Overview

Often when writing code you want to create similar classes or functions that differ only in the type that they operate on. The classic example of this is collection classes. You want to be able to create an Array that can hold objects of a particular type without creating an IntArray, StringArray, etc. This is where generics step in.

A generic class or method takes type parameters — types that the caller provides, much like a method takes value parameters. Pony uses square brackets for type parameters and parentheses for value parameters, so Foo[U32] creates a Foo whose type parameter is U32, and Foo[U32](42) also passes the value 42 to its constructor.

The following sections cover how to use generics in Pony, how generics interact with reference capabilities, and how to constrain what types a generic accepts.