[FR] - Capability to add additional data in a body content

[mmontin]

Internal/External External

Area Plutus

Describe the feature you'd like

At the moment, it does not seem possible to pass additional data in the form of a correspondance between a datum hash and a datum in a transaction body, in addition to the one collected from the script witnesses in inputs, and the outputs of the body.

However, when referencing a utxo which happens to have a hashed datum, this is needed.

Indeed, in function mkShelleyTransactionBody we can see that the HashableScriptData are derived from the body itself, from the two locations mentioned above, as follows:

    scriptdata :: [HashableScriptData]
    scriptdata =
      [d | TxOut _ _ (TxOutDatumInTx _ d) _ <- txOuts]
        ++ [ d
           | ( _
              , AnyScriptWitness
                  ( PlutusScriptWitness
                      _
                      _
                      _
                      (ScriptDatumForTxIn (Just d))
                      _
                      _
                    )
              ) <-
              witnesses
           ]

Then, the verification hash of the body is computed from these data using convPParamsToScriptIntegrityHash, and is wrapped as well in the Shelley transaction body. As a consequence, there seem to be no builtin way to have additional data in the resulting Shelley transaction body.

My feature request is to have such a way of including additional data in the transaction body.

This could take several forms, such as:

• an additional field in the TxBodyContent structure
• an additional parameter in the mkShelleyTransactionBody function

Describe alternatives you've considered

For now, I have used the following work-around:

• generating the transaction body using mkShelleyTransactionBody
• deconstructing the generated transaction body
• manually adding the necessary data in the respective field of the transaction
• manually re-generating the script integrity hash using the new data map with convPParamsToScriptIntegrityHash
• wrapping back those changed elements into a new Shelley transaction body.

This does allow the ledger to generate the proper script context but it does not feel like the proper way to achieve this result.

[smelc]

cc @Jimbo4350 @carbolymer

[github-actions[bot]]

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[mmontin]

Following a discussion I had on Discord, it was confirmed that this feature is not currently present. I will work on a contribution to integrate it.