Interfaces

std/interfaces declares the common standards as interfaces, for typed query and defer against other contracts, for exposing a module's externals with use M exposing, and for announcing them with supports. Each interface is exactly its standard's functions, so its ERC-165 id is the standard's. Interfaces for part of a module's externals, such as IERC20Views or IOwnable, are declared next to the module, not here. The ERC-20, ERC-721, and ERC-1155 files also declare the errors of ERC-6093 that OpenZeppelin, and the standard library's tokens, raise, and the ERC-6909 file declares OpenZeppelin's ERC6909 errors.

import "std/evm";
import "std/interfaces/erc20";
 
error TooLittle of uint;
 
storage is record {
  token : IERC20;
}
 
external function pay(const to : address; const amount : uint)
  raises TooLittle
is {
  const balance : uint := query storage.token.balanceOf(Evm.self());
  assert(balance >= amount, TooLittle(balance));
  defer storage.token.transfer(to, amount);
}