Sending ether

(* `defer Evm.transfer(to, amount)` sends ether after this call's own work is done.
   It forwards all remaining gas and reverts the transaction if the transfer fails. *)
 
import "std/evm";
 
error Insufficient of record {
  available : wei;
  required : wei;
}
 
storage is record {
  balances : map(address, wei);
}
 
external payable function deposit() is {
  storage.balances[Evm.sender()] := storage.balances[Evm.sender()] + Evm.value();
}
 
external nonreentrant function withdraw(const amount : wei)
  raises Insufficient
is {
  const have : wei := storage.balances[Evm.sender()];
  assert(have >= amount, Insufficient { available = have; required = amount; });
  (* The write lands before the transfer runs, whatever order you write them in. *)
  defer Evm.transfer(Evm.sender(), amount);
  storage.balances[Evm.sender()] := have - amount;
}