Abilities

Abilities are a typing feature in Move that control what actions are permissible for values of a given type. This system grants fine grained control over the "linear" typing behavior of values, as well as if and how values are used in storage (as defined by the specific deployment of Move, e.g. the notion of storage for the blockchain). This is implemented by gating access to certain bytecode instructions so that for a value to be used with the bytecode instruction, it must have the ability required (if one is required at all—not every instruction is gated by an ability).

For Sui, key is used to signify an object. Objects are the basic unit of storage where each object has a unique, 32-byte ID. store is then used to both indicate what data can be stored inside of an object, and is also used to indicate what types can be transferred outside of their defining module.

The Four Abilities

The four abilities are:

• copy
  • Allows values of types with this ability to be copied.
• drop
  • Allows values of types with this ability to be popped/dropped.
• store
  • Allows values of types with this ability to exist inside a value in storage.
  • For Sui, store controls what data can be stored inside of an object. store also controls what types can be transferred outside of their defining module.
• key
  • Allows the type to serve as a "key" for storage. Ostensibly this means the value can be a top-level value in storage; in other words, it does not need to be contained in another value to be in storage.
  • For Sui, key is used to signify an object.

copy

The copy ability allows values of types with that ability to be copied. It gates the ability to copy values out of local variables with the copy operator and to copy values via references with dereference *e.

If a value has copy, all values contained inside of that value have copy.

drop

The drop ability allows values of types with that ability to be dropped. By dropped, we mean that value is not transferred and is effectively destroyed as the Move program executes. As such, this ability gates the ability to ignore values in a multitude of locations, including:

• not using the value in a local variable or parameter
• not using the value in a sequence via ;
• overwriting values in variables in assignments
• overwriting values via references when writing *e1 = e2.

If a value has drop, all values contained inside of that value have drop.

store

The store ability allows values of types with this ability to exist inside of a value in storage, but not necessarily as a top-level value in storage. This is the only ability that does not directly gate an operation. Instead it gates the existence in storage when used in tandem with key.

If a value has store, all values contained inside of that value have store.

For Sui, store serves double duty. It controls what values can appear inside of an object, and what objects can be transferred outside of their defining module.

key

The key ability allows the type to serve as a key for storage operations as defined by the deployment of Move. While it is specific per Move instance, it serves to gates all storage operations, so in order for a type to be used with storage primitives, the type must have the key ability.

If a value has key, all values contained inside of that value have store. This is the only ability with this sort of asymmetry.

For Sui, key is used to signify an object.

Builtin Types

All primitive, builtin types have copy, drop, and store.

• bool, u8, u16, u32, u64, u128, u256, and address all have copy, drop, and store.
• vector<T> may have copy, drop, and store depending on the abilities of T.
  • See Conditional Abilities and Generic Types for more details.
• Immutable references & and mutable references &mut both have copy and drop.
  • This refers to copying and dropping the reference itself, not what they refer to.
  • References cannot appear in global storage, hence they do not have store.

Note that none of the primitive types have key, meaning none of them can be used directly with storage operations.

Annotating Structs

To declare that a struct has an ability, it is declared with has <ability> after the struct name and either before or after the fields. For example:

public struct Ignorable has drop { f: u64 }
public struct Pair has copy, drop, store { x: u64, y: u64 }
public struct MyVec(vector<u64>) has copy, drop, store;

In this case: Ignorable has the drop ability. Pair and MyVec both have copy, drop, and store.

All of these abilities have strong guarantees over these gated operations. The operation can be performed on the value only if it has that ability; even if the value is deeply nested inside of some other collection!

As such: when declaring a struct’s abilities, certain requirements are placed on the fields. All fields must satisfy these constraints. These rules are necessary so that structs satisfy the reachability rules for the abilities given above. If a struct is declared with the ability...

• copy, all fields must have copy.
• drop, all fields must have drop.
• store, all fields must have store.
• key, all fields must have store.
  • key is the only ability currently that doesn’t require itself.

For example:

// A struct without any abilities
public struct NoAbilities {}

public struct WantsCopy has copy {
    f: NoAbilities, // ERROR 'NoAbilities' does not have 'copy'
}

and similarly:

// A struct without any abilities
public struct NoAbilities {}

public struct MyData has key {
    f: NoAbilities, // Error 'NoAbilities' does not have 'store'
}

Conditional Abilities and Generic Types

When abilities are annotated on a generic type, not all instances of that type are guaranteed to have that ability. Consider this struct declaration:

public struct Cup<T> has copy, drop, store, key { item: T }

It might be very helpful if Cup could hold any type, regardless of its abilities. The type system can see the type parameter, so it should be able to remove abilities from Cup if it sees a type parameter that would violate the guarantees for that ability.

This behavior might sound a bit confusing at first, but it might be more understandable if we think about collection types. We could consider the builtin type vector to have the following type declaration:

vector<T> has copy, drop, store;

We want vectors to work with any type. We don't want separate vector types for different abilities. So what are the rules we would want? Precisely the same that we would want with the field rules above. So, it would be safe to copy a vector value only if the inner elements can be copied. It would be safe to ignore a vector value only if the inner elements can be ignored/dropped. And, it would be safe to put a vector in storage only if the inner elements can be in storage.

To have this extra expressiveness, a type might not have all the abilities it was declared with depending on the instantiation of that type; instead, the abilities a type will have depends on both its declaration and its type arguments. For any type, type parameters are pessimistically assumed to be used inside of the struct, so the abilities are only granted if the type parameters meet the requirements described above for fields. Taking Cup from above as an example:

• Cup has the ability copy only if T has copy.
• It has drop only if T has drop.
• It has store only if T has store.
• It has key only if T has store.

Here are examples for this conditional system for each ability:

Example: conditional copy

public struct NoAbilities {}
public struct S has copy, drop { f: bool }
public struct Cup<T> has copy, drop, store { item: T }

fun example(c_x: Cup<u64>, c_s: Cup<S>) {
    // Valid, 'Cup<u64>' has 'copy' because 'u64' has 'copy'
    let c_x2 = copy c_x;
    // Valid, 'Cup<S>' has 'copy' because 'S' has 'copy'
    let c_s2 = copy c_s;
}

fun invalid(c_account: Cup<signer>, c_n: Cup<NoAbilities>) {
    // Invalid, 'Cup<signer>' does not have 'copy'.
    // Even though 'Cup' was declared with copy, the instance does not have 'copy'
    // because 'signer' does not have 'copy'
    let c_account2 = copy c_account;
    // Invalid, 'Cup<NoAbilities>' does not have 'copy'
    // because 'NoAbilities' does not have 'copy'
    let c_n2 = copy c_n;
}

Example: conditional drop

public struct NoAbilities {}
public struct S has copy, drop { f: bool }
public struct Cup<T> has copy, drop, store { item: T }

fun unused() {
    Cup<bool> { item: true }; // Valid, 'Cup<bool>' has 'drop'
    Cup<S> { item: S { f: false }}; // Valid, 'Cup<S>' has 'drop'
}

fun left_in_local(c_account: Cup<signer>): u64 {
    let c_b = Cup<bool> { item: true };
    let c_s = Cup<S> { item: S { f: false }};
    // Valid return: 'c_account', 'c_b', and 'c_s' have values
    // but 'Cup<signer>', 'Cup<bool>', and 'Cup<S>' have 'drop'
    0
}

fun invalid_unused() {
    // Invalid, Cannot ignore 'Cup<NoAbilities>' because it does not have 'drop'.
    // Even though 'Cup' was declared with 'drop', the instance does not have 'drop'
    // because 'NoAbilities' does not have 'drop'
    Cup<NoAbilities> { item: NoAbilities {} };
}

fun invalid_left_in_local(): u64 {
    let n = Cup<NoAbilities> { item: NoAbilities {} };
    // Invalid return: 'c_n' has a value
    // and 'Cup<NoAbilities>' does not have 'drop'
    0
}

Example: conditional store

public struct Cup<T> has copy, drop, store { item: T }

// 'MyInnerData is declared with 'store' so all fields need 'store'
struct MyInnerData has store {
    yes: Cup<u64>, // Valid, 'Cup<u64>' has 'store'
    // no: Cup<signer>, Invalid, 'Cup<signer>' does not have 'store'
}

// 'MyData' is declared with 'key' so all fields need 'store'
struct MyData has key {
    yes: Cup<u64>, // Valid, 'Cup<u64>' has 'store'
    inner: Cup<MyInnerData>, // Valid, 'Cup<MyInnerData>' has 'store'
    // no: Cup<signer>, Invalid, 'Cup<signer>' does not have 'store'
}

Example: conditional key

public struct NoAbilities {}
public struct MyData<T> has key { f: T }

fun valid(addr: address) acquires MyData {
    // Valid, 'MyData<u64>' has 'key'
    transfer(addr, MyData<u64> { f: 0 });
}

fun invalid(addr: address) {
   // Invalid, 'MyData<NoAbilities>' does not have 'key'
   transfer(addr, MyData<NoAbilities> { f: NoAbilities {} })
   // Invalid, 'MyData<NoAbilities>' does not have 'key'
   borrow<NoAbilities>(addr);
   // Invalid, 'MyData<NoAbilities>' does not have 'key'
   borrow_mut<NoAbilities>(addr);
}

// Mock storage operation
native public fun transfer<T: key>(addr: address, value: T);