Transaction

Transaction building (sc-tools)

This skill documents the sc-tools transaction-building stack centered on Convex.BuildTx.

Workflow

1. Build an unbalanced TxBuilder era using MonadBuildTx helpers (spendPublicKeyOutput, spendPlutus*, payTo*, mint*, addWithdrawal, …).
2. Make the body balancing-friendly:
   • If you create outputs with tokens, inline datums, or reference scripts, apply setMinAdaDepositAll with protocol parameters.
   • If you build custom script witnesses, use buildScriptWitness/buildRefScriptWitness (they set placeholder ex-units that balancing will replace).
3. Balance it with Convex.CoinSelection (adds missing inputs, fees, collateral, execution units, checks min-UTxO, etc).
4. Sign and submit the resulting transaction.

Minimal skeleton

import Cardano.Api qualified as C
import Convex.BuildTx qualified as BuildTx
import Convex.CoinSelection qualified as CoinSelection
import Control.Tracer (nullTracer)

type Era = C.ConwayEra

mkTxBuilder
  :: C.TxIn
  -> C.AddressInEra Era
  -> C.Value
  -> BuildTx.TxBuilder Era
mkTxBuilder input recipient value =
  BuildTx.execBuildTx $ do
    BuildTx.spendPublicKeyOutput input
    BuildTx.payToAddress recipient value

-- Later (in a MonadBlockchain/MonadError context):
--
-- (tx, _changes) <-
--   CoinSelection.balanceForWallet nullTracer wallet walletUtxos (mkTxBuilder input recipient value) CoinSelection.TrailingChange
-- _txIdOrErr <- sendTx tx

Make outputs min-UTxO safe (recommended)

If you can query protocol parameters (e.g. in mockchain or a node-backed environment), apply:

txb <- BuildTx.execBuildTxT $ do
  ...
  pp <- queryProtocolParameters
  BuildTx.setMinAdaDepositAll pp

This avoids common CheckMinUtxoValueError failures during balancing.

If the era becomes ambiguous, prefer a local type alias/annotation (e.g. type Era = C.ConwayEra) rather than visible type applications on execBuildTxT.

Inputs to collect (when writing a tx)

• era: usually C.ConwayEra (specialize if you can).
• NetworkId: required for address construction in many helpers (payToPublicKey, payToScriptInlineDatum, etc).
• Spending inputs: C.TxIn for each UTxO you will spend.
• Script inputs:
  • Validator/minting policy as C.PlutusScript lang (inline) or a reference script C.TxIn + C.PlutusScriptVersion lang.
  • Datum mode (inline datum vs datum hash) and redeemers (must have Plutus.ToData).
• Outputs: recipient addresses, script hashes, datums, and the C.Value for each output.
• Balancing context:
  • Either a Convex.Wallet.Wallet + its UtxoSet, or
  • Payment credentials + MonadUtxoQuery (see Convex.Query).
• Extra requirements: collateral UTxO availability (ADA-only), required signers (addRequiredSignature), validity bounds, min-UTxO.

Use lenses when a helper doesn’t exist

Convex.BuildTx intentionally doesn’t wrap every TxBodyContent field. When you need to set something not covered (validity interval, metadata, governance fields, …), use Convex.CardanoApi.Lenses + addBtx:

import Control.Lens (set)
import Convex.CardanoApi.Lenses qualified as L

BuildTx.addBtx $
  set L.txValidityUpperBound (C.TxValidityUpperBound C.shelleyBasedEra (Just upperSlot))

Gotchas (high-signal)

• Era constraints matter: inline datums, reference inputs, and reference scripts require C.IsBabbageBasedEra era.
• TxBuilder can observe the final tx body: functions like addInputWithTxBody are powerful but can loop if you make the witness depend on itself.
• Order: TxBuilder’s Semigroup instance is intentionally reversed so that do a; b applies a before b.
• Indices are ledger-ordered: inputs are ordered by TxIn, withdrawals by stake address, minting by policy ID. Use lookupIndex* / findIndex* helpers; don’t assume insertion order.
• Lookahead requires placeholders: when constructing Plutus witnesses manually, use BuildTx.buildScriptWitness / BuildTx.buildRefScriptWitness (they use C.ExecutionUnits 0 0); balancing will substitute real ex-units.

Navigation

• API index: TRANSACTION.md
• Recipes: PATTERNS.md
• Debugging: TROUBLESHOOTING.md

Key code modules (in this repo):

• src/base/lib/Convex/BuildTx.hs
• src/coin-selection/lib/Convex/CoinSelection.hs
• src/optics/lib/Convex/CardanoApi/Lenses.hs

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