choice types provide algebraic sum types of all the generic arguments provided to choice. A instance of a choice type is a tagged union of the types provided as actual generic type parameters. This concept is also called variant, sum type or coproduct in other contexts.
Syntactic sugar of the Fuzion language permits an alternative notation for choice types with actual generics as follows
A | B | C | ...
which is equivalent to
choice A B C ...
The parser will directly convert the first notation into a choice type with actual generics.
A field of choice type can be assigned a value of the same choice type or of any of the actual generic type arguments provided to the choice feature.
Two choice types choice A B and choice B A that differ only in the order of their actual generic arguments are treated as different types.
Named choice types can be constructed through inheritance, i.e.,
C : choice A B is {}
creates a choice type of A and B with the name C. Two named choice types D and E that inherit from choice with the same actual generic arguments in the same order are nevertheless different types and their values are not assignable to one another.
Named choice types may declare or inherit additional inner features as long as these features are not fields. Also, declared inner features must not build a closure that accesses outer features. Additional parents must be unit types, i.e., they must not declare fields nor access features of any of their outer features.
Note that all types provided must be distinct, it is not possible to repeat the same type as in choice i32 i32 or float | float. If a sum type of two or more equal types is desired, these types must first be wrapped into a new type as illustrated in the following example:
Say we want to store a temperature that is given as a 32 bit integer in degrees centigrade or degrees Fahrenheit. So we define two wrapper features
centigrade(degrees i32) is {} fahrenheit(degrees i32) is {}
Now we define the choice type using the wrapped i32 types, which are distinct:
has_fever(temp centigrade | fahrenheit) bool is ...
When passing arguments to this feature, we need to wrap them accordingly:
choice types provide algebraic sum types of all the generic arguments
provided to choice. A instance of a choice type is a tagged union of
the types provided as actual generic type parameters. This concept is
also called variant, sum type or coproduct in other contexts.
Syntactic sugar of the Fuzion language permits an alternative notation
for choice types with actual generics as follows
A | B | C | ...
which is equivalent to
choice A B C ...
The parser will directly convert the first notation into a choice type
with actual generics.
A field of choice type can be assigned a value of the same choice type
or of any of the actual generic type arguments provided to the choice
feature.
Two choice types choice A B and choice B A that differ only in the order
of their actual generic arguments are treated as different types.
Named choice types can be constructed through inheritance, i.e.,
C : choice A B is {}
creates a choice type of A and B with the name C. Two named choice types
D and E that inherit from choice with the same actual generic arguments in
the same order are nevertheless different types and their values are not
assignable to one another.
Named choice types may declare or inherit additional inner features as long
as these features are not fields. Also, declared inner features must not
build a closure that accesses outer features. Additional parents must be
unit types, i.e., they must not declare fields nor access features of any
of their outer features.
Note that all types provided must be distinct, it is not possible to
repeat the same type as in choice i32 i32 or float | float. If a sum
type of two or more equal types is desired, these types must first be
wrapped into a new type as illustrated in the following example:
Say we want to store a temperature that is given as a 32 bit integer
in degrees centigrade or degrees Fahrenheit. So we define two wrapper
features
centigrade(degrees i32) is {}
fahrenheit(degrees i32) is {}
Now we define the choice type using the wrapped i32 types, which are
distinct:
has_fever(temp centigrade | fahrenheit) bool is ...
When passing arguments to this feature, we need to wrap them accordingly:
has_fever (centigrade 37)
has_fever (fahrenheit 99)
When matching the choice type, we use the wrapper types and access the
argument field 'degrees' to access the i32 stored inside
match temp
NYI: Once Fuzion's match expression supports destructuring as well, we should
be able to extract the degrees directly as in
match temp
A choice type with no actual generic arguments is isomorphic to 'void', i.e, it
is a type that has an empty set of possible values.