Plutus: Pioneers Program 4th cohort - Lecture 5 - Minting

This weeks' lesson is focused on NFT (Non-fungible token), which are "tokens" (non-divisible, duplicable, or copyable).

Cardano contains three elements around NTFs:

• Currency
• Token
• Asset

Defined as follows:

newtype CurrencySymbol = CurrencySymbol BuiltinByteString

newtype TokenName = TokenName BuiltinByteString

newtype AssetClass = AssetClass (CurrencySymbol, TokenName)

AssetClass represents a token.

Actually, adas (and, by extension, lovelaces, since 1 ada == 1 000 000 lovelace) are simply special tokens:

adaSymbol :: CurrencySymbol
adaSymbol = CurrencySymbol emptyByteString

adaToken :: TokenName
adaToken = TokenName emptyByteString

They are integrated in Value such as:

newtype Value = Value (Map CurrencySymbol (Map TokenName Integer))

Actually, it's important to highlight that, usually scripts don't mint (create) or burnt tokens, but when they do, they have to comply to a minting policy.

Like for scripts, minting policies have an address (which is CurrencySymbol), and is run when working around tokens.

These minting policies look like validator scripts:

mkPolicy :: () -> ScriptContext -> Bool
mkPolicy () _ = True

mkWrappedPolicy :: BuiltinData -> BuiltinData -> ()
mkWrappedPolicy = wrapPolicy mkPolicy

myPolicy :: MintingPolicy
myPolicy = mkMintingPolicyScript $$(PlutusTx.compile [|| mkWrappedPolicy ||])

myCurrencySymbol :: CurrencySymbol
myCurrencySymbol = currencySymbol myPolicy

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

validator :: PlutusV2.Validator
validator = PlutusV2.mkValidatorScript $$(PlutusTx.compile [|| mkValidator ||])

More here:

• https://developers.cardano.org/docs/native-tokens/

Note: At some points, I tried many variations to solve the homeworks, ie.

    checkMintedAmount :: Bool
    checkMintedAmount = case flattenValue (txInfoMint info) of
        [(_, "", 1)] -> True
        _            -> False

    -- or
    checkMintedAmount :: Bool
    checkMintedAmount = case flattenValue (txInfoMint info) of
        [(_, tn'', amt)] -> "" == policyTokenName && amt == 1
        _                -> False

    -- or
    checkMintedAmount :: Bool
    checkMintedAmount = case flattenValue (txInfoMint info) of
        [(_, tn'', amt)] -> tn'' == policyTokenName && amt == 1
        _                -> False

{-# INLINABLE policyTokenName #-}
policyTokenName :: TokenName
policyTokenName = ""

but since Plutus is a subset of Haskell, I ended-up with errors:

Exception: Error: Unsupported feature: Use of == from the Haskell Eq typeclass
Exception: Error: Unsupported feature: Type constructor: GHC.Prim.Char#
Exception: Error: Unsupported feature: Use of fromString on type other than builtin strings or bytestrings: Plutus.V1.Ledger.Value.TokenName

sadly, it's at runtime, which is frustrating to me.