as operator in Pony has two related uses. First, it provides a safe way to increase the specificity of an object’s type (casting). Second, it gives the programmer a way to specify the type of the items in an array literal.
Safely converting to a more specific type (casting)¶
as operator can be used to create a reference to an object with a more specific type than the given reference, if possible. This can be applied to types that are related through inheritance, as well as unions and intersections. This is done at runtime, and if it fails then an error is raised.
Let’s look at an example. The
json package provides a type called
JsonDoc that can attempt to parse strings as fragments of JSON. The parsed value is stored in the
data field of the object, and that field’s type is the union
(F64 | I64 | Bool | None | String | JsonArray | JsonObject). So if there is a
JsonDoc object referenced by
jsonDoc.parse("42") will store an
I64 equal to
jsonDoc.data. If the programmer wants to treat
jsonDoc.data as an
I64 then they can get an
I64 reference to the data by using
jsonDoc.data as I64.
In the following program, the command line arguments are parsed as JSON. A running sum is kept of all of the arguments that can be parsed as
I64 numbers, and all other arguments are ignored.
use "json" actor Main new create(env: Env) => var jsonSum: I64 = 0 let jd: JsonDoc = JsonDoc for arg in env.args.slice(1).values() do try jd.parse(arg)? jsonSum = jsonSum + (jd.data as I64) end end env.out.print(jsonSum.string())
When run with the arguments
2 and 4 et 7 y 15, the program’s output is
The same thing can be done with interfaces, using
as to create a reference to a more specific interface or class. Let’s say, for example, that you have a library for doing things with furry, rodent-like creatures. It provides a
Critter interface which programmers can then use to create specific types of critters.
interface Critter fun wash(): String
The programmer uses this library to create a
Wombat and a
Capybara class. But the
Capybara class provides a new method,
swim(), that is not part of the
Critter class. The programmer wants to store all of the critters in an array, in order to carry out actions on groups of critters. Now assume that when capybaras finish washing they want to go for a swim. The programmer can accomplish that by using
as to attempt to use each
Critter object in the
Array[Critter] as a
Capybara. If this fails because the
Critter is not a
Capybara, then an error is raised; the program can swallow this error and go on to the next item.
interface Critter fun wash(): String class Wombat is Critter fun wash(): String => "I'm a clean wombat!" class Capybara is Critter fun wash(): String => "I feel squeaky clean!" fun swim(): String => "I'm swimming like a fish!" actor Main new create(env: Env) => let critters = Array[Critter].>push(Wombat).>push(Capybara) for critter in critters.values() do env.out.print(critter.wash()) try env.out.print((critter as Capybara).swim()) end end
Specify the type of items in an array literal¶
as operator can be used to tell the compiler what type to use for the items in an array literal. In many cases, the compiler can infer the type, but sometimes it is ambiguous.
For example, in the case of the following program, the method
foo can take either an
Array[U32] ref or an
Array[U64] ref as an argument. If a literal array is passed as an argument to the method and no type is specified then the compiler cannot deduce the correct one because there are two equally valid ones.
actor Main fun foo(xs: (Array[U32] ref | Array[U64] ref)): Bool => // do something boring here true new create(env: Env) => foo([as U32: 1; 2; 3]) // the compiler would complain about this: // foo([1; 2; 3])
The requested type must be a valid type for the items in the array. Since these types are checked at compile time they are guaranteed to work, so there is no need for the programmer to handle an error condition.