Helios language/


Variables in Helios can be defined inside function bodies using assignment expressions, and at the top-level of a script using const statements.


Inside a function body, variables are defined using assignment expressions:

my_number: Int = 42; ...

Here my_number has value 42, and has type Int.

Assignment expressions must be followed by another expression, separated by a semicolon (;).

Note: Int is Helios' only number type, and represents an unbounded integer.

Note: the assignment expression above can be seen as syntactic sugar for the following anonymous function call: ((my_number: Int) -> {...})(42).

Note: an assignment expression can alternatively be seen as a ternary operator: ... = ... ; ...


A variable can be reassigned (though all values in Helios are immutable).

my_number: Int = 42;


my_number = 0 // reassignment

The new value must have the same type. You can reassign a variable inside nested scopes, effectively shadowing the original value. You can also reassign function arguments.

Note: the value of an assignment in a nested scope isn't available in the outer scopes.

const statements

Variables can also be defined at the top-level of a script, or inside struct or enum blocks, with const statements:

const AGE: Int = 123

const statements can be changed using the Helios API (see parameterized contracts).

Note: the right-hand side of const can contain complex expressions and even function calls. The compiler is smart enough to evaluate these at compile-time.

const without right-hand-side

The right-hand-side of a const statement can be omitted, in which case it must be set using the Helios API before compiling (see program.parameters):

const MY_PARAMETER: ValidatorHash

Type annotations

Assignment expressions usually include a type annotation. For literal right-hand sides, type annotations are optional:

list_of_ints = []Int{1, 1, 2, 3, 5}; ...

// instead of the more verbose:

list_of_ints: []Int = []Int{1, 1, 2, 3, 5}; ...

Note: const statements always have a type annotation. The type of the right-hand-side is never inferred.