Davide Gessa @dakk

Davide Gessa @dakk

I’m Davide Gessa, a software developer, computer scientist and sailor.

King of Tezos: a smart-ponzi on Tezos

While writing a new programming language, it is often useful to write some real use-cases to test the syntax, the language expressiveness and the code cleanness comparing to other languages.

So I did for yallo-lang, a (still experimental) smart contract language for Tezos; for starting I chose a contract already existing on ethereum, king_of_ether (that’s right guys, a smart-ponzi).

Our minimal implementation is very simple: a single endpoint which allow the caller to become the king if he sends the contract balance * 2; the old king is then dismissed, but he receives in exchange twice the amount he sent to become a king. And so on.

We start with the yallo contract:

contract KingOfTezos {
	field theKing: address;
	field currentPrice: mutez;

	entry beTheKing() {
		assert (Tezos.amount() >= this.currentPrice);
		let op = Tezos.transfer (this.theKing, this.currentPrice);
		this.theKing = Tezos.sender();
		this.currentPrice = Tezos.amount() * 2n;
		[ op ]
	}
}

We transpile it to ligo: yallo.exe compile king_of_tezos.yallo -target ligo

At that is the result:

type storage = {  theKing: address;  currentPrice: tez;  }

type action = | BeTheKing of unit

let beTheKing (s: storage) = 
  let ovverraidable = if ((Tezos.amount) >= (s.currentPrice)) then () else failwith "Assertion" in
  let op: operation = Tezos.transaction (unit) (s.currentPrice) (match (Tezos.get_contract_opt s.theKing : unit contract option) with| None -> (failwith "invalid contract": unit contract) | Some(c) -> c) in
  let s = { s with theKing=Tezos.sender } in
  let s = { s with currentPrice=(Tezos.amount) * (2n) } in
  (([op]: operation list), (s: storage))


let main(a, s: action * storage): (operation list * storage) = 
  match a with | BeTheKing (arg) -> beTheKing(s)

Then ligo compiler does the rest, and we got king.tz:

{ parameter unit ;
  storage (pair (mutez %currentPrice) (address %theKing)) ;
  code { DUP ;
         CDR ;
         DUP ;
         CAR ;
         AMOUNT ;
         COMPARE ;
         GE ;
         IF { PUSH unit Unit } { PUSH string "Assertion" ; FAILWITH } ;
         DIG 1 ;
         DUP ;
         DUG 2 ;
         CDR ;
         CONTRACT unit ;
         IF_NONE
           { PUSH string "invalid contract" ; FAILWITH }
           { DUP ; DIP { DROP } } ;
         DIG 2 ;
         DUP ;
         DUG 3 ;
         CAR ;
         UNIT ;
         TRANSFER_TOKENS ;
         DIG 2 ;
         DUP ;
         DUG 3 ;
         CAR ;
         SENDER ;
         SWAP ;
         PAIR ;
         DUP ;
         PUSH nat 2 ;
         AMOUNT ;
         MUL ;
         SWAP ;
         CDR ;
         SWAP ;
         PAIR ;
         DUP ;
         NIL operation ;
         DIG 4 ;
         DUP ;
         DUG 5 ;
         CONS ;
         PAIR ;
         DIP { DROP 6 } } }

We then try it on carthagenet:

  1. Deploy 
     tezos-client originate contract kot transferring 1 from my_account running king.tz --init "(Pair 2000000 \"tz1VnNQMNQ796WrY2TyWDaFpwsRTP41mKXWH\")" --force --burn-cap 0.555
  2. We try to become the king with an invalid amount 
     tezos-client transfer 1 from my_account2 to kot --arg "Unit" --burn-cap 0.004

    Obviusly the simulation fails with: script reached FAILWITH instruction

  3. We try to become the king with a valid amount 
     tezos-client transfer 2 from my_account to kot --arg "Unit" --burn-cap 0.004

    And now we are the king of tezos. We can send further transactions doubling the amount every time.

You can inspect the contract and few beTheKing call transactions on bettercalldev.

Operation list

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