Building Cardano Transactions

Scalus provides a fluent API for constructing Cardano transactions — payments, token minting, staking, governance, and smart contract interactions.

What You Can Build

Payments — Send ADA and native tokens
Spending UTxOs — Spending UTxOs from wallet and script addresses
Token Minting — Create and burn native assets
Staking — Delegate stake and withdraw rewards
Governance — DRep registration and voting
Smart Contract Interactions — Spend script UTxOs with Plutus validators

Transaction Builder

TransactionBuilder (TxBuilder) handles the complexity of Cardano transactions — UTxO selection, fee calculation, script evaluation, and balancing:


val tx = TxBuilder(env)
  .payTo(recipient, Value.ada(10))
  .complete(provider, myAddress)
  .await()
  .sign(signer)
  .transaction

Since validators are written in Scala, you can share code between on-chain and off-chain:

Code Reuse — Share data types and validation logic
Type Safety — Scala’s type system across your entire stack
Integrated Testing — Test validators and transactions together

Quick Example


val tx = TxBuilder(env)
  .spend(utxo)                             // Add UTxO as input
  .payTo(recipientAddress, Value.ada(10))  // Send 10 ADA to recipient
  .build(changeTo = changeAddress)         // Finalize: calculate fees, handle change
  .sign(signer)                            // Add signature
  .transaction                             // Get the final transaction

TxBuilder is purely functional — each method returns a new immutable instance. Only build() can throw while evaluating scripts and balancing.

Automatic Completion

The complete() method handles everything that can be determined programmatically:


TxBuilder(env)
  .payTo(recipient, Value.ada(10))
  .complete(provider, sponsorAddress)  // Automatically complete using funds from sponsorAddress

It automatically:

Selects UTxOs from the sponsor address to cover outputs and fees
Adds collateral inputs for script execution
Calculates fees based on transaction size and execution costs
Creates change outputs and iteratively rebalances

Script Evaluation

TxBuilder evaluates Plutus scripts during build to catch errors before submission:


TxBuilder(env, evaluator)
  .spend(scriptUtxo, redeemer, script)
  .payTo(recipient, scriptUtxo.output.value)
  .build(changeTo = changeAddress)  // Scripts evaluated here

Next Steps

First Transaction — Step-by-step guide
First Contract Transaction — Lock and spend from a script address
Payment Methods — Send ADA and tokens
Spending UTxOs — Input selection and script UTxOs
Minting & Burning — Create and destroy native tokens
Staking & Rewards — Delegation and reward withdrawal
Governance — DRep registration and voting

Smart Contracts — Write validators to use in transactions
Testing with Emulator — Test transactions locally
DApp Starter Tutorial — Complete example with transactions