Enumerative Type

Enum types generated by Set. Enum types can be used as-is with type specifications, but further methods can be defined by classifying them into classes or defining patches.

A partially typed system with an enumerated type is called an enumerated partially typed.

Bool = {True, False}
Status = {"ok", "error"}

Since 1..7 can be rewritten as {1, 2, 3, 4, 5, 6, 7}, so when element is finite, the Enum types essentially equivalent the Range types.

Binary! = Class {0, 1}!.
    invert! ref! self =
        if! self == 0:
            do!:
                self.set! 1
            do!:
                self.set! 0

b = Binary!.new !0
b.invert!()

Incidentally, Erg's Enum types are a concept that encompasses enumerative types common in other languages.

#![allow(unused)]
fn main() {
// Rust
enum Status { Ok, Error }
}

# Erg
Status = {"Ok", "Error"}

The difference with Rust is that it uses a structural subtype(SST).

#![allow(unused)]
fn main() {
// There is no relationship between Status and ExtraStatus.
enum Status { Ok, Error }
enum ExtraStatus { Ok, Error, Unknown }

// Methods can be implemented
impl Status {
    // ...
}
impl ExtraStatus {
    // ...
}
}

# Status > ExtraStatus, and elements of Status can use methods of ExtraStatus.
Status = Trait {"Ok", "Error"}
    # ...
ExtraStatus = Trait {"Ok", "Error", "Unknown"}
    # ...

Methods can also be added by patching.

Use the or operator to explicitly indicate inclusion or to add a choice to an existing Enum type.

ExtraStatus = Status or {"Unknown"}

An enumerated type in which all classes to which an element belongs are identical is called a homogenous enumerated type.

By default, a class whose requirement type is an homogeneous enumerated type can be treated as a subclass of the class to which the element belongs.

If you do not wish to do so, you can make it a wrapper class.

Abc = Class {"A", "B", "C"}
Abc.new("A").is_uppercase()

OpaqueAbc = Class {inner = {"A", "B", "C"}}.
    new inner: {"A", "B", "C"} = Self.new {inner;}
OpaqueAbc.new("A").is_uppercase() # TypeError