Run-time Types - Cadence

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 view 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 view 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.


view fun CompositeType(_ identifier: String): Type?
view fun InterfaceType(_ identifier: String): Type?
view fun IntersectionType(types: [String]): Type?

Given a type identifier (or a list of identifiers for interfaces in the case of IntersectionType), 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 IntersectionType) the provided combination of identifiers would not type-check statically, these functions will produce nil.


struct Test: I {}
struct interface I {}
let type: Type = CompositeType("A.0000000000000001.Test")
// `type` is `Type<Test>`
 
let type2: Type = IntersectionType(
    restrictions: ["A.0000000000000001.I"]
)
// `type2` is `Type<{I}>`

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


view fun OptionalType(_ type: Type): Type
view fun VariableSizedArrayType(_ type: Type): Type
view fun ConstantSizedArrayType(type: Type, size: Int): Type
view fun FunctionType(parameters: [Type], return: Type): Type
// returns `nil` if `key` is not valid dictionary key type
view fun DictionaryType(key: Type, value: Type): Type?
// returns `nil` if `type` is not a reference type
view fun CapabilityType(_ type: Type): Type?
view fun ReferenceType(entitlements: [String], type: Type): Type?

Asserting the Type of a Value

The method view 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:


access(all)
resource SimpleSale {
 
    /// The resource for sale.
    /// Once the resource is sold, the field becomes `nil`.
    ///
    access(all)
    var resourceForSale: @AnyResource?
 
    /// The price that is wanted for the purchase of the resource.
    ///
    access(all)
    let priceForResource: UFix64
 
    /// The type of currency that is required for the purchase.
    ///
    access(all)
    let requiredCurrency: Type
    access(all)
    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
    }
 
    /// 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.
    ///
    access(all)
    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
    }
}