feat: replace HelloWorld with GuessTheNumber

[chadoh]

Fixes theahaco/scaffold-stellar#165

• The HelloWorld contract is so simple as to be obtuse. It's confusing in its pointlessness. And it doesn't show off anything interesting about blockchain programming.

• GuessTheNumber represents a most-basic starting point for a contract that can show some interesting blockchain functionality. Already, it shows:

  • storing data
  • constructor
  • admin-guarded methods
  • forthcoming contracttrait composability pattern (feat: add add_impl_type and #[internal] to contracttraits stellar/rs-soroban-sdk#1522)
  • forthcoming admin SEP (https://github.com/AhaLabs/admin-sep)

• This represents the probable starting point for the contract. In its current state, it has many purposeful ommissions/shortcomings. These will be fixed via an onboarding tutorial, with steps roughly as:

  1. Remove unwrap in guess, since it causes error if admin did not yet call reset. Walk learner through:

     • Remove after_deploy script to see this error in action
     • Extract the number-setting logic to separate function (maybe set_number)
     • Call that function in both reset and __constructor
     • Replace the unwrap with an expect
     • Note public vs private methods, and have them explore this

        stellar contract invoke --id guess_the_number --network local -- -h
       

  2. guess is currently a view call; no authentication is needed. This means that people do not need to be signed in and there would be no way to pay them. The first thing to do is require authentication; see feat: add guesser arg #101 for an example.

  3. No XLM is currently owned by contract or awarded to winning guess; need to require XLM transfer from admin in reset as well as nominal fee to call guess (which gets added to pot). With upcoming Stellar Registry functionality, we would like this to look like:

     use stellar_registry::import_asset;
     import_asset!(xlm);
     
     #[contract]
     pub struct GuessTheNumber;
     
     #[contractimpl]
     impl GuessTheNumber {
         pub fn reset(env: &Env) {
             Self::require_admin();
             let x = xlm::client(env);
             x.transfer(10_000_000_0, &admin, env.current_contract_address());
             self.set_number();
         }
     
         pub fn guess(env: &Env, guesser: Address, a_number: u64) -> bool {
             guesser.require_auth();
             if guessed_it = a_number == env.storage().instance().get::<_, u64>(&THE_NUMBER).unwrap() {
                 let tx = x.transfer(
                     x.balance(self.current_contract_address()),
                     self.current_contract_address(),
                     guesser,
                 );
                 if tx.is_err {
                     panic!("transfer failed!");
                 }
             } else {
                 let tx = x.transfer(1_000_000_0, guesser, self.current_contract_address());
                 if tx.is_err {
                     panic!("transfer failed!");
                 }
             }
             guessed_it
         }
     }

  4. Those is_err checks are bad! Introduce #[contracterror], return Result<bool, Error>, flatten in the errors from the XLM cross-contract call into this contract's errors, and do the x.transfer calls with question marks to automatically unwrap the results (rust docs).

  5. Inspect contract storage (for example, via block explorer) to see stored number; need to obfuscate with fun crypto tricks (how???)

• Ideally, this tutorial replaces the main Stellar Developer Docs > Build tutorial, but could also live in Aha-maintained docs (a gitbook? Scaffold Stellar docs?) or possibly in the UI of the app itself

[chadoh]

Just reviewed it and it looks good to me! I can't mark my own PR as "approved", though. @pselle @zachfedor anything you'd like addressed before we merge this?

[zachfedor]

@chadoh the build is failing, other than that it looks good. thanks @pselle for adding the submit button!