21. External calls and interfaces

To call another contract you declare an `interface` describing its external functions, then wrap the target address in that interface's name to get a typed handle.

interface ERC20:
    def transfer(to: address, amount: uint256) -> bool: nonpayable
    def balanceOf(owner: address) -> uint256: view

@external
def pay(token: address, to: address, amount: uint256):
    assert ERC20(token).transfer(to, amount), "transfer failed"

Declaring the correct state mutability (`view`, `nonpayable`, `payable`) on each interface function matters — it affects both correctness and the gas estimation the caller performs.

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