Can add_change_if_needed be called twice on a Transaction Builder?

[gavinharris-dev]

Blockfrost ( provides a way to 'evaluate' a transaction that utilizes a Plutus Script, this evaluation returns the correct ExUnits that should be provided for each Redeemer.

I'm just wondering if it is possible to somehow allow (perhaps with a flag) to recall the add_change_if_needed method, or perhaps provide a means of recalculating the Fee (where we would need to also provide a TX Output to adjust (for change)) now that the Redeemer has been updated?

Example Blockfrost Evaluate response:

{
    "type": "jsonwsp/response",
    "version": "1.0",
    "servicename": "ogmios",
    "methodname": "EvaluateTx",
    "result": {
        "EvaluationResult": {
            "spend:1": {
                "memory": 1669212,
                "steps": 637231177
            }
        }
    },
    "reflection": {
        "id": "..."
    }
}

[lisicky]

Hi @gavinharris-dev! Now it is impossible without modification in the CSL code. Because add_change_if_needed sets fee value in a TransactionBuilder and add_change_if_needed returns error if a TransactionBuilder already has fee value. Calling add_change_if_needed is a final step before calling build or build_tx in a TransactionBuilder. But you can create a new TransactionBuilder instance and do the same steps but with a new ExUnits.