Run-time Types

Types can be represented at run-time. To create a type value, use the constructor function Type<T>(), which accepts the static type as a type argument.

This is similar to e.g. T.self in Swift, T::class/KClass<T> in Kotlin, and T.class/Class<T> in Java.

For example, to represent the type Int at run-time:

let intType: Type = Type<Int>()

This works for both built-in and user-defined types. For example, to get the type value for a resource:

resource Collectible {}

let collectibleType = Type<@Collectible>()

// `collectibleType` has type `Type`

Type values are comparable.

Type<Int>() == Type<Int>()

Type<Int>() != Type<String>()

The method fun isSubtype(of: Type): Bool can be used to compare the run-time types of values.

Type<Int>().isSubtype(of: Type<Int>()) // true

Type<Int>().isSubtype(of: Type<String>()) // false

Type<Int>().isSubtype(of: Type<Int?>()) // true

To get the run-time type's fully qualified type identifier, use the let identifier: String field:

let type = Type<Int>()

type.identifier // is "Int"

// in account 0x1

struct Test {}

let type = Type<Test>()

type.identifier // is "A.0000000000000001.Test"

Getting the Type from a Value

The method fun getType(): Type can be used to get the runtime type of a value.

let something = "hello"

let type: Type = something.getType()

// `type` is `Type<String>()`

This method returns the concrete run-time type of the object, not the static type.

// Declare a variable named `something` that has the *static* type `AnyResource`

// and has a resource of type `Collectible`

//

let something: @AnyResource <- create Collectible()

// The resource's concrete run-time type is `Collectible`

//

let type: Type = something.getType()

// `type` is `Type<@Collectible>()`

Constructing a Run-time Type

Run-time types can also be constructed from type identifier strings using built-in constructor functions.

fun CompositeType(_ identifier: String): Type?

fun InterfaceType(_ identifier: String): Type?

fun RestrictedType(identifier: String?, restrictions: [String]): Type?

Given a type identifier (as well as a list of identifiers for restricting interfaces in the case of RestrictedType), these functions will look up nominal types and produce their run-time equivalents. If the provided identifiers do not correspond to any types, or (in the case of RestrictedType) the provided combination of identifiers would not type-check statically, these functions will produce nil.

struct Test {}

struct interface I {}

let type: Type = CompositeType("A.0000000000000001.Test")

// `type` is `Type<Test>`

let type2: Type = RestrictedType(

identifier: type.identifier,

restrictions: ["A.0000000000000001.I"]

)

// `type2` is `Type<Test{I}>`

Other built-in functions will construct compound types from other run-types.

fun OptionalType(_ type: Type): Type

fun VariableSizedArrayType(_ type: Type): Type

fun ConstantSizedArrayType(type: Type, size: Int): Type

fun FunctionType(parameters: [Type], return: Type): Type

// returns `nil` if `key` is not valid dictionary key type

fun DictionaryType(key: Type, value: Type): Type?

// returns `nil` if `type` is not a reference type

fun CapabilityType(_ type: Type): Type?

fun ReferenceType(authorized: bool, type: Type): Type

Asserting the Type of a Value

The method fun isInstance(_ type: Type): Bool can be used to check if a value has a certain type, using the concrete run-time type, and considering subtyping rules,

// Declare a variable named `collectible` that has the *static* type `Collectible`

// and has a resource of type `Collectible`

//

let collectible: @Collectible <- create Collectible()

// The resource is an instance of type `Collectible`,

// because the concrete run-time type is `Collectible`

//

collectible.isInstance(Type<@Collectible>()) // is `true`

// The resource is an instance of type `AnyResource`,

// because the concrete run-time type `Collectible` is a subtype of `AnyResource`

//

collectible.isInstance(Type<@AnyResource>()) // is `true`

// The resource is *not* an instance of type `String`,

// because the concrete run-time type `Collectible` is *not* a subtype of `String`

//

collectible.isInstance(Type<String>()) // is `false`

Note that the concrete run-time type of the object is used, not the static type.

// Declare a variable named `something` that has the *static* type `AnyResource`

// and has a resource of type `Collectible`

//

let something: @AnyResource <- create Collectible()

// The resource is an instance of type `Collectible`,

// because the concrete run-time type is `Collectible`

//

something.isInstance(Type<@Collectible>()) // is `true`

// The resource is an instance of type `AnyResource`,

// because the concrete run-time type `Collectible` is a subtype of `AnyResource`

//

something.isInstance(Type<@AnyResource>()) // is `true`

// The resource is *not* an instance of type `String`,

// because the concrete run-time type `Collectible` is *not* a subtype of `String`

//

something.isInstance(Type<String>()) // is `false`

For example, this allows implementing a marketplace sale resource:

pub resource SimpleSale {

/// The resource for sale.

/// Once the resource is sold, the field becomes `nil`.

///

pub var resourceForSale: @AnyResource?

/// The price that is wanted for the purchase of the resource.

///

pub let priceForResource: UFix64

/// The type of currency that is required for the purchase.

///

pub let requiredCurrency: Type

pub let paymentReceiver: Capability<&{FungibleToken.Receiver}>

/// `paymentReceiver` is the capability that will be borrowed

/// once a valid purchase is made.

/// It is expected to target a resource that allows depositing the paid amount

/// (a vault which has the type in `requiredCurrency`).

///

init(

resourceForSale: @AnyResource,

priceForResource: UFix64,

requiredCurrency: Type,

paymentReceiver: Capability<&{FungibleToken.Receiver}>

) {

self.resourceForSale <- resourceForSale

self.priceForResource = priceForResource

self.requiredCurrency = requiredCurrency

self.paymentReceiver = paymentReceiver

}

destroy() {

// When this sale resource is destroyed,

// also destroy the resource for sale.

// Another option could be to transfer it back to the seller.

destroy self.resourceForSale

}

/// buyObject allows purchasing the resource for sale by providing

/// the required funds.

/// If the purchase succeeds, the resource for sale is returned.

/// If the purchase fails, the program aborts.

///

pub fun buyObject(with funds: @FungibleToken.Vault): @AnyResource {

pre {

// Ensure the resource is still up for sale

self.resourceForSale != nil: "The resource has already been sold"

// Ensure the paid funds have the right amount

funds.balance >= self.priceForResource: "Payment has insufficient amount"

// Ensure the paid currency is correct

funds.isInstance(self.requiredCurrency): "Incorrect payment currency"

}

// Transfer the paid funds to the payment receiver

// by borrowing the payment receiver capability of this sale resource

// and depositing the payment into it

let receiver = self.paymentReceiver.borrow()

?? panic("failed to borrow payment receiver capability")

receiver.deposit(from: <-funds)

let resourceForSale <- self.resourceForSale <- nil

return <-resourceForSale

}

}