TUTORIAL.md

SilverScript Tutorial

Table of Contents

1. Introduction
2. Compiling Contracts
   • Using the CLI (silverc)
   • Programmatic Compilation
3. Language Basics
   • Contract Structure
   • Pragma Directives
   • Data Types
   • Variables
   • Comments
4. Functions
   • Function Definition
   • Entrypoint Functions
   • Function Parameters and Return Types
5. Operators
   • Arithmetic Operators
   • Comparison Operators
   • Logical Operators
   • Bitwise Operators
6. Control Flow
   • If Statements
   • Require Statements
   • For Loops
7. Working with Data
   • Literals
   • Number Units
   • Date Literals
   • Arrays
   • String Operations
   • Bytes Operations
8. Type Casting
9. Built-in Functions
   • Cryptographic Functions
   • Type Conversion Functions
10. Transaction Introspection
    • Transaction Fields
    • Input Introspection
    • Output Introspection
11. Covenants
    • Creating ScriptPubKey
    • State Transition Builtins
    • Covenant Examples
12. Advanced Features
    • Constants
    • Tuple Unpacking
    • Split and Slice Operations
13. Complete Examples
    • Pay-to-Public-Key (P2PK)
    • Transfer with Timeout
    • Recurring Payment (Mecenas)

---

Introduction

SilverScript is a CashScript-inspired smart contract language that compiles to Kaspa script. It enables you to write Kaspa smart contracts with a high-level, Solidity-like syntax. SilverScript contracts can enforce complex spending conditions, create covenants, and enable advanced cryptocurrency applications on the Kaspa network.

---

Compiling Contracts

Using the CLI (silverc)

The silverc command-line tool compiles .sil source files into JSON artifacts containing the compiled bytecode and ABI.

Basic Usage:

silverc contract.sil

This reads contract.sil and outputs contract.json by default.

Specify Output File:

silverc contract.sil -o output.json

With Constructor Arguments:

If your contract has constructor parameters, you can provide their values via a JSON file:

silverc contract.sil --constructor-args args.json

The args.json file should contain an array of constructor argument expressions. For example:

[
  {"kind": "byte[]", "data": [1, 2, 3, 4]},
  {"kind": "int", "data": 12345}
]

The compiled JSON output includes:

• contract_name: The name of the contract
• script: The compiled bytecode (as an array of bytes)
• ast: The abstract syntax tree of the parsed contract
• abi: An array of entrypoint functions with their parameter types

Programmatic Compilation

You can also compile contracts programmatically using the SilverScript Rust library:

use silverscript_lang::compiler::{compile_contract, CompileOptions};
use silverscript_lang::ast::Expr;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let source = r#"
        pragma silverscript ^0.1.0;
        
        contract MyContract(int x) {
            entrypoint function spend(int y) {
                require(y > x);
            }
        }
    "#;
    
    // Constructor arguments (x = 100)
    let constructor_args = vec![Expr::Int(100)];
    
    // Compile with default options
    let options = CompileOptions::default();
    let compiled = compile_contract(source, &constructor_args, options)?;
    
    println!("Contract name: {}", compiled.contract_name);
    println!("Script length: {} bytes", compiled.script.len());
    println!("ABI: {:?}", compiled.abi);
    
    Ok(())
}

Building Signature Scripts Programmatically:

After compiling a contract, you can build signature scripts for its entrypoint functions:

use silverscript_lang::ast::Expr;

let source = r#"
    pragma silverscript ^0.1.0;
    
    contract TransferWithTimeout(pubkey sender, pubkey recipient, int timeout) {
        entrypoint function transfer(sig recipientSig) {
            require(checkSig(recipientSig, recipient));
        }
        
        entrypoint function reclaim(sig senderSig) {
            require(checkSig(senderSig, sender));
            require(tx.time >= timeout);
        }
    }
"#;

let sender_pk = vec![3u8; 32];
let recipient_pk = vec![4u8; 32];
let timeout = 1640000000i64;
let compiled = compile_contract(
    source,
    &[sender_pk.into(), recipient_pk.into(), timeout.into()],
    CompileOptions::default()
)?;

// Build sigscript for multiple entrypoints
let sig = vec![5u8; 65];

// For 'transfer' function (selector = 0)
let transfer_sigscript = compiled.build_sig_script(
    "transfer",
    vec![sig.clone().into()]
)?;
// transfer_sigscript contains: <signature> <0> (selector 0)

// For 'reclaim' function (selector = 1)
let reclaim_sigscript = compiled.build_sig_script(
    "reclaim",
    vec![sig.into()]
)?;
// reclaim_sigscript contains: <signature> <1> (selector 1)

The build_sig_script method automatically:

• Validates argument count and types
• Encodes arguments properly for the Kaspa script stack
• Appends the function selector for contracts with multiple entrypoints
• Omits the selector for contracts with a single entrypoint

---

Language Basics

Contract Structure

Every SilverScript program defines a single contract. A contract has a name, optional constructor parameters, and one or more functions:

pragma silverscript ^0.1.0;

contract MyContract(int param1, byte[32] param2) {
    // Contract constants (optional)
    int constant MAX_VALUE = 1000;
    
    // Functions
    entrypoint function spend(sig s, pubkey pk) {
        require(checkSig(s, pk));
    }
}

Pragma Directives

Every contract should start with a pragma directive specifying the SilverScript version:

pragma silverscript ^0.1.0;

Version operators:

• ^0.1.0 - Compatible with 0.1.x
• ~0.1.0 - Compatible with 0.1.0 only
• >=0.1.0 - Greater than or equal
• >0.1.0 - Greater than
• <0.2.0 - Less than
• <=0.1.5 - Less than or equal
• =0.1.0 - Exactly this version

Data Types

SilverScript supports the following data types:

Type	Description	Example
int	64-bit signed integer	42, -100, 1000
bool	Boolean value	true, false
string	UTF-8 string	"hello", 'world'
byte	Single byte	byte
pubkey	Public key (32 bytes)	pubkey
sig	Signature (65 bytes)	sig
datasig	Data signature (64 bytes)	datasig

Array Types:

You can create arrays by appending [] or [N] to any type:

int[] numbers;
int[4] fixedNumbers;
byte[] data;
byte[32] hash;
byte[32][] hashes;
pubkey[] publicKeys;

• type[] = array type where the size may be inferred from initialization.
• type[N] = fixed-size array type with compile-time size N.

When a type[] variable is initialized with a literal, SilverScript infers a fixed size from context:

byte[] data = 0x1234abcd;  // inferred as byte[4]
int[] nums = [1, 2, 3];    // inferred as int[3]

Variables

Variables must be declared with their type before use:

entrypoint function example() {
    // Variable declaration
    int myNumber = 42;
    bool flag = true;
    string message = "Hello World";

    // Array initialization
    byte[] data = 0x1234abcd;
    int[] nums = [1, 2, 3];
    int[4] fixed = [10, 20, 30, 40];
    
    // Declaration without initialization
    int uninitializedValue;
    
    // Variable reassignment
    myNumber = 100;
}

Comments

SilverScript supports both single-line and multi-line comments:

// This is a single-line comment

/*
 * This is a multi-line comment
 * It can span multiple lines
 */

int x = 10; // Comments can appear at the end of lines

---

Functions

Function Definition

Functions are defined with the function keyword:

function helper(int x, int y) {
    // function body
}

Entrypoint Functions

Entrypoint functions are callable from outside the contract. Mark them with the entrypoint keyword:

entrypoint function spend(sig s, pubkey pk) {
    require(checkSig(s, pk));
}

A contract must have at least one entrypoint function. Contracts with multiple entrypoints use function selectors automatically.

Function Parameters and Return Types

Functions can have multiple parameters and return values:

// Function with return type
function add(int a, int b): (int) {
    return (a + b);
}

// Multiple return values
function split(byte[32] data): (byte[16], byte[16]) {
    byte[16] left, byte[16] right = data.split(16);
    return (left, right);
}

// Using the return value
entrypoint function example() {
    int result = add(5, 10);
    require(result == 15);
}

---

Operators

Arithmetic Operators

int a = 10;
int b = 3;

int sum = a + b;        // 13
int difference = a - b;  // 7
int product = a * b;     // 30
int quotient = a / b;    // 3
int remainder = a % b;   // 1
int negative = -a;       // -10

Comparison Operators

bool eq = (a == b);   // false (equality)
bool ne = (a != b);   // true (inequality)
bool lt = (a < b);    // false (less than)
bool le = (a <= b);   // false (less than or equal)
bool gt = (a > b);    // true (greater than)
bool ge = (a >= b);   // true (greater than or equal)

Logical Operators

bool t = true;
bool f = false;

bool and = t && f;  // false (logical AND)
bool or = t || f;   // true (logical OR)
bool not = !t;      // false (logical NOT)

Bitwise Operators

Note: Bitwise operators require covenant features to be enabled.

int x = 0x0F;  // 00001111
int y = 0xF0;  // 11110000

int bitAnd = x & y;  // 0x00 (bitwise AND)
int bitOr = x | y;   // 0xFF (bitwise OR)
int bitXor = x ^ y;  // 0xFF (bitwise XOR)

---

Control Flow

If Statements

Basic if-else structure:

entrypoint function example(int x) {
    if (x > 10) {
        require(true);
    } else if (x < 0) {
        require(false);
    } else {
        require(x == 5);
    }
}

Single-statement branches don't require braces:

if (x > 0)
    require(true);
else
    require(false);

Require Statements

The require statement enforces conditions. If the condition is false, the contract execution fails:

require(x > 0);  // Passes if x > 0, fails otherwise

// With error message
require(x > 0, "x must be positive");

Time-based require statements:

// Require transaction time
require(tx.time >= 1640000000);

// Require contract age
require(this.age >= 86400);  // 1 day in seconds

For Loops

For loops iterate over a runtime range of integers, but the unroll bound must be known at compile time:

contract ForLoop() {
    int constant MAX_ITERATIONS = 4;
    int constant MIN_OUT = 1000;

    entrypoint function check(int start, int end) {
        for(i, start, end, MAX_ITERATIONS) {
            require(tx.outputs[i].value >= MIN_OUT + i);
        }
    }
}

The loop variable i takes values from start to end - 1 (exclusive end), but the compiler emits exactly MAX_ITERATIONS guarded iterations.

---

Working with Data

Literals

Integer Literals:

int decimal = 42;
int negative = -100;
int withUnderscore = 1_000_000;  // Underscores for readability
int exponential = 1e6;  // 1,000,000

Boolean Literals:

bool t = true;
bool f = false;

String Literals:

string s1 = "Hello World";
string s2 = 'Single quotes work too';
string escaped = "Line 1\nLine 2\tTabbed";
string quote = "He said \"Hello\"";
string apostrophe = 'It\'s working';

Hex Literals:

byte[] data = 0x1234abcd;
byte[] empty = 0x;
byte[] pubkeyBytes = 0x0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef;

Hex literals are always parsed as byte sequences. That means 0x00 is not a scalar byte; use an explicit cast when you want one:

byte bad = 0x00;        // compiler error: use byte(0x00) to cast a one-byte hex literal to byte
byte good = byte(0x00); // compiles

Number Units

SilverScript supports convenient number units for values and time:

Value Units:

int amount1 = 1000 litras;
int amount2 = 10 grains;
int amount3 = 1 kas;

Time Units:

int time1 = 30 seconds;
int time2 = 5 minutes;  // 300 seconds
int time3 = 2 hours;    // 7200 seconds
int time4 = 7 days;     // 604800 seconds
int time5 = 4 weeks;    // 2419200 seconds

Example usage:

entrypoint function withdraw() {
    require(this.age >= 30 days);
    require(tx.outputs[0].value >= 10000 litras);
}

Date Literals

Convert ISO 8601 date strings to Unix timestamps:

int timestamp = date("2021-02-17T01:30:00");
require(tx.time >= timestamp);

Format: YYYY-MM-DDThh:mm:ss

Arrays

SilverScript supports both direct array initialization and dynamic building with .push():

// Direct initialization (size inferred from literals)
int[] nums = [1, 2, 3];           // inferred as int[3]
byte[] data = 0x1234abcd;         // inferred as byte[4]

// Explicit fixed-size initialization
int[4] fixedNums = [1, 2, 3, 4];
byte[4] tag = 0x01020304;

// Dynamic building with push
int[] numbers;
numbers.push(1);
numbers.push(2);
numbers.push(3);
numbers.push(4);
numbers.push(5);

// Build byte[32] array dynamically
byte[32][] hashes;
hashes.push(0x1111111111111111111111111111111111111111111111111111111111111111);
hashes.push(0x2222222222222222222222222222222222222222222222222222222222222222);

// Access array elements
int first = numbers[0];
int second = numbers[1];

// Array length
int count = numbers.length;

// For fixed-size arrays (including inferred ones), length is compile-time
require(nums.length == 3);
require(data.length == 4);

Array Concatenation:

You can concatenate arrays with + when element types are compatible.

This works for array types whose element size is known at compile time, including:

• byte[] (element type byte)
• int[] (element type int)
• bool[] (element type bool)
• pubkey[] (element type pubkey)
• byte[N][] (element type byte[N])

Examples:

// int[] + int[]
int[] a = [1, 2];
int[] b = [3, 4];
int[4] c = a + b;

require(c.length == 4);
require(c[0] == 1);
require(c[1] == 2);
require(c[2] == 3);
require(c[3] == 4);

// byte[] + byte[]
byte[] p = 0x0102;
byte[] q = 0x0304;
byte[4] r = p + q;
require(r == 0x01020304);

// bool[] + bool[]
bool[] f1 = [true, false];
bool[] f2 = [true, false];
bool[4] f = f1 + f2;
require(f[0]);
require(!f[1]);
require(f[2]);
require(!f[3]);

// pubkey[] + pubkey[]
pubkey k1 = 0x0202020202020202020202020202020202020202020202020202020202020202;
pubkey k2 = 0x0303030303030303030303030303030303030303030303030303030303030303;
pubkey[] ks1 = [k1];
pubkey[] ks2 = [k2];
pubkey[2] ks = ks1 + ks2;
require(ks[0] == k1);
require(ks[1] == k2);

// byte[N][] + byte[N][]
byte[2][] x = [0x0102, 0x0304];
byte[2][] y = [0x0506];
byte[2][3] z = x + y;
require(z.length == 3);
require(z[2] == 0x0506);

String Operations

Concatenation:

string hello = "Hello";
string world = "World";
string message = hello + " " + world;  // "Hello World"

// Length
int len = message.length;  // 11

Bytes Operations

Concatenation:

byte[] a = 0x1234;
byte[] b = 0x5678;
byte[] combined = a + b;  // 0x12345678

Split:

Split byte[] at a specific index:

byte[] data = 0x1234567890abcdef;
byte[] left = data.split(4)[0];   // 0x12345678
byte[] right = data.split(4)[1];  // 0x90abcdef

Slice:

Extract a range of bytes:

byte[] data = 0x123456789abcdef;
byte[] middle = data.slice(2, 5);  // byte[] from index 2 to 5 (exclusive)

Length:

byte[] data = 0x1234;
int size = data.length;  // 2

---

Type Casting

SilverScript supports explicit type casting:

// Cast to bytes
byte[] fromInt = byte[](42);
byte[] fromString = byte[]("hello");

// Cast to specific byte size
byte[32] hash = byte[32](data);
byte[65] signatureBytes = byte[65](sigBytes);

// Cast a one-byte hex literal to scalar byte
byte b = byte(0x00);

// Cast to pubkey or sig
pubkey pk = pubkey(keyBytes);
sig signature = sig(signatureBytes);

// Cast to int
int number = int(someData);

Example:

entrypoint function example(pubkey pk, byte[65] sigBytes) {
    sig s = sig(sigBytes);
    require(checkSig(s, pk));
}

---

Built-in Functions

Cryptographic Functions

blake2b(byte[] data): byte[32]

Compute the BLAKE2b hash of the input:

byte[32] hash = blake2b(data);
byte[32] pkh = blake2b(pk);

sha256(byte[] data): byte[32]

Compute the SHA-256 hash:

byte[32] hash = sha256(data);

checkSig(sig signature, pubkey publicKey): bool

Verify a signature against a public key:

require(checkSig(s, pk));

Type Conversion Functions

byte[](value): bytes

Convert to bytes:

byte[] b1 = byte[](42);
byte[] b2 = byte[]("hello");

byte[](int value, int size): bytes

Convert integer to byte[] with specific size:

byte[8] b = byte[](1234, 8);

int(bool value): int

Convert boolean to integer (true = 1, false = 0):

int x = int(false);  // 0

length(byte[] value): int

Get the length of a byte array:

int size = length(data);

---

Transaction Introspection

Transaction introspection allows contracts to examine the transaction that is spending them.

Transaction Fields

Nullary Operations (no parameters):

// Current active input index
int inputIdx = this.activeInputIndex;

// Active bytecode (current contract's scriptPubKey)
byte[] script = this.activeScriptPubKey;

// Number of inputs
int inputCount = tx.inputs.length;

// Number of outputs
int outputCount = tx.outputs.length;

// Transaction version
int version = tx.version;

// Transaction locktime
int locktime = tx.locktime;

Time-based Fields:

// Age of the UTXO being spent (in seconds)
require(this.age >= 0);

// Transaction locktime
require(tx.time >= 0);

Input Introspection

Access properties of transaction inputs:

// Access input at index i
int inputValue = tx.inputs[i].value;
byte[] inputScript = tx.inputs[i].scriptPubKey;

Example:

entrypoint function spend() {
    int currentValue = tx.inputs[this.activeInputIndex].value;
    require(currentValue >= 1000);
}

Output Introspection

Access properties of transaction outputs:

// Access output at index i
int outputValue = tx.outputs[i].value;
byte[] outputScriptPubKey = tx.outputs[i].scriptPubKey;

Example:

entrypoint function transfer() {
    // Ensure first output has at least 10000 litras
    require(tx.outputs[0].value >= 10000);
}

---

Covenants

Covenants are contracts that enforce conditions on how funds can be spent. They use transaction introspection to validate outputs.

Creating ScriptPubKey

new ScriptPubKeyP2PK(pubkey pk): byte[34]

Create a Pay-to-Public-Key scriptPubKey:

byte[34] outputScriptPubKey = new ScriptPubKeyP2PK(recipientPubkey);
require(tx.outputs[0].scriptPubKey == outputScriptPubKey);

new ScriptPubKeyP2SH(byte[32] scriptHash): byte[35]

Create a Pay-to-Script-Hash scriptPubKey:

byte[32] redeemScriptHash = blake2b(redeemScript);
byte[35] outputScriptPubKey = new ScriptPubKeyP2SH(redeemScriptHash);
require(tx.outputs[0].scriptPubKey == outputScriptPubKey);

new ScriptPubKeyP2SHFromRedeemScript(byte[] redeemScript): byte[35]

Create a P2SH scriptPubKey directly from a redeem script:

byte[35] outputScriptPubKey = new ScriptPubKeyP2SHFromRedeemScript(redeemScript);

State Transition Builtins

SilverScript provides four builtins for state routing and cross-template state inspection.

• Validate Output State: validate continuation into the same contract template. newState must provide every state field exactly once in the local State layout.

validateOutputState(int outputIndex, object newState)

• Validate Output State With Template: validate continuation into a foreign contract template. newState is encoded using the struct layout implied by the value you pass, then inserted between templatePrefix and templateSuffix.

validateOutputStateWithTemplate(
    int outputIndex,
    object newState,
    byte[] templatePrefix,
    byte[] templateSuffix,
    byte[32] expectedTemplateHash
)

• Read Input State: read another input as this contract's own State.

readInputState(int inputIndex)

• Read Input State With Template: read another input using a foreign struct layout. It checks the foreign template hash and the foreign input's P2SH commitment before decoding.

readInputStateWithTemplate(
    int inputIndex,
    int templatePrefixLen,
    int templateSuffixLen,
    byte[32] expectedTemplateHash
)

Use it with a direct struct binding or destructuring assignment:

OtherState other = readInputStateWithTemplate(inputIndex, templatePrefixLen, templateSuffixLen, expectedTemplateHash);

Same-template example:

pragma silverscript ^0.1.0;

contract Counter(int initCount, byte[2] initTag) {
    int count = initCount;
    byte[2] tag = initTag;

    entrypoint function step() {
        validateOutputState(0, { count: count + 1, tag: tag });
    }
}

Input-side note:

• readInputState(...) and readInputStateWithTemplate(...) are input-state decoders. They read bytes from another input's sigscript and decode them as state.
• readInputState(...) is appropriate when the surrounding covenant domain guarantees a single allowed contract/layout for the foreign input.
• readInputStateWithTemplate(...) is appropriate when multiple templates may share a covenant domain; it additionally validates the foreign input's template hash and checks that the claimed redeem-script bytes match the foreign input's P2SH scriptPubKey.
• Without those surrounding guarantees, plain readInputState(...) would also need extra correlation checks between the foreign input and the inspected part of its sigscript.

Covenant Examples

Simple Covenant (Send to Specific Address):

pragma silverscript ^0.1.0;

contract SimpleCovenant(pubkey recipient) {
    entrypoint function spend() {
        // First output must go to the recipient
        byte[34] recipientScriptPubKey = new ScriptPubKeyP2PK(recipient);
        require(tx.outputs[0].scriptPubKey == recipientScriptPubKey);
    }
}

Recurring Payment Covenant:

pragma silverscript ^0.1.0;

contract RecurringPayment(pubkey recipient, int paymentAmount, int period) {
    entrypoint function withdraw() {
        // Must wait for the period to elapse
        require(this.age >= period);
        
        // First output must pay the recipient
        byte[34] recipientScriptPubKey = new ScriptPubKeyP2PK(recipient);
        require(tx.outputs[0].scriptPubKey == recipientScriptPubKey);
        require(tx.outputs[0].value >= paymentAmount);
        
        // Calculate change
        int inputValue = tx.inputs[this.activeInputIndex].value;
        int minerFee = 1000;
        int changeValue = inputValue - paymentAmount - minerFee;
        
        // If sufficient funds remain, send change back to contract
        if (changeValue >= paymentAmount + minerFee) {
            byte[] changeScriptPubKey = tx.inputs[this.activeInputIndex].scriptPubKey;
            require(tx.outputs[1].scriptPubKey == changeScriptPubKey);
            require(tx.outputs[1].value == changeValue);
        }
    }
}

---

Advanced Features

Constants

Define contract-level constants:

contract MyContract() {
    int constant MAX_VALUE = 1000;
    int constant MIN_VALUE = 100;
    string constant MESSAGE = "hello";
    
    entrypoint function check(int x) {
        require(x >= MIN_VALUE);
        require(x <= MAX_VALUE);
    }
}

Constants can also be declared inside functions:

entrypoint function example() {
    string constant greeting = "Hello";
    require(sha256(greeting) != 0x);
}

Tuple Unpacking

Unpack multiple values from function returns or split operations:

// Function with multiple returns
function getPair(): (int, int) {
    return (10, 20);
}

// Unpack split results and function results
entrypoint function example(byte[32] data) {
    byte[16] left, byte[16] right = data.split(16);
    (int x, int y) = getPair();
}

In Function Parameters:

entrypoint function example(byte[32] data) {
    byte[16] x, byte[16] y = data.split(16);
    require(x == y);
}

Split and Slice Operations

Split:

Divide byte[] into two parts at a given index:

byte[] data = 0x1122334455667788;

// Split at byte 4
byte[] left = data.split(4)[0];   // 0x11223344
byte[] right = data.split(4)[1];  // 0x55667788

// Direct tuple unpacking with types
byte[4] a, byte[4] b = data.split(4);

Slice:

Extract a substring of bytes:

byte[] data = 0x1122334455667788;

// Get byte[] from index 2 to 5 (exclusive)
byte[] middle = data.slice(2, 5);  // 0x334455

// Variable indices
int start = 1;
int end = 4;
byte[] extracted = data.slice(start, end);

---

Complete Examples

Pay-to-Public-Key (P2PK)

pragma silverscript ^0.1.0;

contract P2PK(pubkey pk) {
    entrypoint function spend(sig s) {      
        // Verify the signature
        require(checkSig(s, pk));
    }
}

Constructor arguments:

• pk: The recipient's public key

Spend arguments:

• s: A signature from the private key corresponding to pk

Transfer with Timeout

pragma silverscript ^0.1.0;

contract TransferWithTimeout(
    pubkey sender,
    pubkey recipient,
    int timeout
) {
    // Recipient can spend at any time
    entrypoint function transfer(sig recipientSig) {
        require(checkSig(recipientSig, recipient));
    }

    // Sender can reclaim after timeout
    entrypoint function reclaim(sig senderSig) {
        require(checkSig(senderSig, sender));
        require(tx.time >= timeout);
    }
}

Constructor arguments:

• sender: Public key of the sender (who can reclaim)
• recipient: Public key of the recipient (who can spend)
• timeout: Unix timestamp after which sender can reclaim

Spend paths:

1. Transfer: Recipient signs to claim funds
2. Reclaim: Sender signs after timeout to reclaim funds

Recurring Payment (Mecenas)

A contract that releases periodic payments to a beneficiary:

pragma silverscript ^0.1.0;

contract Mecenas(pubkey recipient, byte[32] funder, int pledge, int period) {
    // Periodic payment to recipient
    entrypoint function receive() {
        // Must wait for the period to elapse
        require(this.age >= period);

        // Check that the first output sends to the recipient
        byte[34] recipientScriptPubKey = new ScriptPubKeyP2PK(recipient);
        require(tx.outputs[0].scriptPubKey == recipientScriptPubKey);

        // Calculate the value that's left
        int minerFee = 1000;
        int currentValue = tx.inputs[this.activeInputIndex].value;
        int changeValue = currentValue - pledge - minerFee;

        // If there is not enough left for another pledge after this one,
        // send the remainder to the recipient. Otherwise send the
        // pledge to the recipient and the change back to the contract
        if (changeValue <= pledge + minerFee) {
            require(tx.outputs[0].value == currentValue - minerFee);
        } else {
            require(tx.outputs[0].value == pledge);
            byte[] changeScriptPubKey = tx.inputs[this.activeInputIndex].scriptPubKey;
            require(tx.outputs[1].scriptPubKey == changeScriptPubKey);
            require(tx.outputs[1].value == changeValue);
        }
    }

    // Funder can reclaim at any time
    entrypoint function reclaim(pubkey pk, sig s) {
        require(blake2b(pk) == funder);
        require(checkSig(s, pk));
    }
}

Constructor arguments:

• recipient: Public key of the beneficiary
• funder: Hash of the funder's public key (for reclaim)
• pledge: Amount to pay per period
• period: Time in seconds between payments

Spend paths:

1. Receive: Anyone can trigger a payment after the period elapses
2. Reclaim: Funder can reclaim all funds at any time

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Best Practices

1. Always use pragma directives to specify the language version
2. Use descriptive variable and function names for better readability
3. Add comments to explain complex logic
4. Validate all inputs with require statements
5. Be mindful of miner fees when calculating output values in covenants
6. Test extensively before deploying to mainnet
7. Use constants for magic numbers and repeated values
8. Keep contracts simple - complexity increases the risk of bugs