05-lecture.md

label	icon	author	order
Lecture 5	file	name email Antonio Hernandez contacto@antoniohernandez.mx	-8

Minting Policy

Lecture Videos :icon-link-external:

Topics covered:

1. Introduction
2. Values
3. A simple minting policy
4. A more realistic minting policy
5. NFT's
6. Homework

Native tokens and value

The topic of this lecture is how native tokens can be minted and burned in Cardano.

Recall that each UTxO has an address and a value (in adition of a Datum). In Plutus, the Value is defined as a map from CurrencySymbols to maps from TokenNames to Integers.

The Value module is defined in Plutus.V1.Ledger.Value.

Each native token (includying ADA) is identified by a currency symbol and a token name. Both are ByteStrings.

A value tells how many units are contained in each asset class. The latter is identified by a pair of currency symbol and token name.

Minting policies

Recall that, in general, the validation of a script requires a Datum, a Redeemer and a Context. For a minting policy, the Datum is not required.

The ScriptContext contains the TxInfo and the ScriptPurpose.

The ScriptContext is defined in Plutus.V1.Ledger.Contexts.

A minting policy is triggered if the txInfoMint field of the TxInfo has a non-zero value (a "bag" of different asset classes, each identified by a currency symbol and a token name), specified in the field txInfoMint.

For each currency symbol in the value specified by txInfoMint a corresponding minting policy script is run. Each currency symbol is the hash of one such minting policy script.

A minting policy script only has two inputs: a redeemer and a context (no datum). Recall that the transaction provides the redeemer.

A parametrized minting policy

In part 4, Lars discusses a minting policy that can only be triggered by a specified public key. This is achieved by parametrizing a minting policy so that it depends on a value of type PaymentPubKeyHash. The corresponding policy in PlutusCore takes an argument that has to be lifted.

NFT's

NFT's are discussed in part 5. An NFT is a token that can only exist once. The trick to ensure uniqueness of the token is to include the ID of the UTxO as a parameter in the minting policy. (There can not be two different UTxO's with the same ID; Lars gives an inductive argument proving this claim based on the fact that fees are always present.)

Notice that it is not enough to constrain a transaction to only mint once, since nothing prevents having many transactions with such a constraint.

Homework (spoiler alert)

Two problems:

1. Write a contract with a Shelly-era solution for creating an NFT, based on a deadline not too far in the future.

2. Write a minting policy for an NFT where the TokenName is an empty ByteString.

Solution to HW 1

Probably the main challenge was to create a minting policy with two parameters. My solution can be found in file mySolution1.hs .

Solution to HW 2

The only difficulty that I found was how to represent the empty ByteString. It is expressed using the data constructor TokenName, so that the empty ByteString is represented by TokenName emptyByteString.

Something that puzzles me is that apparently it is sufficient to put the empty ByteString constraint only on the off-chain part of the code. To be precise, I found that my solution still works if I sustitute the code

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

with the alternative

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

In any case, my solution can be found in file mySolution2.hs .