Variables may be aliased or evaluated:

  • Aliased
    • Value Reference: let a = 5.
    • Expression Reference: let a = x.
      • An alias may be evaluated with assert a = b (checks x equals b).
  • Evaluated
    • Temporary variable: tempvar a = 5 * b.
    • Local: local a = 5 * b.
    • Constant: const a = 5.

A variable may be assigned to a function output:

  • let (a) = foo().
  • let (local a) = foo().
%lang starknet
%builtins pedersen range_check

from starkware.cairo.common.cairo_builtins import HashBuiltin

# Constant variables defined here are available to all functions.
# E.g., const my_const_2 = 10

# All persistent state appears in @storage_var
@storage_var
func persistent_state() -> (res : felt):
end

@external
func use_variables{
        syscall_ptr : felt*,
        pedersen_ptr : HashBuiltin*,
        range_check_ptr
    }():
    alloc_locals  # needed for "local" variables.

    # Transient, revocable felt (reference).
    let my_reference = 50
    # Redefine reference.
    let my_reference = 51

    # Transient, revocable expression (temporary variable).
    tempvar my_tempvar = 2 * my_reference
    # Redefine tempvar.
    tempvar my_tempvar = 3 * my_reference

    # Transient, non-revocable felt (constant).
    const my_const = 60
    # Cannot redefine const (const my_const = 61)

    # Transient, non-revoacable expression (local). Requires alloc_locals.
    local my_local = 70
    # Cannot redefine local (local my_local = 71).

    # Persistent (@storage_var) storage, without a variable.
    persistent_state.write(80)
    # Redefine state.
    persistent_state.write(81)

    # A variable can be assigned to a function output:
    # let (my_var) = func().
    let (my_reference_2) = persistent_state.read()
    let (local my_local_2) = persistent_state.read()

    assert my_local_2 = 81

    return ()
end

Things to note:

  • Locals are good when you want to assert that a variable has a certain value. If you use a reference with an assert statement, the reference is redefined (it assigns rather than checks). By using a local, the contract would perform the check and halt with an error.

Save as contracts/variables.cairo

Compile

Compile

nile compile

Or compile this specific contract

nile compile contracts/variables.cairo

Local Deployment

Deploy to the local devnet.

nile deploy variables --alias variables

Interact

Read

nile call variables use_variables

Public deployment

Will default to the Goerli/alpha testnet until mainnet is available.

nile deploy variables --alias variables --network mainnet
🚀 Deploying variables
🌕 artifacts/variables.json successfully deployed to 0x025dd2b60d1439b9e65a972930b8c5a35b3fdb0f50d523abdf3faa87dd42c79d
📦 Registering deployment as variables in mainnet.deployments.txt

Deployments can be viewed in the voyager explorer https://voyager.online