Obscure error while trying to unlock funds from a script from the CLI

Question

I have been trying to interact with a simple script I created using the CLI, and I am getting an error that I don't understand:

This is the error:

The Plutus script evaluation failed: An error has occurred:  User error:
The provided Plutus code called 'error'.
Caused by: [ (builtin unConstrData) (con data #80) ]

This is the script:

{-|
Module      : Horrocubes.Counter.
Description : Plutus script that keeps track of an internal counter.
License     : Apache-2.0
Maintainer  : [email protected]
Stability   : experimental

This script keeps a counter and increases it everytime the eUTXO is spent.
-}

-- LANGUAGE EXTENSIONS --------------------------------------------------------

{-# LANGUAGE DataKinds                  #-}
{-# LANGUAGE DeriveAnyClass             #-}
{-# LANGUAGE DeriveGeneric              #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE FlexibleContexts           #-}
{-# LANGUAGE MultiParamTypeClasses      #-}
{-# LANGUAGE NoImplicitPrelude          #-}
{-# LANGUAGE OverloadedStrings          #-}
{-# LANGUAGE ScopedTypeVariables        #-}
{-# LANGUAGE TemplateHaskell            #-}
{-# LANGUAGE TypeApplications           #-}
{-# LANGUAGE TypeFamilies               #-}
{-# LANGUAGE TypeOperators              #-}
{-# LANGUAGE DerivingStrategies         #-}
{-# LANGUAGE LambdaCase                 #-}
{-# LANGUAGE NamedFieldPuns             #-}
{-# LANGUAGE ViewPatterns               #-}

-- MODULE DEFINITION ----------------------------------------------------------

module Horrocubes.Counter
(
  counterScript,
  counterScriptShortBs,
  CounterParameter(..)
) where

-- IMPORTS --------------------------------------------------------------------

import           Cardano.Api.Shelley      (PlutusScript (..), PlutusScriptV1)
import           Codec.Serialise
import qualified Data.ByteString.Lazy     as LBS
import qualified Data.ByteString.Short    as SBS
import           Ledger                   hiding (singleton)
import qualified Ledger.Typed.Scripts     as Scripts
import           Ledger.Value             as Value
import qualified PlutusTx
import           PlutusTx.Prelude         as P hiding (Semigroup (..), unless)
import           Data.Aeson               (FromJSON, ToJSON)
import           GHC.Generics             (Generic)
import qualified Ledger.Contexts          as Validation
import           Text.Show
import           PlutusTx.Builtins


-- DATA TYPES -----------------------------------------------------------------

-- | The parameters for the counter contract.
data CounterParameter = CounterParameter {
        ownerPkh    :: !PubKeyHash, -- ^ The transaction that spends this output must be signed by the private key
        identityNft :: !AssetClass  -- ^ The NFT that identifies the correct eUTXO.
    } deriving (Show, Generic, FromJSON, ToJSON)

PlutusTx.makeLift ''CounterParameter

-- PlutusTx.unstableMakeIsData ''CounterDatum

-- | The Counter script type. Sets the Redeemer and Datum types for this script.
data Counter 
instance Scripts.ValidatorTypes Counter where
    type instance DatumType Counter = BuiltinData
    type instance RedeemerType Counter = ()
    
-- DEFINITIONS ----------------------------------------------------------------

-- | Maybe gets the datum from the transatcion output.
{-# INLINABLE counterDatum #-}
counterDatum :: TxOut -> (DatumHash -> Maybe Datum) -> Maybe Integer
counterDatum o f = do
    dh      <- txOutDatum o
    Datum d <- f dh
    PlutusTx.fromBuiltinData d

-- | Checks that the identity NFT is locked again in the contract.
{-# INLINABLE isIdentityNftRelocked #-}
isIdentityNftRelocked:: CounterParameter -> Value -> Bool
isIdentityNftRelocked params valueLockedByScript = assetClassValueOf valueLockedByScript (identityNft params) == 1

-- | Creates the validator script for the outputs on this contract.
{-# INLINABLE mkCounterValidator #-}
mkCounterValidator :: CounterParameter -> BuiltinData -> () -> ScriptContext -> Bool
mkCounterValidator parameters oldDatum _ ctx = 
    let oldCounterValue        = oldDatumIntegerValue
        isRightNexCounterValue = (newDatumValue == (oldCounterValue + 1))
    in traceIfFalse "Wrong counter value"           isRightNexCounterValue
    where
        info :: TxInfo
        info = scriptContextTxInfo ctx

        ownOutput :: TxOut
        ownOutput = case getContinuingOutputs ctx of
            [o] -> o
            _   -> traceError "Expected exactly one output"

        newDatumValue :: Integer
        newDatumValue = case counterDatum ownOutput (`findDatum` info) of
            Nothing -> traceError "Counter output datum not found"
            Just datum  -> datum

        oldDatumIntegerValue :: Integer
        oldDatumIntegerValue = case PlutusTx.fromBuiltinData oldDatum of
            Nothing -> traceError "Counter output datum not found"
            Just datum  -> datum

        valueLockedByScript :: Value
        valueLockedByScript = Validation.valueLockedBy info (Validation.ownHash ctx)

        isTransactionSignedByOwner :: Bool
        isTransactionSignedByOwner = txSignedBy info (ownerPkh parameters)

-- | The script instance of the counter. It contains the mkCounterValidator function
--   compiled to a Plutus core validator script.
counterInstance :: CounterParameter -> Scripts.TypedValidator Counter
counterInstance counter = Scripts.mkTypedValidator @Counter
    ($$(PlutusTx.compile [|| mkCounterValidator ||]) `PlutusTx.applyCode` PlutusTx.liftCode counter) $$(PlutusTx.compile [|| wrap ||])
    where
        wrap = Scripts.wrapValidator @BuiltinData @()

-- | Gets the counter validator script that matches the given parameters.
counterValidator :: CounterParameter -> Validator
counterValidator params = Scripts.validatorScript . counterInstance $ params

-- | Generates the plutus script.
counterPlutusScript :: CounterParameter -> Script
counterPlutusScript params = unValidatorScript $ counterValidator params

-- | Serializes the contract in CBOR format.
counterScriptShortBs :: CounterParameter -> SBS.ShortByteString
counterScriptShortBs params = SBS.toShort . LBS.toStrict $ serialise $ counterPlutusScript params

-- | Gets a serizlized plutus script from the given parameters.
counterScript :: PubKeyHash -> AssetClass -> PlutusScript PlutusScriptV1
counterScript pkh ac = PlutusScriptSerialised $ counterScriptShortBs $ CounterParameter { ownerPkh = pkh,  identityNft = ac }

This is how I am creating the transaction:

cardano-cli transaction build --alonzo-era --testnet-magic 1097911063  --change-address $(cat counter/payment.addr) --tx-in-collateral 7d65ca46aca44532d94da57ec6b7297efdda2e523626f90bbdc780f3767202f2#0 --tx-in 0be3e055fa3e51b34baa1cc5589397520ee79670dbf2428f7884713ba0066f4f#1 --tx-in-script-file ./counter/out2.plutus --tx-in-datum-file ./datum_0.json --tx-in-redeemer-value [] --tx-out "addr_test1vpfvmwfl8eucm8rnsej9pehzh7628k53raczagz4uvzzm2csx7sfl+1000000" --tx-out-datum-embed-file ./datum_1.json  --protocol-params-file protocol.json --out-file tx-script2.build

I have been trying to find the root cause of the error to no avail. It seems it is related to one of the Datum inputs, but I don't the error on the script or the datum data.

This is what is on the datum.json files:

datum_0.json

{"constructor":0,"fields":[{"int":0}]}

datum_1.json

{"constructor":0,"fields":[{"int":1}]}

Answer 1

I finally made it work, I will leave it here in case anyone encounters the same issue.

The problem was the redeemer. You have to be careful with the encoding of the datums and the redeemer (even if you are not using the redeemer), this one took me a really long time, as I didn’t understand why I was getting some constructor error for a built-in type, I thought there was something wrong with the datums, it turned out to be the redeemer, I was using [] as a dummy value, but in reality, you have to pass a valid value as it will try to construct a value for the redeemer even if you only defined it as unit ().