iso as an argument. It's stubbed out since it doesn't need to do anything for the example. The main code creates a Box, updates it, and calls the utility function on it.

class Box
  var a: String iso

  new create(x: String iso) =>
    a = consume x

  fun ref update(x: String iso): String iso =>
    let b = a = consume x
    consume b

primitive Foo
  fun doit(s: String iso) =>
    None

actor Main
  new create(env: Env) =>
    let a = Box(recover iso String end)
    let b = a.update(recover iso String end)
    Foo.doit(consume b)

Some things to note based on prior discussion. The create method consumes the argument to prevent aliasing. The update function also consumes the argument to prevent aliasing. It uses the destructive read syntax to assign the argument x to the field a and assign the old value of a to b to avoid aliasing. Unfortunately this example fails to compile:

Error:
f3/main.pony:19:14: argument not a subtype of parameter
    Foo.doit(consume b)
             ^
    Info:
    f3/main.pony:12:12: parameter type: String iso
      fun doit(s: String iso) =>
               ^
    f3/main.pony:19:14: argument type: String iso!
        Foo.doit(consume b)
                 ^
    f3/main.pony:7:34: String iso! is not a subtype of String iso: iso! is not a subtype of iso
      fun ref update(x: String iso): String iso =>
                                     ^

From the discussion previously on ! this error tells us that we are aliasing b. We can narrow it down by explicitly declaring the type of b:

let b: String iso = a.update(recover iso String end)

The error is due to update returning a String iso. We are consuming b and returning it as a String iso which then gets aliased when assigned to b in the main routine. Changing the return type to use hat resolves the issue. consume b returns the ephmeral type which is an object with no variable referencing it. It is safe to assign to a String iso so the change compiles:

class Box
  var a: String iso

  new create(x: String iso) =>
    a = consume x

  fun ref update(x: String iso): String iso^ =>
    let b = a = consume x
    consume b

primitive Foo
  fun doit(s: String iso) =>
    None

actor Main
  new create(env: Env) =>
    let a = Box(recover iso String end)
    let b = a.update(recover iso String end)
    Foo.doit(consume b)

Another approach would be to return a String iso but change doit to be a String tag (the type that can alias a String iso). This compiles but because doit now takes String tag it is limited in what it can do with the string. The approach of using an ephemeral type allows obtaining the mutable object from the Box.

Hat in parameters

Sometimes you'll see hat in parameter lists. The Array builtin has an init constructor that looks like:

new init(from: A^, len: USize)

This initializes an array so that all elements are the from value. To explore how this works, here's a smaller example that does something similar:

class Box[A]
  var a: A
  var b: A

  new create(x: A^) =>
    a = x
    b = x

Without the hat in A^ there is an error due to aliasing. We can't assign x to both a and b in case A is an iso. With the hat it compiles. The is because an epehemeral reference capability is a way of saying "a reference capability that, when aliased, results in the base reference capability". So a String iso^ can be assigned to a String iso, a String ref^ can be assigned to a String ref, etc. This means the generic class itself compiles but using it for a String iso will fail due to aliasing but it can be used for other reference capability types. Compare this to a plain x: A where the generic class itself won't compile since a String iso can't be assigned to another String iso due to aliasing.

Code demonstrating this is:

actor Main
  new create(env: Env) =>
    let a = Box[String iso](recover iso String end)
    let c = Box[String ref](recover ref String end)
    let e = Box[String val](recover val String end)

Arrow

The arrow syntax (or ->) is known as viewpoint adapter types and is related to viewpoint adaption.

Arrow in error messages

Viewpoint adaption defines what the reference capability of a field looks like to some caller based on the reference capability of the object the field is being read from. This is important to maintain the reference capability guarantees. A val object should not be able to access an iso field as iso or it breaks the constraints of val - it should be immutable but obtaining it as iso allows mutation of the field. There is a table in viewpoint adaption that shows what the mapping is.

An example of an error that can occur by ignoring viewpoint adaption is in the following code:

class Something
  var a: String iso

  new create() =>
    a = recover iso String end

  fun doit(s: String) =>
    a.append(s) 

actor Main
  new create(env: Env) =>
    let a = Something
    a.doit("hello")

The error here is calling append on a the a field in the doit method. By default methods have a receiver reference capability of box. Anything that happens inside the method cannot affect the state of the object. This is why you see methods that modify object fields start with fun ref - it's to change the receiver reference capability to something mutable. Even though the field a is iso and therefore mutable because we are inside a box method it appears as a non-mutable reference capability. The viewpoint adaption table shows that a box origin with an iso field gives a tag type. So a looks like a String tag within the method. The compiler gives:

Error:
v/main.pony:8:13: receiver type is not a subtype of target type
    a.append(s) 
            ^
    Info:
    v/main.pony:8:5: receiver type: this->String iso!
        a.append(s) 
        ^
    ny/ponyc/packages/builtin/string.pony:622:3: target type: String ref
      fun ref append(seq: ReadSeq[U8], offset: USize = 0, len: USize = -1)
      ^
    v/main.pony:2:10: String tag is not a subtype of String ref: tag is not a subtype of ref
      var a: String iso
             ^

The 'receiver type' of this->String iso! is an example of arrow usage. It's saying that an object of type String iso! (an alias to a String iso) as seen by an origin of this. The reference capability of this in a method is that of the receiver reference capability on the function - in this case box. So this->String iso! is String tag. That's why the last error description line refers to String tag.

The solution here is to change the reference capability for the method to something that allows mutation:

fun ref doit(s: String) =>

Arrow in type declarations

When writing generic code it's sometimes required to be explicit in what viewpoint adaption to use for generic types. Returning to the Box example used previously we'll make it generic and make it usable for any reference capability:

class Box[A]
  var a: A

  new create(x: A) =>
    a = consume x

  fun apply(): this->A! =>
    a

  fun ref update(x: A): A^ =>
    let b = a = consume x
    consume b

  fun clone(): Box[this->A!] =>
    Box[this->A!](a)

Notice the use of this->A! in the return type of apply. We want to return what is held in the Box. If it is a Box[String val] val then we can return a String val since it is immutable and the box is immutable. If it is a Box[String ref] val we still want to return a String val, not a String ref. The latter would allow modifying an immutable box. If it's a Box[String ref] ref then it's safe to return a String ref. This is what the arrow type handles for us. The this refers to the reference capability of the object. The A! refers to the field type - note that it is being aliased here so we want a type that can hold an alias to an A. The viewpoint adaption gives the resulting reference capability of the type.

Looking up the table of viewpoint adaption gives:

Box[String val] val => val->val => val => String val
Box[String ref] val => val->ref => val => String val
Box[String ref] ref => ref->ref => ref => String ref
Box[String iso] ref => ref->iso => iso => String iso
Box[String ref] iso => iso->ref => tag => String tag

That last one is interesting in that the Box[String ref] iso says that only one reference of the Box can exist. If we allow a String ref to be obtained from it then it breaks this condition since both the original reference to the box can modify the string and so can the returned reference. This is why the viewpoint adaption gives a String tag. A tag only allows identity operations so it's safe to have this type of alias of an iso.

Note that the table above gives the mapping for this->A. Because it's a this->A! it has to be a type that can hold an alias to the type of the table. So we have another mapping:

String val! => String val
String ref! => String ref
String iso! => String tag

In this way a Box[String iso] ref will give out a String tag - the only safe way of aliasing the original string in the box.

The other use of an arrow type in this example is in the clone function. This must do a shallow copy of the object. It returns a new Box holding a reference to the same value. Because we need to alias the value the same constraints as described for the apply method exist. We want to return a Box[this->A!] to ensure the value object for that box instance is a safe alias to the original. For a Box[String iso] ref this returns a Box[String tag] for example.

The following code can be used with the Box class above to test it:

primitive Foo
  fun doit(s: String tag) =>
    None

actor Main
  new create(env: Env) =>
    let a = Box[String iso](recover iso String end)
    let b = a.clone()
    Foo.doit(b())

    let c = Box[String ref](recover ref String end)
    let d = c.clone()
    Foo.doit(d())

    let e = Box[String val](recover val String end)
    let f = e.clone()
    Foo.doit(f())

A->B arrows

Arrow types don't need to always use this on the receiver side. They can use an explicit reference capability like box->A or they can use another parameterized type. Examples of this are in some of the library code:

class ArrayValues[A, B: Array[A] #read] is Iterator[B->A]

An ArrayValues is returned by the values method on Array. It's an iterator over the objects in the array. The B->A syntax means that the type of the generic argument to Iterator is of type "A as seen by B" using viewpoint adaption. It's not an iterator over A, it's an iterator over "A as seen by B". This allows iteration over arrays whether they are val or ref and produces a compatible type for the Iterator that works with both.

Conclusion

Most of the functionality described here is some of the more esotoric Pony functionality. It is mainly hit when using generics. The best current reference for generics is a video by Sylvan Clebsch for the virtual Pony users group - Writing Generic Code.

A good way to learn is to try some of the examples in this post and play around with them. Try aliasing, using different types, different reference capabilities and see what happens. The Pony library code, Array.pony for example, is a useful reference.

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