Constitution Script

The overall purpose of a "Constitution Script" is to define in Plutus, that part of the Cardano Constitution which is possible to be automated as a smart contract.

Currently, in this repository, we are focusing on defining a constitution script to check that a ParameterChange proposal or TreasuryWithdrawals proposal is "constitutional". In the future, we may enhance the script to automate more parts of the Cardano Constitution.

The script is written in the high-level PlutusTx language, which is subsequently compiled to Untyped Plutus Core and executed on then chain upon every new Governance Proposal.

FIXME: any governance proposal?

Being a smart contract, the constitution script is a validator function of the form:

const_script :: BuiltinData -> BuiltinUnit

The sole argument to this function is the BuiltinData-encoded V3.ScriptContext. Note the absence of the 2 extra arguments, previously known as Datum argument and the Redeemer argument. Since V3 and CIP-69, the Datum and Redeemer values are not passed anymore as separate function arguments, but embedded inside the V3.ScriptContext argument. The "proposal under investigation" is also embedded inside the ScriptContext.

Datum is not provided for the "constitution script", since it is not a spending validator. Redeemer will be provided to the "constitution script", but the current script implementations ignore any value given to it (see Clause D).

When the script is fully applied, one of the 2 cases can happen:

1. The script executes successfully by returning BuiltinUnit, which means that the Proposal under investigation is constitutional (the next step of the process would be to vote on this proposal, but this is out-of-scope of this repository).

2. The script fails with an error. There can be many different reasons for a script error:

• logical error in the constitution script (in config and/or engine, see next section)
• Proposal is malformed
• Proposal violates a constitution rule.
• bug in the CEK evaluator

Irregardless of the specific error, the outcome is the same: the proposal would be un-constitutional (and no further steps will be taken for this proposal).

Constitution Rules (a.k.a. GuardRails)

The constitution rules (a.k.a. guardrails) may change in the future, which may require that the constitution script be also accordingly updated, and re-submitted on the chain so as to be "enacted".

To minimise the chances of introducing bugs when the constititution script has to be updated, we decided to separate the fixed "logic part" of the script from the possibly evolving part, i.e. the "constitution rules". For this reason, the constitution rules are separately given as a PlutusTx ADT (with type Config), which when applied to the fixed-logic part (named engine for short) yields the actual constitution script:

data Config = ... -- see Config/Types.hs
const_engine :: Config -> V3.ScriptContext -> BuiltinUnit
const_engine = ...fixed logic...

const_script :: V3.ScriptContext -> BuiltinUnit
const_script = const_engine my_config

In other words, the Config is "eliminated" statically at compile-time, by partially applying it to the constitution engine.

These constitution rules can be thought of as predicates (PlutusTx functions that return Bool) over the proposed values. We currently have 3 such predicates:

minValue jsonValue proposedValue = jsonValue Tx.=< proposedValue
maxValue jsonValue proposedValue = jsonValue Tx.>= proposedValue
notEqual jsonValue proposedValue = jsonValue Tx./= proposedValue

An alternative & preferred method than constructing a Config value, is to edit a configuration file that contains the "constitution rules" laid out in JSON. Its default location is at data/defaultConstitution.json, with its expected JSON schema specified at data/defaultConstitution.schema.json. After editing this JSON configuration file and re-compiling the cabal package, the JSON will be statically translated to a Config PlutusTx-value and applied to the engine to yield a new script. This is the preferred method because, first, it does not require any prior PlutusTx/Haskell knowledge and second, there can be extra sanity checks applied: e.g. when parsing/translating the JSON to Config or when using an external JSON schema validator.

ChangedParameters Format

In case of a ParameterChange governance action, the ledger will construct out of the proposed parameters, a ChangedParameters value, encode it as BuiltinData, then pass it onto us (the Constitution script) inside the V3.ScriptContext. The ChangedParameters valueis decoded as aTx.AssocMap`:

ChangedParameters => Tx.AssocMap ChangedParamId ChangedParamValue
ChangedParamId => I Integer
ChangedParamValue => I Integer
          , or => (I Integer, Cons(I Integer, Nil)) -- a Rational numerator, denominator, a.k.a unit_interval
          , or => List(ChangedParamValue) -- an arbitrary-length, heterogeneous list of (integer, unit_interval) values

Integer is the usual arbitrary-precision Integer of Haskell/PlutusTx. There is no other type of a changed parameter (e.g. nested-list parameter), so the script implementations will fail on any other format.

Specification of script implementation

There can be many different implementations of the logic (engine) made for the constitution script. A particular script implementation behaves correctly (is valid), when it complies with all the following clauses:

Clauses

• S01. If thisGovAction is TreasuryWithdrawals _ _, then PASS and no checks left.
• S02. If thisGovAction is (ParameterChange _ proposedParams _) and the decoded proposedParams is an empty list (a.k.a. Tx.AssocMap), then UNSPECIFIED.
• S03. If thisGovAction is (ParameterChange _ proposedParams _) and decoded proposedParams is a non-empty list, start checking each proposed parameter in the list against the Config.
• S04. The Redeemer BuiltinData value is UNSPECIFIED.
• S05. In all other cases of decoded ScriptContext return FAIL.
• S06. Lookup in the Config the rules associated to the current proposedParam's id and test these rules against the proposedParam's value. If one or more tests fail => FAIL. Otherwise, set the next proposedParam in the list as the current one and continue to (F).
• S07. If no proposedParam to check is left in the proposedParams list, PASS and no more checks left.
• S08. If a proposedParam's id is not found in the Config => FAIL. This can happen if the parameter is unknown, or is known but wrongfully omitten from the config file.
• S09. If the Config says {type: any} under a given proposedParam , then do not try to decode the value of the proposedParam, but simply PASS and continue to next check.
• S10. In all other cases of {type: integer/unit_interval/list}, decode the proposedParam value according to the expected type (see "ChangedParameters Format"). If the encoding of the proposedParam value does not match the expected encoding of that type, FAIL.
• S11. In case of expected type list, if more or less than the expected length of the list elements are proposed, FAIL.

Legend

• thisGovAction : (fromBuiltinData(v3_context) -> scriptcontextScriptInfo -> (ProposingScript _ (ProposalProcedure _ _ thisGovAction)))
• PASS: An implementation accepts the check, and continues with the rest of the checks. If there are no checks left, the script returns BuiltinUnit, thus deeming this proposal constitutional.
• FAIL: An implementation must make the overall script fail with an error (explicitly by calling Tx.error () or implicitly e.g. 1/0), thus deeming the proposal un-constitutional.
• UNSPECIFIED: The behavior is explicitly left unspecified, meaning that implementations may decide to PASS or FAIL or loop indefinitely --- note that in reality, looping indefinitely behaves the same as FAIL since plutus scripts are "guarded" by certain resource limits.

Ledger guarantees

Although not part of the specification, the ledger provides us extra guarantees, which a valid implementation may optionally rely upon (i.e. take it as an assumption):

• G01. The underlying AssocMap does not contain duplicate-key entries.
• G02. The underlying AssocMap is sorted on the keys (the usual Ordering Integer).
• G03. The underlying AssocMap is not empty.
• G04. Unit_Interval's denominator is strictly positive.
• G05. Unit_Interval's numerator and denominator are co-prime.
• G06. Unit_Interval's value range is [0,1] (i.e. both sides inclusive).
• G07. Protocol parameter IDs are positive.
• G08. Redeemer is encoded as (). FIXME: any governance proposal?

Current script implementations

There are 2 engine implementations:

• Unsorted
• Sorted

Unsorted and Sorted must be valid implementations, so they must comply to all clauses [S01..].

Unsorted does not rely on any ledger guarantees. Sorted as the name implies relies on sortedness to work and thus assumes the G01,G02 guarantees. Sorted further requires that the Config is also sorted, which must be guaranteed by-construction when using the JSON Config format (this is not currently guaranteed when manually constructing a Config ADT value).

Note that, although all implementations could theoretically work without problem with negative proposedParam ids, the Config JSON format (not the ADT) and the Ledger are limited only to positive ids (see G07).

The Sorted implementation will most likely be the implementation to be used on the mainnet chain.

Testing the implementations

The testing infrastructure generates artificial proposals and unit/random tests them against the 2 valid implementations, unsorted and sorted.

The artificial proposals are built such as to satisfy all specification clauses [S01..] (required for validity).

Since this repository's testing infrastructure cannot be aware or test the ledger's behavior, we have to make explicit the ledger guarantees that the test code needs to rely upon. To keep things simple and uniform, we decided that the (random) testing infrastructure has to rely on the union (Sum) of the ledger guarantees required by all our current implementations, i.e. G01,G02.

Repository layout

• src/Cardano/Constitution/Config/* : types and instances for the Config ADT
• src/Cardano/Constitution/Config.hs : "predicate meanings" and umbrella module for the Config
• src/Cardano/Constitution/Validator/Sorted.hs : sorted engine
• src/Cardano/Constitution/Validator/Unsorted.hs : unsorted engine
• src/Cardano/Constitution/Validator/Common.hs : common code between the 2 engines
• data/* : contains the JSON configuration files
• test/*: testing code