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Week 2

For anything to happen on the blockhain, it has to be externally triggered. For example, in order to finish an auction, a transaction that will close it has to be submitted.

  • Each Cardano epoch consists of a number of slots, where each slot lasts for one second.
  • Block is a slot that contains a set of recent transactions on the network.
  • Slots that are inhabited by blocks are called active slots

Untyped on-chain script

Data

data Data =
      Constr Integer [Data]
    | Map [(Data, Data)]
    | List [Data]
    | I Integer
    | B BS.ByteString
    deriving stock (Show, Eq, Ord, Generic)
    deriving anyclass (NFData)

Low-level type for Plutus Core script, analogous to JSON. See [[Haskell Primer#Data types]] and [[Haskell Primer#Typeclasses]].

  • Data is defined in PlutusCore.Data module (re-exported from PlutusTx module).

Gift contract

mkValidator :: Data -> Data -> Data -> ()
mkValidator _ _ _ = ()

The most simple Validator that will always pass the validation. Any token amount at this script address can be consumed by anyone.

  • mkValidator function declaration and definition, see [[Haskell Primer#Functions]].
  • The parameters are Datum, Redeemer and Context (transaction info).
  • The return type is [[Haskell Primer#Unit]].
  • The arguments are ignored with [[Haskell Primer#Wildcard]].
validator :: Validator
validator = mkValidatorScript $$(PlutusTx.compile [|| mkValidator ||])

Transform the Haskell expression to the Plutus expression, this relies on [[Haskell Primer#Template Haskell]].

  • mkValidatorScript is declared in Plutus.V1.Ledger.Scripts module.
    • mkValidatorScript :: CompiledCode (Data -> Data -> Data -> ()) -> Validator.
  1. [|| mkValidator ||] - quote the mkValidator expression to get the abstract syntax tree (AST) of it.
  2. PlutusTx.compile - compile the Haskell AST of the mkValidator and produce the Plutus Core AST.
  3. $$(...) - splice the Plutus Core AST to the Plutus Core expression.
  4. mkValidatorScript - transform the Plutus Core expression of mkValidator to Validator .
{-# INLINABLE mkValidator #-}`

This pragma makes the mkValidator function inlineable for [|| ... ||]. See [[Haskell Primer#Pragmas]].

valHash :: Ledger.ValidatorHash
valHash = Scripts.validatorHash validator

Transform Validator (Plutus Core script) into a validator hash represented by ValidatorHash type.

  • validatorHash is declared in Plutus.V1.Ledger.Scripts module.
    • validatorHash :: Validator -> ValidatorHash
scrAddress :: Ledger.Address
scrAddress = scriptAddress validator

Transform Validator into an actual blockchain address represented by Address type.

  • scriptAddress is declared in Plutus.V1.Ledger.Address module.
    • scriptAddress :: Validator -> Address

Burn contract

mkValidator :: Data -> Data -> Data -> ()
mkValidator _ _ _ = error()

Such Validator will always fail validation.

  • error is declared in PlutusTx.Builtins module.
    • error :: () -> a - where a is any type, see [[Haskell Primer#Type variables]]
{-# LANGUAGE NoImplicitPrelude #-}

This pragma excludes Haskell Prelude from being auto-imported to avoid name collisions with PlutusTx.Prelude.

mkValidator :: Data -> Data -> Data -> ()
mkValidator _ _ _ = traceError "BURNT!"

This Validator will always fail with the specified error message.

  • traceError is declared in PlutusTx.Prelude module.
    • traceError :: Builtins.BuiltinString -> a
{-# LANGUAGE OverloadedStrings #-}

This pragma allows to produce Plutus strings from string literals.

FortyTwo contract

mkValidator :: Data -> Data -> Data -> ()
mkValidator _ r _
    | r == I 42 = ()
    | otherwise = traceError "wrong redeemer"

This Validator will pass validation only if Redeemer is 42.

  • [[Haskell Primer#Guards]] are used to test the redeemer argument.
  • r == I 42 - construct a new Data value as Integer of 42 and check whether it matches the Redeemer Data value.

Typed on-chain script

Typed contract

mkValidator :: () -> Integer -> ScriptContext -> Bool
mkValidator = _ r _ = traceIfFalse "wrong redeemer" $ r == 42

This Validator requires specific types of Datum, Redeemer, and Context. It will also log the message, if Redeemer is not 42.

  • traceIfFalse is declared in PlutusTx.Prelude module.
    • traceIfFalse :: Builtins.BuiltinString -> Bool -> Bool.
    • This function returns the given Bool argument, and logs the specified string, if it is False.
  • $ r == 42 - is equivalent to (r == 42), see [[Haskell Primer#Dollar operator]].
data Typed
instance Scripts.ValidatorTypes Typed where
	type instance DatumType Typed = ()
	type instance RedeemerType Typed = Integer

Define a dummy data type Type and make it an instance of ValidatorTypes typeclass. See [[Haskell Primer#Data types]] and [[Haskell Primer#Typeclasses]].

  • ValidatorTypes is declared in Ledger.Typed.Scripts.Validators module.
class ValidatorTypes (a :: Type) where
    type RedeemerType a :: Type
    type DatumType a :: Type
  • type instance DatumType Typed = () - define the type synonym for Datum as ()
  • type instance RedeemerType Typed = Integer - define the type synonym for Redeemer as Integer
typedValidator :: Scripts.TypedValidator Typed
typedValidator = Scripts.mkTypedValidator @Typed
		$$(PlutusTx.compile [|| mkValidator ||])
		$$(PlutusTx.compile [|| wrap ||])
	where
		wrap = Scripts.wrapValidator @() @Integer

Transform the Haskell type validator expression to the Plutus typed validator expression.

  • TypedValidator (a :: Type) type is declared in Ledger.Typed.Scripts.Validators module.
  • mkTypedValidator function is also declared in Ledger.Typed.Scripts.Validators module.
    • mkTypedValidator :: CompiledCode (ValidatorType a) -> CompiledCode (ValidatorType a -> WrappedValidatorType) -> TypedValidator a
  1. typedValidator :: Scripts.TypedValidator Typed - declare typedValidator function with TypedValidator Typed return type (where Typed is the type argument for TypedValidator).
  2. typedValidator = Scripts.mkTypedValidator @Typed - give Typed as the type argument to mkTypedValidator function.
  3. $$(PlutusTx.compile [|| mkValidator ||]) - splice the Haskell expression to the Plutus expression.
  4. $$(PlutusTx.compile [|| wrap ||]) - splice the typed expression to the untyped one using the wrap function definition.
  5. wrap = Scripts.wrapValidator @() @Integer - define wrap function as wrapValidator function, which transforms Data arguments to the typed arguments of Datum and Redeemer - () and Integer respectively.
validator :: Validator
validator = Scripts.validatorScript typedValidator

Transform the typed validator to the untyped one.

  • validatorScript is declared in Ledger.Typed.Scripts.Validators module.
    • validatorScript :: TypedValidator a -> Scripts.Validator
valHash :: Ledger.ValidatorHash
valHash = Scripts.validatorHash typeValidator

Transform TypedValidator Typed (Plutus Core script) into a validator hash represented by ValidatorHash type.

  • The typed version of validatorHash is declared in Ledger.Typed.Scripts.Validators module.
    • validatorHash :: TypedValidator a -> Scripts.ValidatorHash

IsData contract

Conversion between Data and specific types is achieved by implementing an instance of IsData typeclass declared in PlutusTx.IsData.Class module.

class IsData (a :: Type) where
    toData :: a -> Data
    fromData :: Data -> Maybe a

For some types like Data, Integer, ByteString, [a], Void the instances are already predefined.

newtype MySillyRedeemer = MySillyRedeemer Integer

Define a simple data type MySillyRedeemer.

PlutusTx.unstableMakeIsData ''MySillyRedeemer

At compile time, implementation of IsData typeclass instance for MySillyRedeemer will be generated.

  • unstableMakeIsData is [[Haskell Primer#Template Haskell]] function declared in PlutusTx.IsData.TH module.
    • unstableMakeIsData :: TH.Name -> TH.Q [TH.Dec]
  • ''MySillyRedeemer - quote MySillyRedeemer type as TH.Name