Components

Components are the building blocks of Miden accounts. Each component defines a storage layout, exposes public methods, and can be composed with other components on the same account — for example, a wallet component + an auth component + custom logic. This modularity lets you reuse a wallet component across many accounts and test components independently. Updating a deployed account replaces its complete account code and must preserve its storage layout; see Account code upgrades.

The #[component] macro

A component has three Rust pieces: a #[component_storage] struct for storage fields, a #[component] trait for the public interface, and a #[component] impl Trait for Storage block for behavior:

use miden::{component, component_storage, felt, Felt, StorageMap, Word};

#[component_storage]
struct CounterContractStorage {
    #[storage(description = "counter contract storage map")]
    count_map: StorageMap<Word, Felt>,
}

#[component]
trait CounterContract {
    #[account_procedure]
    fn get_count(&self) -> Felt;
    #[account_procedure]
    fn increment_count(&mut self) -> Felt;
}

#[component]
impl CounterContract for CounterContractStorage {
    fn get_count(&self) -> Felt {
        let key = Word::new([felt!(0), felt!(0), felt!(0), felt!(1)]);
        self.count_map.get(key)
    }

    fn increment_count(&mut self) -> Felt {
        let key = Word::new([felt!(0), felt!(0), felt!(0), felt!(1)]);
        let current_value: Felt = self.count_map.get(key);
        let new_value = current_value + felt!(1);
        self.count_map.set(key, new_value);
        new_value
    }
}

The macros generate:

1. Public API exports describing the component's callable methods
2. Storage metadata mapping slot names to slot IDs (derived from the component package + field name)
3. Runtime bindings for the Miden execution environment

Project manifest

Every account component crate also needs a miden-project.toml next to Cargo.toml:

miden-project.toml

[package]
name = "counter-contract"
version = "0.1.0"

[lib]
kind = "account-component"
namespace = "miden:counter-contract/[email protected]"
path = "src/lib.rs"

[dependencies]
miden-core = "*"
miden-protocol = "*"

The namespace interface segment must match the kebab-cased #[component] trait name. For generated WIT dependencies, see Cross-Component Calls.

Storage struct

The storage struct defines the component's storage layout:

use miden::{component_storage, AccountId, Felt, StorageMap, StorageValue, Word};

#[component_storage]
struct MyContractStorage {
    #[storage(description = "owner account identifier")]
    owner: StorageValue<Word>,

    #[storage(description = "initialization flag")]
    initialized: StorageValue<Word>,

    #[storage(description = "user balances")]
    balances: StorageMap<AccountId, Felt>,
}

Storage fields

Each field must be either StorageValue<T> (single-slot) or StorageMap<K, V> (map-slot), annotated with #[storage]:

#[storage(description = "human-readable description")]
field_name: StorageValue<Word>,

#[storage(description = "human-readable description")]
field_name: StorageMap<Word, Word>,

The description is optional and becomes part of the generated metadata. Slot IDs are derived from the component package name and the field name, so renaming a field changes the slot ID. Ordering does not matter, and slot(N) is not supported.

Trait and impl block — methods

Declare callable methods on the #[component] trait, mark each one with #[account_procedure], then implement that trait for the storage struct. Unmarked trait methods are not exported from the account interface. Authentication entrypoints use #[auth_script] instead; the two attributes cannot be combined.

Read methods (&self)

Methods that take &self can read component storage but cannot mutate it through self:

fn get_balance(&self, depositor: AccountId) -> Felt {
    self.balances.get(depositor)
}

Write methods (&mut self)

Methods that take &mut self can update component storage and call mutating methods on self:

fn deposit(&mut self, asset: Asset) {
    self.add_asset(asset);
}

Private methods

Helpers that are not declared on the #[component] trait are private. Define them on the storage struct with a normal inherent impl, then call them from the component trait implementation:

impl MyContractStorage {
    fn require_initialized(&self) {
        let state: Word = self.initialized.get();
        assert!(state[0] == felt!(1));
    }
}

#[component]
impl MyContract for MyContractStorage {
    fn do_something(&mut self) {
        self.require_initialized();
        // ...
    }
}

Supported parameter and return types

Public methods can use SDK types (Felt, Word, Asset, AccountId, NoteIdx) and custom types annotated with #[export_type].

Auto-generated methods

The #[component] macro automatically provides methods on self for account operations.

Mutation methods (&mut self)

// Add an asset to the account vault
self.add_asset(asset: Asset) -> Word

// Remove an asset from the account vault
self.remove_asset(asset: Asset) -> Word

// Increment the account nonce (only inside #[auth_script])
self.incr_nonce() -> Nonce

The kernel allows incr_nonce only from the account's authentication procedure, and only once per transaction. Calling it from an ordinary account procedure fails. Standard authentication components handle this increment automatically.

Read-only methods (&self)

// Get the account ID
self.get_id() -> AccountId

// Get the account nonce
self.get_nonce() -> Nonce

// Get the value word stored under an asset ID
self.get_asset(asset_id: AssetId) -> Word

// Check fungible or non-fungible asset ownership
self.has_asset(asset_id: AssetId) -> bool

// Compute commitment of account state changes
self.compute_delta_commitment() -> Word

// Check if a procedure was called during this transaction
self.was_procedure_called(proc_root: Word) -> bool

// Get storage and vault commitments
self.get_vault_root() -> Word
self.compute_commitment() -> Word // native account only
self.compute_storage_commitment() -> Word
// ... and more (see API Reference)

If storage or the vault has changed, compute_delta_commitment requires the nonce to have been incremented. Compute such a delta inside the authentication procedure after the increment; calling it earlier fails.

For the full list of auto-generated methods, see Account Operations. To export your own types for use in public method signatures, see Custom Types.

Full API docs on docs.rs: miden (top-level — #[component] macro)