generics

runnable examples

The ghul-examples repository has fuller, runnable generics examples — open it in a GitHub Codespace or a dev container to build and run them.

ghūl supports generic type arguments on

• classes
• structs
• traits
• methods
• unions
• global functions

Type arguments declare a named type, which can be used anywhere within its scope in type expressions.

For example in the following global function, T is a type argument, and it can be used within the function's definition and body. When a particular specialization of print_something[T](T) is called, T will have whatever actual type argument was supplied

ghul

print_something[T](t: T) => write_line("something is {t}");

ghul

print_something[int](1234);

print_something[string]("hello");

something is 1234
something is hello

ghul

struct HOLD_SOMETHING[T] is

value: T;

init(value: T) is

self.value = value;

si

si

ghul

let holds_int = HOLD_SOMETHING(1234);

let holds_string = HOLD_SOMETHING("hello");

ghul

union Option[T] is

SOME(value: T);

NONE;

si

let some_int = Option.SOME(1234);

Generic argument types are largely opaque: the operations permitted on values of generic argument types are limited to:

• storing in a variable
• passing as a method or function argument
• returning from a method or function
• method calls supported by object

Generic argument types can be inferred from context for generic constructor invocations as well as generic function and method calls

ghul

print_something(1234);

print_something("hello");

something is 1234
something is hello