3.8 Mutable Variables and Assignment

8.15.0.12

3.8 Mutable Variables and Assignment🔗ℹ

binding operator

Binds id so that its value can be changed using an assignment operator such as :=.

If an annot is present using ::, then the value of every assignment to id must satisfy the annotation, and the value installed into id is the converted value if annot is a converter annotation. Static information from annot is associated with uses of id whether attached by :: or :~.

> def mutable count = 0
> count := count + 1
> count
1
> count := "string"

> def mutable count :: Int = 0
> count := count + 1
> count := "string"
count: value does not satisfy annotation
value: "string"
annotation: Int

expression

id := expr

An assignment operator that changes the value of id to the result of expr and returns #void. The id must be bound with mutable.

The := operator is also recognized by other forms, such as . and #%index, for changing mutable components of some values.

> def mutable count = 0
> count := count + 1
> count
1

1	Notation and Conventions
2	Implicits and Context
3	Names and Definitions
4	Functions and Operators
5	Comparison and Branching
6	Objects and Annotations
7	Basic Data
8	Collections and Iteration
9	Object Protocols
10	Higher-Order Control
11	Code as Data
12	String Formatting and Matching
13	Input and Output
14	Operating System
15	Threads and Concurrency
16	Runtime System
17	Meta and Macros

3.1	Modules and Submodules
3.2	Namespaces
3.3	Import
3.4	Export
3.5	Block
3.6	Definitions
3.7	Multiple Values
3.8	Mutable Variables and Assignment