By default, Enumerated Cases have no scalar equivalent. They are simply singleton objects. However, there are ample cases where an Enumerated Case needs to be able to round-trip to a database or similar datastore, so having a built-in scalar [and thus trivially serializable] equivalent defined intrinsically is useful.
To define a scalar equivalent for an Enumeration, the syntax is as follows:
A case that has a scalar equivalent is called a Backed Case, as it is "Backed" by a simpler value. An Enum that contains all Backed Cases is called a "Backed Enum." A Backed Enum may contain only Backed Cases. A Pure Enum may contain only Pure Cases.
A Backed Enum may be backed by types of int
or string
, and a given enumeration supports only a single type at a time [that is, no union of int|string
]. If an enumeration is marked as having a scalar equivalent, then all cases must have a unique scalar equivalent defined explicitly. There
are no auto-generated scalar equivalents [e.g., sequential integers]. Backed cases must be unique; two backed enum cases may not have the same scalar equivalent. However, a constant may refer to a case, effectively creating an alias. See Enumeration constants.
Equivalent values must be literals or literal expressions. Constants and constant expressions are not supported. That is, 1 + 1
is allowed, but 1 + SOME_CONST
is not.
Backed Cases have an additional read-only property,
value
, which is the value specified in the definition.
In order to enforce the value
property as read-only, a variable cannot be assigned as a reference to it. That is, the following throws an error:
Backed enums implement an internal BackedEnum interface, which exposes two additional methods:
-
from[int|string]: self
will take a scalar and return the corresponding Enum Case. If one is not found, it will throw a ValueError. This is mainly useful in cases where the input scalar is trusted and a missing enum value should be considered an application-stopping error. -
tryFrom[int|string]: ?self
will take a scalar and return the corresponding Enum Case. If one is not found, it will returnnull
. This is mainly useful in cases where the input scalar is untrusted and the caller wants to implement their own error handling or default-value logic.
The from[]
and tryFrom[]
methods follow
standard weak/strong typing rules. In weak typing mode, passing an integer or string is acceptable and the system will coerce the value accordingly. Passing a float will also work and be coerced. In strict typing mode, passing an integer to from[]
on a string-backed enum [or vice versa] will result in a TypeError, as will a float in all circumstances. All other parameter types will throw a TypeError in both modes.
Manually defining a from[]
or tryFrom[]
method on a Backed Enum will result in a fatal error.