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As a newbie in Plutus and Haskell, I write code by printing/logging variables to see their value, which is the only reason why I managed to write Contract, off-chain Plutus part. My question is, are there any possible ways (even hacky ones, doesn't matter), to inspect variable value in on-chain, validator Plutus part.

I am aware of trace, traceIfFalse, etc. All of the functions can be found here: https://playground.plutus.iohkdev.io/doc/haddock/plutus-tx/html/PlutusTx-Trace.html But it doesn't help, as you can't inspect variables.

This enhancement seems to be dead: https://github.com/input-output-hk/plutus/issues/3164 Searched a lot of sources and found nothing useful. I am aware why it's difficult (BuiltInString, absence of Show), only asking for possible solutions/tricks/hacks and not for explanation why it's impossible.

This question is related: Logging in validator

Thanks

EDIT:

let's image I have a validator:

mkValidator :: Order -> Order -> OrderAction -> ScriptContext -> Bool
mkValidator o oDatum act ctx = True

how can I print the value of for example ctx (ScriptContext), or oDatum, then executing the whole script with Emulator, so that whole script compiles and I can see ctx Value somewhere in EmulatorTrace (or any other way)?

I mean something like this:

mkValidator o oDatum act ctx = trace "test" ctx && True
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  • Why is impossible to inspect variables with trace? Could you include specific examples of what you're trying to do? Nov 20 at 22:28
  • let's image I have a validator: mkValidator :: Order -> Order -> OrderAction -> ScriptContext -> Bool mkValidator o oDatum act ctx = True. how can I print the value of for example ctx (ScriptContext), or oDatum, then executing the whole script with Emulator, so that whole script compiles and I can see ctx Value somewhere in EmulatorTrace? I mean something like this: mkValidator o oDatum act ctx = trace "test" ctx && True
    – serx
    Nov 21 at 10:26
1

I could not find a way to work around this yet. Validation is a side effect free function (a pure function) so it can't write to any log.

What I do is create a debug endpoint in my contract log info there

So I will have something like this

debug :: AsContractError e => () -> Contract w Schema e ()
debug _ = do
        utxos <- utxosAt address
        Contract.logInfo @String $ show utxos
        Contract.logInfo @String $ show (length utxos)
        wallet1Utxos <- utxosAt . walletAddress $ knownWallet 2
        Contract.logInfo @String $ show wallet1Utxos
        wallet2Utxos <- utxosAt . walletAddress $ knownWallet 1
        Contract.logInfo @String $ "Utxos for wallet 2: " ++ show wallet2Utxos

And then use that in a EmulatorTrace()

mintAndTransfer :: EmulatorTrace ()
mintAndTransfer = do
        let tn = "USDC"
        h1 <- activateContractWallet (knownWallet 1) (endpoints @ContractError)
        callEndpoint @"mint" h1 $ MintParams
                { mintParamsTokenName = tn
                , mintParamsAmount = 100 }
        void $ waitNSlots 1
        callEndpoint @"mint" h1 $ MintParams
                { mintParamsTokenName = tn
                , mintParamsAmount = 200 }
        void $ waitNSlots 1
        callEndpoint @"transferToWallet" h1 $ TransferToWalletParams
                { tpTo = walletPubKeyHash $ knownWallet 2
                , tpAmount = 50
                , tpTokenName = tn }
        void $ waitNSlots 1
        callEndpoint @"debug" h1 ()
        void $ waitNSlots 1

Not ideal I know but it helps sometime.

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There is a recent update regarding the error message from validator onchain here https://github.com/input-output-hk/cardano-node/releases If you update your cardano-node to version 1.31.0, you will be able to see log message from traceIfFalse. Something like

The provided Plutus code called 'error'.
Script debugging logs: UTxO not consumed

Like @Calin answered, you can always run any test with the Emulator Trace, which gives you where it fails onchain.

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  • It is still just a pre-prepared string logging, not variable logging though
    – serx
    yesterday
  • 1
    Yes, that's right, but at least it gets improved and now while testing on testnet/mainnet we at least know which condition doesn't meet. yesterday

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