Move Language

---
name: aptos-move-language
description: Expert on Move programming language fundamentals including abilities (copy/drop/store/key), generics, phantom types, references, global storage operations (move_to/move_from/borrow_global), signer pattern, visibility modifiers, and advanced type system features.
allowed-tools: Read, Write, Edit, Grep, Glob, Bash
model: sonnet
license: MIT
metadata:
  author: raintree
  version: "1.0"
---

# Move Language Expert

Deep expertise on the Move programming language for Aptos blockchain.

## Triggers

- move language, abilities, generics
- phantom type, borrow_global
- signer, friend, inline
- copy, drop, store, key
- move_to, move_from, acquires

## Abilities

The four abilities control what can be done with types:

| Ability | Meaning | Use Case |
|---------|---------|----------|
| `copy` | Can be copied | Primitives, configs |
| `drop` | Can be discarded | Temporary data |
| `store` | Can be stored in structs | Most data types |
| `key` | Can be top-level resource | Account resources |

```move
struct Resource has key, store { value: u64 }
struct Point has copy, drop, store { x: u64, y: u64 }
struct Capability {}  // No abilities - hot potato
```

### Critical Rules

1. Fields must have compatible abilities
2. Structs without `drop` must be explicitly handled
3. `copy` requires all fields to have `copy`

## Generics

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

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

### Phantom Types (Zero-Cost Type Safety)

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

struct BTC {}
struct ETH {}

// Coin<BTC> != Coin<ETH> at compile time
```

## References

```move
// Immutable reference
fun read(x: &u64): u64 { *x }

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

### Reference Rules

- Can't have mutable + immutable refs simultaneously
- Only one mutable reference at a time
- References can't outlive values

## Global Storage

```move
// Store resource
move_to(account, MyResource { value: 0 });

// Remove resource
let resource = move_from<MyResource>(addr);

// Immutable borrow
let r = borrow_global<MyResource>(addr);

// Mutable borrow
let r = borrow_global_mut<MyResource>(addr);

// Check existence
exists<MyResource>(addr);
```

### The `acquires` Annotation

```move
public fun get_value(addr: address): u64 acquires MyResource {
    borrow_global<MyResource>(addr).value
}
```

## Signer

`signer` is Move's authentication primitive:

```move
public entry fun initialize(account: &signer) {
    move_to(account, Resource { value: 0 });
}

let addr = signer::address_of(account);
```

## Visibility

```move
fun private_fn() { }                    // Module only
public fun public_fn() { }              // Anywhere
public(friend) fun friend_fn() { }      // Friends only
public entry fun entry_fn(s: &signer) { }  // Transaction entry
entry fun local_entry(s: &signer) { }   // Local entry
```

### Friend Declarations

```move
module admin {
    friend user_module;
    public(friend) fun admin_function() { }
}
```

## Inline Functions

```move
inline fun min(a: u64, b: u64): u64 {
    if (a < b) a else b
}
```

## Common Patterns

### Capability Pattern

```move
struct AdminCap has key, store {}

public fun admin_only(admin: &signer) acquires AdminCap {
    assert!(exists<AdminCap>(signer::address_of(admin)), ERROR);
}
```

### Witness Pattern

```move
struct MyModule has drop {}

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

### Hot Potato Pattern

```move
struct Receipt { amount: u64 }  // No abilities!

public fun buy(): Receipt { Receipt { amount: 100 } }
public fun redeem(r: Receipt) { let Receipt { amount } = r; }
// Must call both - can't drop Receipt
```

## Best Practices

- Use abilities explicitly
- Leverage phantom types for type safety
- Constrain generics only as needed
- Use references to avoid copies
- Check exists before borrow_global
- Always annotate acquires