Variables
Variables in Sway are immutable by default. This means that, by default, once a variable is declared, its value cannot change. This is one of the ways how Sway encourages safe programming, and many modern languages have this same default. Let's take a look at variables in detail.
Declaring a Variable
Let's look at a variable declaration:
let foo = 5;
Great! We have just declared a variable, foo
. What do we know about foo
?
- It is immutable.
- Its value is
5
. - Its type is
u64
, a 64-bit unsigned integer.
u64
is the default numeric type, and represents a 64-bit unsigned integer. See the section Built-in Types for more details.
We can also make a mutable variable. Let's take a look:
let mut foo = 5;
foo = 6;
Now, foo
is mutable, and the reassignment to the number 6
is valid. That is, we are allowed to mutate the variable foo
to change its value.
Type Annotations
A variable declaration can contain a type annotation. A type annotation serves the purpose of declaring the type, in addition to the value, of a variable. Let's take a look:
let foo: u32 = 5;
We have just declared the type of the variable foo
as a u32
, which is an unsigned 32-bit integer. Let's take a look at a few other type annotations:
let bar: str[4] = "sway";
let baz: bool = true;
If the value declared cannot be assigned to the declared type, there will be an error generated by the compiler.
Configuration-time Constants
It is possible to define and initialize constant variables in the manifest file Forc.toml
of a Sway project. These constants then become visible and usable in the corresponding Sway program. Such variables are called configuration-time constants and have to be defined in their own section called [constants]
in the manifest file. The syntax for declaring such constants is as follows:
[constants]
some_contract_addr = { type = "b256", value = "0x580acb6ee759d9be0c0f78d3ef24e1b59300c625b3c61999967366dbbebad31c" }
some_num = { type = "u64", value = "42" }
some_string = { type = "str[4]", value = "\"fuel\"" }
true_bool = { type = "bool", value = "true" }
Notice that each constant requires two fields: a type
and a value
.
Note Because configuration-time constants are constants, they are immutable and cannot be made otherwise.
The constants defined above can now be used in a Sway program that uses the manifest file as follows:
script;
struct S {
x: u64,
}
fn main() -> u64 {
let addr = some_contract_addr;
let string = some_string;
return if true_bool { some_num } else { 0 };
}
Note Currently, it is only possible to define configuration-time constants that have primitive types and that are initialized using literals. This will change in the future.