Programming Languages

Playing Rock, Paper, Scissors (rps.glow)

Rock, paper, scissors is the classic children's game where each child shows what they have chosen at the same time. The decision on who has won is based on the following rules:

  • Both selected the same: Draw
  • Rock beats scissors
  • Paper beats rock
  • Scissors beats paper

How con you code this in Glow if there is no at the same time. How does a player communicate their choice to the blockchain during the game?



Glow code

1 #lang glow
2 data Hand = | Rock | Paper | Scissors;
3 data Outcome = | B_Wins | Draw | A_Wins;
5 let winner = (handA : Hand, handB : Hand) : Outcome => {
6 Outcome.ofNat((Hand.toNat(handA) + (4 - Hand.toNat(handB))) % 3) }
8 /* The somewhat magic formula above is equivalent to the tedious definition below:
9 let winner = (handA : Hand, handB : Hand) : Outcome => {
10 switch (handA) {
11 case Rock:
12 switch(handB) {
13 case Rock: return Draw;
14 case Paper: return B_Wins;
15 case Scissors: return A_Wins;
16 }
17 case Paper:
18 switch(handB) {
19 case Rock: return A_Wins;
20 case Paper: return Draw;
21 case Scissors: return B_Wins;
22 }
23 case Scissors:
24 switch(handB) {
25 case Rock: return B_Wins;
26 case Paper: return A_Wins;
27 case Scissors: return Draw;
28 }
29 }
30 }
31 */
34 @interaction([A, B])
35 let rockPaperScissors = (wagerAmount) => {
36 @A assert! canReach(end, end.outcome == A_Wins);
37 @A let handA = Hand.input("First player, pick your hand");
38 @A let salt = randomUInt256();
39 @verifiably!(A) let commitment = digest([salt, handA]);
40 publish! A -> commitment; deposit! A -> wagerAmount;
42 @B assert! canReach(end, end.outcome == B_Wins);
43 @B let handB = Hand.input("Second player, pick your hand");
44 publish! B -> handB; deposit! B -> wagerAmount;
46 publish! A -> salt, handA;
47 verify! commitment;
48 let outcome = winner(handA, handB);
50 // end:
51 switch(outcome) {
52 | A_Wins => withdraw! A <- 2*wagerAmount
53 | B_Wins => withdraw! B <- 2*wagerAmount
54 | Draw => withdraw! A <- wagerAmount; withdraw! B <- wagerAmount };
56 outcome };
  • A Hand can only be Rock, Paper or Scissors
  • There are only three possible Outcomes either B_wins, A_wins, or its a Draw
  • Now define a function winner, that when given two hands can determine the Outcome
  • This is an arithmetic trick that translates each of the nine possible hand combinations to three possible outcomes
  • Alice and Bob use this contract
  • Declare the rockPaperScissors contract that has the wagerAmount
  • @Alice makes sure (assert!) that it's possible to reach the end: label
  • @Alice asks (input) and stores the value of her Hand.
  • @Alice creates a random value (salt) that will be used to obfuscate her Hand
  • @Alice store the obfuscated value of her Hand in a verifiably commitment
  • @Alice makes her commitment public and deposit! her wage.
  • @Bob makes sure he can reach the end.outcome where he wins
  • @Bob can input what hand he chooses to play
  • @Bob publishes his Hand and deposits his wager
  • Now it's possible to publish the salt and in the next step, use it to
  • verify! that the commitment was obfuscated with the salt
  • now we calculate the outcome as the result of evaluating the winner function with both hands.
  • switch for pattern matching, it's possible to select the appropriate outcome.
  • if outcome is A_wins withdraw to Alice both wages.
  • if outcome is B_wins withdraw to Bob both wages.
  • if outcome is Draw withdraw to give back their money to Alice and Bob.

Lessons learned

  • You can define your data types with data Hand
  • You can define smaller functions that are used later in the contract. Like: let winner = (handA:Hand, handB:Hand)
  • You can use switch to do pattern matching

© IOHK 2015 - 2021

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