Is there a way to go from a TxId to a Tx?

Question

I looked through the Plutus GitHub and found some info, but I cannot decode it.plutus/plutus-ledger/src/Ledger/Tx.hs

Here it is:

  encode (SomeTx tx eraInMode) = encodedMode eraInMode <> Encoding (TkBytes (C.serialiseToCBOR tx))
    where
      encodedMode :: C.EraInMode era C.CardanoMode -> Encoding
      -- 0 and 1 are for ByronEraInByronMode and ShelleyEraInShelleyMode
      encodedMode C.ByronEraInCardanoMode   = Encoding (TkSimple 2)
      encodedMode C.ShelleyEraInCardanoMode = Encoding (TkSimple 3)
      encodedMode C.AllegraEraInCardanoMode = Encoding (TkSimple 4)
      encodedMode C.MaryEraInCardanoMode    = Encoding (TkSimple 5)
      encodedMode C.AlonzoEraInCardanoMode  = Encoding (TkSimple 6)
  decode = do
    w <- decodeSimple
    case w of
      2 -> decodeTx C.AsByronEra C.ByronEraInCardanoMode
      3 -> decodeTx C.AsShelleyEra C.ShelleyEraInCardanoMode
      4 -> decodeTx C.AsAllegraEra C.AllegraEraInCardanoMode
      5 -> decodeTx C.AsMaryEra C.MaryEraInCardanoMode
      6 -> decodeTx C.AsAlonzoEra C.AlonzoEraInCardanoMode
      _ -> fail "Unexpected value while decoding Cardano.Api.EraInMode"
    where
      decodeTx :: C.IsCardanoEra era => C.AsType era -> C.EraInMode era C.CardanoMode -> Decoder s SomeCardanoApiTx
      decodeTx asType eraInMode = do
        bytes <- decodeBytes
        tx <- either (const $ fail "Failed to decode Cardano.Api.Tx") pure $ C.deserialiseFromCBOR (C.AsTx asType) bytes
        pure $ SomeTx tx eraInMode

How do you go about using something like this?

Thanks!

Answer 1

The answer is no.

A TxId is hash digest (blake2b-256) of the serialized transaction body. By construction, a hash is non-reversible (https://en.wikipedia.org/wiki/Cryptographic_hash_function).

In order to find back a transaction from a TxId, you need an index which keeps track of transactions and enable looking them up.

Answer 2

The validator script has only access to the ScriptContext which contains TxInfo and ScriptPurpose.

TxInfo contains the following:

data TxInfo = TxInfo
  { txInfoInputs      :: [TxInInfo] -- ^ Transaction inputs
  , txInfoOutputs     :: [TxOut] -- ^ Transaction outputs
  , txInfoFee         :: Value -- ^ The fee paid by this transaction.
  , txInfoMint        :: Value -- ^ The 'Value' minted by this transaction.
  , txInfoDCert       :: [DCert] -- ^ Digests of certificates included in this transaction
  , txInfoWdrl        :: [(StakingCredential, Integer)] -- ^ Withdrawals
  , txInfoValidRange  :: POSIXTimeRange -- ^ The valid range for the transaction.
  , txInfoSignatories :: [PubKeyHash] -- ^ Signatures provided with the transaction, attested that they all signed the tx
  , txInfoData        :: [(DatumHash, Datum)]
  , txInfoId          :: TxId
  } deriving stock (Generic, Haskell.Show, Haskell.Eq)

So it looks to me as if you cannot access the Tx in your validator.