Select only one UTxO from script address

Question

I need to get only one UTxO from the script address, where its datum hash matches the redeemer hash. For example I have locked two UTxO with 10 Ada and the DatumHash "house" at the script and only want to unlock one of them.

I use the guessing game example of the Plutus Playground, but it unlocks both UTxO when the Redeemer has the same hash:

guess = endpoint @"guess" @GuessParams $ \(GuessParams theGuess) -> do
    -- Wait for script to have a UTxO of a least 1 lovelace
    utxos <- fundsAtAddressGeq gameAddress (Ada.lovelaceValueOf 1)

    let redeemer = clearString theGuess
        tx       = collectFromScript utxos redeemer

    let hashedSecretWord = findSecretWordValue utxos
        isCorrectSecretWord = fmap (`isGoodGuess` redeemer) hashedSecretWord == Just 

    void (submitTxConstraintsSpending gameInstance utxos tx)

-- | Find the secret word in the Datum of the UTxOs
findSecretWordValue :: Map TxOutRef ChainIndexTxOut -> Maybe HashedString
findSecretWordValue =
  listToMaybe . catMaybes . Map.elems . Map.map secretWordValue

-- | Extract the secret word in the Datum of a given transaction output is possible
secretWordValue :: ChainIndexTxOut -> Maybe HashedString
secretWordValue o = do
  Datum d <- either (const Nothing) Just (_ciTxOutDatum o)
  PlutusTx.fromBuiltinData d

At which part do I have to change the code, so it only unlocks one UTxO (best case: a random choice of both)?

Answer 1

You may need to update this part: utxos <- fundsAtAddressGeq gameAddress (Ada.lovelaceValueOf 1)

Try: utxos <- Map.take 1 <$> fundsAtAddressGeq gameAddress (Ada.lovelaceValueOf 1)

You need to change the import on top to import PlutusTx.Prelude hiding (pure) so as to use <$>.

I have tried it using 4 wallets (somehow the Playground does not allow one wallet to make multiple calls) and it worked.