calling_a_contract.md

Chapter 10: Calling a Contract

In Chapter 8, we added WildPokemons interface in the trainer contract. In this chapter, we will call the battle function of the WildPokemon interface.

If you recall from Chapter 8, we need a contract address to interact with WildPokemons.

We will learn how to deploy a contract to Ethereum blockchain and get the contract address in future lessons. For the purpose of this chapter, I have deployed the WildPokemons contract to the Rinkeby Testnet

Here is the contract address: 0x66f4804E06007630e1aF0a7B0b279e6F27A3FdE5

Using the contract address and the interface, you can make external calls to the interface functions (which was discussed in depth in Chapter 8):

interface Car:
    def start() -> bool: nonpayable
    def getSpeed() -> uint256: view

@external
def Driver(some_address: address):
    Car(some_address).start()

Handling Multiple Returns using Tuple Assignment

Unlike Python, you cannot directly declare tuple types in Vyper. However, in certain cases you can use literal tuples during assignment. For example, when a function returns multiple values:

@internal
def getNameAndAge() -> (String[32], int128):
    return "vasa", 22

@external
def getPerson():
    name: String[32] = empty(String[32])
    age: int128 = empty(int128)

    # the return value of `getNameAndAge` is assigned using a tuple
    (name, age) = self.getNameAndAge()

    # Can also skip the parenthesis
    name, age = self.getNameAndAge()

Put it to the test

The battle function returns the following 4 arguments:

• battleResult: bool
• newPokemonName: String[32]
• newPokemonDNA: uint256
• newPokemonHP: uint256

1. Inside battleWildPokemon, define these 4 function variables and initialize them using the empty built-in function.

2. Define a constant address storage variable named WILD_POKEMON having the value 0x66f4804E06007630e1aF0a7B0b279e6F27A3FdE5.

3. Inside battleWildPokemon, use WildPokemons and WILD_POKEMON to call the battle function that takes a single parameter of type Pokemon. This parameter can be obtained using the trainerToPokemon mapping by passing the 2 keys: msg.sender as the first key and pokemonIndex as the second key.

4. Use the function variables created in step 1 to create a tuple in order to handle/capture the multiple return values.

** Template **

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** Solution **

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** Previous Chapter Solution **

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