External state tracking on the validator level which is passed to the payouts

[paluh]

There may be substantial utility in expanding the Validator datum type, enabling it to convey additional data that could be passed as datum to the payout UTxOs and payout script. The Marlowe validator should solely ensure the preservation of this value across its execution thread on the chain.

In essence, I am proposing that we expand MarloweData to include externalData:

data MarloweData a = MarloweData {
        marloweParams   :: MarloweParams,
        marloweState    :: State,
        marloweContract :: Contract,
        externalData    :: a
    } deriving stock (Haskell.Show, Haskell.Eq, Generic)
      deriving anyclass (ToJSON, FromJSON)

The only extension required in the validator would be an equality check for this data, along with logic to pass this data to any payout. The role payout script could also potentially receive this additional state, but we would need to conduct a more in-depth analysis of this use case.

The primary objective of these external data pieces is to facilitate the transmission of information regarding certain preconditions. In conjunction with a blockchain level thread token (which naturally integrates with the Marlowe account model - we can store the thread token there), this data could serve as a means to provide witnesses and implement intriguing protocols on top.