Structure of a script

Helios validator scripts have a function called main that returns a boolean (true or false) when validating the spending of a UTxO.

For a spending validator, main takes three arguments:

  • Datum: data stored on-chain that is linked to the locked UTxO (not avaiable for minting/staking scripts)
  • Redeemer: data specified by the user attempting to spend the locked UTxO
  • ScriptContext: information about the transaction spending the locked UTxO

Datum and Redeemer are user-defined but ScriptContext is a builtin type.

The structure of a validator script looks as follows:

// --- (1) ---
spending my_validator       

// --- (2) ---
struct Datum {..}           

// --- (3) ---
enum Redeemer {..}          
// --- (4) ---
func main(datum: Datum, redeemer: Redeemer, ctx: ScriptContext) -> Bool {

// --- (5) ---
const MY_DATUM = Datum {...}

Script purpose (1)

In Helios all scripts start with a script purpose, followed by the name of the script. There are four script purposes currently:

  • spending
  • minting
  • staking
  • testing
  • module

On this page we are only concerned with the spending script purpose:

spending my_validator


module is covered in the next section.

minting, staking and testing will be covered in the advanced concepts chapter.

Note: the name of each Helios source is registered in the global scope, so these names can't be used by statements, nor for the lhs of assignments. So eg. the entrypoint script can't be named main as that would conflict with the entrypoint function.

Datum (2)

Each UTxO locked at a script address will also have an associated datum. The script can choose to use the datum as part of the spending validation, or it can choose to ignore the datum if it is irrelevant.

If the script uses the datum then a struct or enum must be defined above main that is named Datum.

Redeemer (3)

Each UTxO used as an input for a transaction also has a redeemer attached. This is data specified by the user attempting to spend that UTxO. The script can again choose to use or ignore the redeemer during validation.

If the script uses the redeemer then a struct or enum must be defined above main that is named Redeemer.

main function (4)

The main function (4) of a validator script accepts up to three optional arguments and returns a Bool:

  • datum (2)
  • redeemer (3)
  • script context

Each main argument is optional, but must appear in that order.

spending my_validator


func main(datum: Datum, redeemer: Redeemer, context: ScriptContext) -> Bool {


Most of the data needed for writing useful validators is contained in the ScriptContext.

Note: The datum and the redeemer are user-defined types (structs or enums) that currently must be named Datum and Redeemer.

Data generators and test functions (5)

After the main function you can define functions and constants for:

  • generating data structures (eg. datums or redeemers)
  • testing the main function

The API has special functionality for working with these:

  • program.parameters an object that evaluates/sets any top-level constant in a Helios source

Some compiler restrictions are lifted in this part of the script:

  • not all names need to be used (relevant for function arguments and assignments)
  • structs can be empty

Special constructors, that aren't available in main, become available in this part of the script: