Table of Contents

• Aiken Library for Common Design Patterns in Cardano Smart Contracts
  • How to Use
  • How to Run Package Tests
  • Provided Patterns
    • Stake Validator
      • Endpoints
    • UTxO Indexers
      • Singular UTxO Indexer
        • One-to-One
        • One-to-Many
      • Multi UTxO Indexer
    • Transaction Level Validator Minting Policy
    • Validity Range Normalization
    • Merkelized Validator

Aiken Library for Common Design Patterns in Cardano Smart Contracts

To help facilitate faster development of Cardano smart contracts, we present a collection of tried and tested modules and functions for implementing common design patterns.

How to Use

Install the package with aiken:

aiken package add anastasia-labs/aiken-design-patterns --version main

And you'll be able to import functions of various patterns:

use aiken_design_patterns/merkelized_validator as merkelized_validator
use aiken_design_patterns/multi_utxo_indexer as multi_utxo_indexer
use aiken_design_patterns/singular_utxo_indexer as singular_utxo_indexer
use aiken_design_patterns/stake_validator as stake_validator
use aiken_design_patterns/tx_level_minter as tx_level_minter

Check out validators/ to see how the exposed functions can be used.

How to Run Package Tests

Here are the steps to compile and run the included tests:

1. Clone the repo and navigate inside:

git clone https://github.com/Anastasia-Labs/aiken-design-patterns
cd aiken-design-patterns

2. Run the build command, which both compiles all the functions/examples and also runs the included unit tests:

aiken build

3. Execute the test suite:

aiken check

Test results:

Provided Patterns

Stake Validator

This module offers two functions meant to be used within a multi-validator for implementing a "coupled" stake validator logic.

The primary application for this is the so-called "withdraw zero trick," which is most effective for validators that need to go over multiple inputs.

With a minimal spending logic (which is executed for each UTxO), and an arbitrary withdrawal logic (which is executed only once), a much more optimized script can be implemented.

Endpoints

spend merely looks for the presence of a withdrawal (with arbitrary amount) from its own reward address.

withdraw takes a custom logic that requires 3 arguments:

1. Redeemer (arbitrary Data)
2. Script's validator hash (Hash<Blake2b_224, Script>)
3. Transaction info (Transaction)

UTxO Indexers

The primary purpose of this pattern is to offer a more optimized solution for a unique mapping between one input UTxO to one or many output UTxOs.

Singular UTxO Indexer

One-to-One

By specifying the redeemer type to be a pair of integers ((Int, Int)), the validator can efficiently pick the input UTxO, match its output reference to make sure it's the one that's getting spent, and similarly pick the corresponding output UTxO in order to perform an arbitrary validation between the two.

Note

Neither of singular UTxO indexer patterns provide protection against the double satisfaction vulnerability, as this can be done in multiple ways depending on the contract.

The provided example validates that the two are identical, and each carries a single state token apart from Ada.

One-to-Many

Here the validator looks for a set of outputs for the given input, through a redeemer of type (Int, List<Int>) (output indices are required to be in ascending order to disallow duplicates). To make the abstraction as efficient as possible, the provided higher-order function takes 3 validation logics:

1. A function that validates the spending Input (single invocation).
2. A function that validates the input UTxO against a corresponding output UTxO. Note that this is executed for each associated output.
3. A function that validates the collective outputs. This also runs only once. The number of outputs is also available for this function (its second argument).

Multi UTxO Indexer

While the singular variant of this pattern is primarily meant for the spending endpoint of a contract, a multi UTxO indexer utilizes the stake validator provided by this package. And therefore the spending endpoint can be taken directly from stake_validator.

Subsequently, spend redeemers are irrelevant here. The redeemer of the withdrawal endpoint is expected to be a properly sorted list of pairs of indices (for the one-to-one case), or a list of one-to-many mappings of indices.

It's worth emphasizing that it is necessary for this design to be a multi-validator as the staking logic filters inputs that are coming from a script address which its validator hash is identical to its own.

The distinction between one-to-one and one-to-many variants here is very similar to the singular case, so please refer to its section above for more details.

The primary difference is that here, input indices should be provided for the filtered list of inputs, i.e. only script inputs, unlike the singular variant where the index applies to all the inputs of the transaction. This slight inconvenience is for preventing extra overhead on-chain.

Transaction Level Validator Minting Policy

Very similar to the stake validator, this design pattern utilizes a multi-validator comprising of a spend and a minting endpoint.

The role of the spendig input is to ensure the minting endpoint executes. It does so by looking at the mint field and making sure a non-zero amount of its asset (where its policy is the same as the multi-validator's hash, and its name is specified as a parameter) are getting minted/burnt.

The arbitrary logic is passed to the minting policy so that it can be executed a single time for a given transaction.

Validity Range Normalization

The datatype that models validity range in Cardano currently allows for values that are either meaningless, or can have more than one representations. For example, since the values are integers, the inclusive flag for each end is redundant and can be omitted in favor of a predefined convention (e.g. a value should always be considered inclusive).

In this module we present a custom datatype that essentially reduces the value domain of the original validity range to a smaller one that eliminates meaningless instances and redundancies.

The datatype is defined as following:

pub type NormalizedTimeRange {
  ClosedRange { lower: Int, upper: Int }
  FromNegInf  {             upper: Int }
  ToPosInf    { lower: Int             }
  Always
}

The exposed function of the module (normalize_time_range), takes a ValidityRange and returns this custom datatype.

Merkelized Validator

Since transaction size is limited in Cardano, some validators benefit from a solution which allows them to delegate parts of their logics. This becomes more prominent in cases where such logics can greatly benefit from optimization solutions that trade computation resources for script sizes (e.g. table lookups can take up more space so that costly computations can be averted).

This design pattern offers an interface for off-loading such logics into an external withdrawal script, so that the size of the validator itself can stay within the limits of Cardano.

Note

While currently the sizes of reference scripts are essentially irrelevant, they'll soon impose additional fees. See here for more info.

The exposed spend function from merkelized_validator expects 3 arguments:

1. The hash of the withdrawal validator that performs the computation.
2. The list of arguments expected by the underlying logic.
3. The Dict of all redeemers within the current script context.

This function expects to find the given stake validator in the redeemers list, such that its redeemer is of type WithdrawRedeemer (which carries the list of input arguments and the list of expected outputs), makes sure provided inputs match the ones given to the validator through its redeemer, and returns the outputs (which are carried inside the withdrawal redeemer) so that you can safely use them.

For defining a withdrawal logic that carries out the computation, use the exposed withdraw function. It expects 3 arguments:

1. The computation itself. It has to take a list of generic inputs, and return a list of generic outputs.
2. A redeemer of type WithdrawRedeemer<a, b>. Note that a is the type of input arguments, and b is the type of output arguments.
3. The script context.

It validates that the puropse is withdrawal, and that given the list of inputs, the provided function yields identical outputs as the ones provided via the redeemer.