Plutus: Pioneers Program 4th cohort - Lecture 7 - Marlowe and hosted smart contracts

While Plutus is the "main" entry-point for smart contracts on Cardano, the only requirement is to produce some Plutus Core, to be run on Cardano. It's an amazing opportunity since Plutus isn't that easy and error-prone.

It leads to the creation of some alternatives to Plutus:

• OpShin: A Python-based Smart Contracts language
• Aiken: A dedicate functional programming language for Smart Contracts
• Marlowe: A(n) (E)DSL (Haskell/JavaScript) focused on financial Smart Contract

One thing which struck me, is the provide GUI-based builder which looks like Scratch, but it actually express the simplicity of the language. Actually, it relies on the functional programming trinity embodying program-as-expression, relying on one big type:

data Contract 
  = Close
  | Pay Party Payee Token Value Contract
  | If Observation Contract Contract
  | When [Case] Timeout Contract
  | Let ValueId Value Contract
  | Assert Observation Contract

On thing you should notice is, except Close, each value have Contract as last element, which is a kind of continuation-passing-style way to build programs.

There are few interesting concepts:

• Values represent values including volatile/contextual info (e.g. current slot interval, token account balance, previous choices)
• Observation which are Values-based comparisons results
• Action which can be money deposit, choices between alternatives, contract notification

Here are the associated types:

data Timeout
  = TimeParam String
  | POSIXTime Integer
  deriving stock (Show,Generic,Eq)

instance Num Timeout where
  -- ...

data Value
  = AvailableMoney AccountId Token
  | Constant Integer
  | ConstantParam String
  | NegValue Value
  | AddValue Value Value
  | SubValue Value Value
  | MulValue Value Value
  | DivValue Value Value
  | ChoiceValue ChoiceId
  | TimeIntervalStart
  | TimeIntervalEnd
  | UseValue ValueId
  | Cond Observation Value Value
  deriving stock (Show,Generic,Eq)

data Observation
  = AndObs Observation Observation
  | OrObs Observation Observation
  | NotObs Observation
  | ChoseSomething ChoiceId
  | ValueGE Value Value
  | ValueGT Value Value
  | ValueLT Value Value
  | ValueLE Value Value
  | ValueEQ Value Value
  | TrueObs
  | FalseObs
  deriving stock (Show,Generic,Eq)

data Action
  = Deposit AccountId Party Token Value
  | Choice ChoiceId [Bound]
  | Notify Observation
  deriving stock (Show,Generic,Eq)

data Payee
  = Account AccountId
  | Party Party
  deriving stock (Show,Generic,Eq)

data Case = Case Action Contract
  deriving stock (Show,Generic,Eq)

We can try a very simple token swap token:

When
    [Case
        (Deposit
            (Role "Seller")
            (Role "Buyer")
            (Token "" "")
            (Constant 10)
        )
        (Pay
            (Role "Buyer")
            (Party (Role "Seller"))
            (Token "" "T")
            (Constant 1)
            Close
        )]
    1682892000000 Close

Which could be explain it as follows:

• When Buyer deposit 10 adas to Seller
• Then Seller pays 1T to Buyer

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