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plutarch-context-builder

Public discussion and assistance can be found on discord @ #liqwid-plutarch-extra. Issues and project-management-related information are tracked on Notion.

What is this?

A library containing builders for ScriptContexts. Currently, we support building ScriptContexts with the Spending and Minting ScriptPurposes.

What exactly does this do for me?

Building a ScriptContext manually is sometimes necessary: testing is a good example. However, doing this directly is tedious, error-prone and extremely unclear in many places; this leads to significant wasted time and confusion.

plutarch-context-builder aims to make this as easy as possible. We do this through a combination of higher-level operations to describe what the script should do, along with a Monoid-based API to put things together. Users of this library don't have to deal with the particulars of how ScriptContexts are defined if they don't want to, but they can still generate them reliably and with minimal pain. More, the monoidal API allows overrides on specific fields; for example, overriding Transaction ID. This allows users to create a slightly different script context without laborious record overriding or reconstructing contexts from scratch.

plutarch-context-builder fits into the testing strategy of Liqwid Labs by providing an ergonomic interface for "sample tests" in conjunction with plutarch-unit(a tasty provider). Sample tests are useful for

  • Demonstrating the intended "happy path" of a script or transaction family
  • Unit testing individual validators in isolation (i.e., not testing a full transaction). This can be useful, for instance, if multiple related validators are required to test a full transaction, but not all of them are fully implemented.
  • Unit testing helper functions that work on a subset of the fields of a ScriptContext or TxInfo, but that don't fit the Datum -> Redeemer -> ScriptContext -> () of a validator
  • Regression testing -- making specific, individual test cases when things break.

For other testing libraries that complement the functionality of plutarch-context-builder, see:

How do I use this?

Everything is done monoidally in plutarch-context-builder, from setting UTXO to adding values. Users can use <> to combine interface functions into larger script contexts.

There are two main types to consider: UTXO and Builder a. The former constructs single UTXO in transactions:

data UTXO = UTXO
    { utxoCredential :: Maybe Credential
    , utxoStakingCredential :: Maybe StakingCredential
    , utxoValue :: Value
    , utxoData :: Maybe DatumType
    , utxoReferenceScript :: Maybe ScriptHash
    , utxoTxId :: Maybe TxId
    , utxoTxIdx :: Maybe Integer
    }

There are a few things to note. First, a UTXO can be shared in different contexts as both input or output; TxOutRef-related fields get ignored when a UTXO is being used as an output. Second, besides Value, the binary operator(<>) will replace previous values. It allows users to reuse pre-defined UTXOs with updated information. Note that the binary operator will always update the last given data.

let utxo = address "aabbcc" <> withStakingCredential (StakingPtr 1 2 3)
in utxo <> withStakingCredential (StakingPtr 7 8 9)
-- utxo is "updated" and now has 'StakingPtr 7 8 9'

Builder a primarily appends instead of replacing. It will collect multiple inputs, reference inputs, outputs, signatures, et cetra and build the context out of it. User can expect it to combine similarly to regular lists. Builder a will only replace the TxId and time range.

Builder a is a typeclass, containing "common" script-building interfaces. Using this, plutarch-context-builder defines specific builders such as SpendingBuilder, MintingBuilder, and TxInfoBuilder. These specific builders are specialized on handling the ScriptPurpose when a full ScriptContext is generated; they will require calling some special interfaces --still Monoidal-- to build the ScriptContext properly.

Example

The example source can be found in the example directory.

-- What can be shared between contexts
commonContext :: (Monoid a, Builder a) => a
commonContext =
	mkOutRefIndices $ -- automatically assigns OutRefIdx
    mconcat
        [ input $
            pubKey "aabb"
                <> withValue (singleton "cc" "hello" 123)
                <> withRefIndex 5
                <> withStakingCredential (StakingPtr 0 0 0)
        , input $
            address (Address (PubKeyCredential $ PubKeyHash "aa") (Just $ StakingPtr 1 2 3))
                <> withValue (singleton "cc" "hello" 123)
                <> withDatum (123 :: Integer)
                <> withRefTxId "eeff"
        , output $
            script "cccc"
                <> withValue (singleton "dd" "world" 123)
        , mint $ singleton "aaaa" "hello" 333
        ]
		
spendingContext :: SpendingBuilder
spendingContext = commonContext <> withSpendingOutRefIdx 5

spendingContext2 :: SpendingBuilder
spendingContext2 = commonContext <> withSpendingOutRefId "eeff"

mintingContext :: Mintingbuilder
mintingContext = generalSample <> withMinting "aaaa"

-- Build contexts
buildMinting mempty mintingContext
buildSpending mempty spendingContext

-- Builders that will return a product type instead of throwing error.
tryBuildMinting mempty mintingContext
tryBuildSpending mempty spendingContext

Checkers

plutarch-context-builder provides various checkers and some combinators to construct custom ones. It comes with quasi-phase-1 validation that will ensure all bytestrings are of the correct length, the inflow and outflow of the context is equal, at least one signature is provided, et cetra.

Users can also construct a custom checker with a common contravariant interface:

data MyError = MoreThanOneInput

ensureOnlyOneInput :: Builder a => Checker MyError a
ensureOnlyOneInput = 
	checkAt AtInput $
	mconcat $
	[ contramap ((== 1) . length . bbInputs . unpack) (checkBool $ OtherError MoreThanOneInput)
	, ... -- Multiple checkers can be combined as well.
	, ...
	]
	
-- Will return `Acc` list of errors
-- Empty list when no error is found
runChecker ensureOnlyOneInput context

-- User can run phase-1 validation with
buildSpending checkPhase1 context

If more sophisticated validation is required, plutus-simple-model should be used. plutus-simple-model uses functions directly from cardano-ledger, and thus is more true-to-life than plutarch-context-builder. plutarch-context-builder aims to test simpler functionalities very quickly and intuitively, while plutus-simple-model almost simulates an entire contract flow.

Standards

The plutarch-context-builder code base follows strict standards to increase consistency, to minimize the impact of legacy, to properly use automated tools, and more. The standards document can be found here.