Move Language Expert

Purpose

Provide deep expertise on the Move programming language, focusing on its unique type system, abilities, generics, resource safety, and Aptos-specific features. Move is designed for safe digital asset programming with linear types and formal verification support.

When to Use

Auto-invoke when users mention:

Abilities - copy, drop, store, key, ability constraints
Generics - type parameters, phantom types, constraints
References - borrowing, &T, &mut T, borrow_global
Global Storage - move_to, move_from, exists, borrow_global_mut
Signer - authentication, signer pattern, access control
Visibility - public, public(friend), entry, private
Advanced - inline, friend functions, spec blocks

Move Language Fundamentals

Type System Overview

Move is a statically typed, compiled language with:

Linear types (resources can't be copied or dropped arbitrarily)
Generics with ability constraints
No null/undefined - explicit Option<T>
No dynamic dispatch - all calls resolved at compile time
Memory safety without garbage collection

Primitive Types

// Integers
let x: u8 = 255;         // 0 to 255
let y: u16 = 65535;      // 0 to 65,535
let z: u32 = 4294967295; // 0 to 4,294,967,295
let w: u64 = 18446744073709551615; // 0 to 2^64-1
let v: u128 = 340282366920938463463374607431768211455; // 0 to 2^128-1
let t: u256 = 115792089237316195423570985008687907853269984665640564039457584007913129639935; // 0 to 2^256-1

// Boolean
let flag: bool = true;

// Address
let addr: address = @0x1;
let named: address = @aptos_framework;

// Vectors
let nums: vector<u64> = vector[1, 2, 3];
let empty: vector<address> = vector::empty();

No Implicit Conversions

let x: u8 = 10;
let y: u64 = 20;

// ❌ Error: type mismatch
// let z = x + y;

// ✅ Correct: explicit casting
let z = (x as u64) + y;

Abilities - Move's Type Superpowers

The Four Abilities

struct Resource has key, store {
    value: u64
}
// key:   Can be stored in global storage as a top-level resource
// store: Can be stored inside other structs
// copy:  Can be copied (duplicated)
// drop:  Can be dropped/discarded

Ability Semantics

| Ability | Meaning | Example Use Case | |---------|---------|------------------| | copy | Type can be copied by value | Primitives, small configs | | drop | Type can be discarded | References, temporary data | | store | Can be stored in structs/global storage | Most data types | | key | Can be top-level resource in global storage | Account resources, NFTs |

Ability Constraints

// No abilities - can't copy, drop, store, or use as resource
struct Capability {}

// Only store - can be inside structs, but not global storage
struct InnerData has store {
    value: u64
}

// store + key - can be global resource
struct Account has store, key {
    balance: u64,
    inner: InnerData,  // ✅ InnerData has store
}

// copy + drop + store - behaves like primitive
struct Point has copy, drop, store {
    x: u64,
    y: u64,
}

Critical Rules

Rule 1: Fields must have compatible abilities

// ❌ ERROR: InnerData doesn't have 'key'
struct Account has key {
    data: InnerData  // InnerData needs 'store' at minimum
}

// ✅ CORRECT
struct Account has key {
    data: InnerData  // InnerData has 'store'
}

Rule 2: Structs without drop must be explicitly handled

struct NoDrop has store, key {
    value: u64
}

fun use_no_drop(nd: NoDrop) {
    // ❌ ERROR: Can't drop NoDrop
    // Function ends and nd is dropped implicitly
}

fun use_no_drop_correct(nd: NoDrop) {
    // ✅ Must explicitly destructure or store
    let NoDrop { value: _ } = nd;  // Unpack and drop fields
}

Rule 3: copy requires all fields to have copy

struct NotCopyable has store {
    x: u64
}

// ❌ ERROR: Can't have copy because NotCopyable doesn't
struct Container has copy {
    inner: NotCopyable
}

// ✅ CORRECT
struct Container has store {
    inner: NotCopyable
}

Generics and Type Parameters

Basic Generics

struct Box<T> has store {
    value: T
}

struct Pair<T1, T2> has store {
    first: T1,
    second: T2,
}

public fun create_box<T: store>(value: T): Box<T> {
    Box { value }
}

Ability Constraints on Type Parameters

// T must have 'store' ability
public fun store_in_box<T: store>(value: T): Box<T> {
    Box { value }
}

// T must have 'copy + drop'
public fun duplicate<T: copy + drop>(value: T): (T, T) {
    (value, copy value)
}

// T must have all abilities
public fun full_featured<T: copy + drop + store + key>(value: T) {
    // Can do anything with T
}

// No constraints (very limited - can only pass around)
public fun unconstrained<T>(value: T): T {
    value  // Can't copy, can't drop, can't store
}

Phantom Type Parameters

Phantom types don't appear in struct fields but affect type safety:

struct Coin<phantom CoinType> has store {
    value: u64  // CoinType doesn't appear here!
}

struct BTC {}
struct ETH {}

// These are different types!
let btc: Coin<BTC> = Coin { value: 100 };
let eth: Coin<ETH> = Coin { value: 50 };

// ❌ ERROR: Type mismatch
// let mixed = btc + eth;

Why Phantom Types?

Zero runtime overhead (erased at compile time)
Type-level guarantees (can't mix BTC and ETH)
Ability inheritance doesn't require CoinType to have abilities

// Even if SomeCoin doesn't have 'store', Coin<SomeCoin> can have it
struct SomeCoin {}  // No abilities!

struct Coin<phantom CoinType> has store {
    value: u64
}

// ✅ Works! Phantom type doesn't affect abilities
let coin: Coin<SomeCoin> = Coin { value: 100 };

Multiple Type Parameters

struct Pool<phantom X, phantom Y> has key {
    reserve_x: u64,
    reserve_y: u64,
    lp_supply: u64,
}

public fun create_pool<X, Y>(account: &signer) {
    move_to(account, Pool<X, Y> {
        reserve_x: 0,
        reserve_y: 0,
        lp_supply: 0,
    });
}

// Pool<BTC, ETH> ≠ Pool<ETH, BTC>

References and Borrowing

Immutable References (&T)

fun read_value(x: &u64): u64 {
    *x  // Dereference to read
}

let val = 42;
let ref = &val;
let copy = *ref;  // copy is 42, val is still 42

Mutable References (&mut T)

fun increment(x: &mut u64) {
    *x = *x + 1;
}

let mut val = 42;
increment(&mut val);
// val is now 43

Reference Rules

Can't have mutable + immutable refs simultaneously

let mut x = 10;
let r1 = &x;
let r2 = &mut x;  // ❌ ERROR: Can't have both

Only one mutable reference at a time

let mut x = 10;
let r1 = &mut x;
let r2 = &mut x;  // ❌ ERROR: x already borrowed mutably

References can't outlive their values

fun get_ref(): &u64 {
    let x = 42;
    &x  // ❌ ERROR: Can't return ref to local variable
}

Reference Copying for `copy` Types

fun copy_from_ref(x: &u64): u64 {
    *x  // ✅ u64 has copy, so this works
}

struct NoCopy has store {}

fun copy_from_ref_no_copy(x: &NoCopy): NoCopy {
    *x  // ❌ ERROR: NoCopy doesn't have copy ability
}

Global Storage Operations

The Five Global Storage Functions

// 1. move_to<T> - Store resource at signer's address
public fun initialize(account: &signer) {
    move_to(account, MyResource { value: 0 });
}

// 2. move_from<T> - Remove and return resource
public fun destroy(account: &signer): MyResource {
    move_from<MyResource>(signer::address_of(account))
}

// 3. borrow_global<T> - Immutable borrow
public fun read_value(addr: address): u64 acquires MyResource {
    let resource = borrow_global<MyResource>(addr);
    resource.value
}

// 4. borrow_global_mut<T> - Mutable borrow
public fun update_value(addr: address, new_val: u64) acquires MyResource {
    let resource = borrow_global_mut<MyResource>(addr);
    resource.value = new_val;
}

// 5. exists<T> - Check if resource exists
public fun has_resource(addr: address): bool {
    exists<MyResource>(addr)
}

The `acquires` Annotation

Critical: Functions that use borrow_global or borrow_global_mut must declare acquires:

struct Balance has key {
    coins: u64
}

// ✅ Correct
public fun get_balance(addr: address): u64 acquires Balance {
    borrow_global<Balance>(addr).coins
}

// ❌ ERROR: Missing 'acquires Balance'
public fun get_balance_wrong(addr: address): u64 {
    borrow_global<Balance>(addr).coins
}

// Multiple acquires
public fun transfer(from: address, to: address) acquires Balance {
    let from_balance = borrow_global_mut<Balance>(from);
    let to_balance = borrow_global_mut<Balance>(to);
    // ...
}

Resource Existence Patterns

// Pattern 1: Ensure resource exists
public fun ensure_initialized(account: &signer) {
    let addr = signer::address_of(account);
    if (!exists<MyResource>(addr)) {
        move_to(account, MyResource { value: 0 });
    }
}

// Pattern 2: Get or create
public fun get_or_create(account: &signer): &mut MyResource acquires MyResource {
    let addr = signer::address_of(account);
    if (!exists<MyResource>(addr)) {
        move_to(account, MyResource { value: 0 });
    };
    borrow_global_mut<MyResource>(addr)
}

// Pattern 3: Assert exists
public fun must_exist(addr: address) acquires MyResource {
    assert!(exists<MyResource>(addr), ERROR_NOT_INITIALIZED);
    let resource = borrow_global<MyResource>(addr);
    // ...
}

Signer - Authentication Primitive

What is Signer?

signer is Move's authentication primitive - represents authority to act on behalf of an account.

// ✅ Only the signer can authorize operations on their account
public entry fun initialize(account: &signer) {
    // 'account' proves the caller owns this address
    move_to(account, MyResource { value: 0 });
}

// ❌ Can't fake a signer - runtime provides it
public fun fake_signer(addr: address) {
    // No way to create signer from address!
}

Signer Operations

use std::signer;

public fun get_address(account: &signer): address {
    signer::address_of(account)
}

// Common pattern: get address for storage lookup
public fun update_resource(account: &signer, val: u64) acquires MyResource {
    let addr = signer::address_of(account);
    let resource = borrow_global_mut<MyResource>(addr);
    resource.value = val;
}

Access Control with Signer

const ERROR_UNAUTHORIZED: u64 = 1;

public entry fun admin_only(admin: &signer) {
    assert!(signer::address_of(admin) == @admin_address, ERROR_UNAUTHORIZED);
    // Only @admin_address can call this
}

public entry fun owner_only(owner: &signer, resource_addr: address) acquires Owner {
    let owner_addr = signer::address_of(owner);
    let owner_resource = borrow_global<Owner>(resource_addr);
    assert!(owner_resource.owner == owner_addr, ERROR_UNAUTHORIZED);
    // Only the owner can call this
}

Visibility Modifiers

Function Visibility

module my_module {
    // Private (default) - only callable within module
    fun private_function() { }

    // Public - callable from anywhere, but not as entry point
    public fun public_function() { }

    // Public(friend) - only callable from this module + friend modules
    public(friend) fun friend_function() { }

    // Entry - callable as transaction entry point (public entry)
    public entry fun entry_function(account: &signer) { }

    // Entry (module-local) - entry point but not callable from other modules
    entry fun local_entry(account: &signer) { }
}

Friend Functions

module admin {
    friend user_module;  // Declare friend

    public(friend) fun admin_function() {
        // Only callable from admin module or user_module
    }
}

module user_module {
    use admin::admin_function;

    public fun user_function() {
        admin_function();  // ✅ Allowed (we're a friend)
    }
}

module other_module {
    use admin::admin_function;

    public fun other_function() {
        admin_function();  // ❌ ERROR: Not a friend
    }
}

Entry Functions

Entry functions can be called directly as transaction entry points:

// ✅ Valid entry function signatures
public entry fun simple() { }
public entry fun with_signer(account: &signer) { }
public entry fun with_args(account: &signer, amount: u64, recipient: address) { }

// ❌ Invalid - entry functions can't return values
public entry fun returns_value(): u64 { 0 }

// ❌ Invalid - entry functions can't have reference parameters (except &signer)
public entry fun ref_param(x: &u64) { }

Advanced Features

Inline Functions

Mark functions for inlining to save gas:

inline fun add(a: u64, b: u64): u64 {
    a + b
}

public fun calculate(): u64 {
    add(5, 10)  // Inlined: becomes 5 + 10 directly
}

When to use inline:

Small functions called frequently
Wrappers around simple operations
Gas-critical paths

Constant Values

const MAX_SUPPLY: u64 = 1_000_000;
const ERROR_INSUFFICIENT_BALANCE: u64 = 1;
const MODULE_NAME: vector<u8> = b"MyModule";

public fun check_supply(amount: u64) {
    assert!(amount <= MAX_SUPPLY, ERROR_INSUFFICIENT_BALANCE);
}

Module Initialization

fun init_module(deployer: &signer) {
    // Called exactly once when module is published
    move_to(deployer, ModuleConfig {
        admin: signer::address_of(deployer),
        version: 1,
    });
}

Struct Unpacking

struct Point has copy, drop {
    x: u64,
    y: u64,
}

// Unpack all fields
let Point { x, y } = point;

// Unpack some fields, ignore others
let Point { x, y: _ } = point;

// Unpack and rename
let Point { x: x_coord, y: y_coord } = point;

Vector Operations

use std::vector;

let mut v = vector::empty<u64>();
vector::push_back(&mut v, 10);
vector::push_back(&mut v, 20);

let len = vector::length(&v);
let first = vector::borrow(&v, 0);
let mut last = vector::borrow_mut(&mut v, 1);
*last = 30;

let popped = vector::pop_back(&mut v);

vector::append(&mut v, vector[40, 50]);

// Iteration
let i = 0;
while (i < vector::length(&v)) {
    let elem = vector::borrow(&v, i);
    // Use elem
    i = i + 1;
}

Common Patterns

Pattern 1: Capability Pattern

struct AdminCap has key, store {}

public fun initialize_admin(account: &signer) {
    move_to(account, AdminCap {});
}

public fun admin_only_function(admin: &signer) acquires AdminCap {
    let admin_addr = signer::address_of(admin);
    assert!(exists<AdminCap>(admin_addr), ERROR_NO_ADMIN_CAP);
    // Admin has proven they have AdminCap
}

// Transfer admin capability
public fun transfer_admin(admin: &signer, new_admin: address) acquires AdminCap {
    let cap = move_from<AdminCap>(signer::address_of(admin));
    // In practice, you'd need new_admin's signer
    // This is simplified for demonstration
}

Pattern 2: Witness Pattern

struct MyModule has drop {}  // Witness type

struct Config<phantom T> has key {
    value: u64
}

// Only callable once - witness can only be created in init_module
fun init_module(account: &signer) {
    initialize(account, MyModule {});
}

public fun initialize<T: drop>(account: &signer, _witness: T) {
    move_to(account, Config<T> { value: 0 });
}

Pattern 3: Hot Potato Pattern

// No abilities - can't copy, drop, or store
struct Receipt {
    amount: u64
}

public fun buy(): Receipt {
    Receipt { amount: 100 }
}

public fun redeem(receipt: Receipt) {
    let Receipt { amount } = receipt;  // Must unpack
    // Process redemption
}

// Caller MUST call both buy() and redeem()
// Can't drop Receipt, so must be consumed

Type System Best Practices

✅ Do

Use abilities explicitly - Think about copy/drop/store/key
Leverage phantom types - Zero-cost type safety
Constrain generics - Add ability constraints as needed
Use references - Avoid unnecessary copies
Check exists before borrow - Prevent runtime errors
Mark admin functions - Use signer for access control

❌ Avoid

Over-constraining generics - Don't require abilities you don't use
Ignoring ability requirements - Compiler errors indicate design issues
Returning references to locals - Lifetime violation
Missing acquires - Always annotate global storage access
Copying large structs - Use references when possible

Common Errors

Error: "Field requires ability 'store'"

// ❌ Problem
struct Container has key {
    inner: Inner  // Inner needs 'store'
}

struct Inner {  // Missing 'store'
    value: u64
}

// ✅ Solution
struct Inner has store {
    value: u64
}

Error: "Missing acquires annotation"

// ❌ Problem
public fun get_value(addr: address): u64 {
    borrow_global<Resource>(addr).value
}

// ✅ Solution
public fun get_value(addr: address): u64 acquires Resource {
    borrow_global<Resource>(addr).value
}

Error: "Type parameter T requires constraint"

// ❌ Problem
public fun store<T>(value: T) {
    // Can't do anything with T
}

// ✅ Solution
public fun store<T: store>(value: T) {
    // Now T can be stored
}

Testing Move Code

#[test]
fun test_abilities() {
    let point = Point { x: 10, y: 20 };
    let copy = point;  // ✅ Has copy
    let Point { x, y } = point;  // ✅ Has drop
}

#[test(account = @0x123)]
fun test_signer(account: &signer) {
    let addr = signer::address_of(account);
    assert!(addr == @0x123, 0);
}

#[test]
#[expected_failure(abort_code = ERROR_INVALID)]
fun test_failure() {
    assert!(false, ERROR_INVALID);
}

Response Style

Type-first - Explain type implications before code
Ability-aware - Always discuss ability constraints
Safety-focused - Highlight Move's safety guarantees
Pattern-driven - Show idiomatic Move patterns
Error-preventing - Explain common mistakes upfront

Follow-up Suggestions

After helping with Move language, suggest:

Ability design for custom types
Generic function optimization
Move Prover specifications
Gas optimization techniques
Testing strategies for complex types
Migration from other smart contract languages