# What is a Smart Asset

Any user can not only create their own token on the Waves blockchain but also endow it with certain functionality by attaching a script to it. A token with an attached script is called a smart asset, and an attached script is called an asset script. Examples of practical use of smart assets:

Freezing. Transactions with a smart asset can be prohibited before (or after) the moment when the blockchain reaches a certain height.
Black / white lists. Transfer of a smart asset can be prohibited to the specified addresses or, conversely, allowed only to the specified addresses.
Commission. The creator of the smart asset can set a commission that will be paid to him for each operation with his smart asset.
Multi-signature. A smart asset may require the signing of multiple accounts to complete a transaction.
Purchase restriction. A rule can be established according to which a smart asset can only be purchased for certain tokens. Buying a smart asset for other tokens may be prohibited.
Restriction on the matcher. A rule may be established according to which the purchase and sale transaction of a smart asset can be carried out only by a specific matcher.
Gaming. For a smart asset used as game currency, permission can be set to make transactions only in specified game locations or only between characters with certain properties.

The features of smart assets are not limited to the examples given.

# Creating a Smart Asset

You can attach a script to a token only at the time the token is created. The script can be updated at any time. To create a token, you can use both Waves IDE and libraries (WavesJ, WavesCS, and others). The asset script must be written in Ride and attached to the token using the set asset script transaction. A commission of 1 WAVES is charged for the execution of an asset script installation transaction.

# Asset Script Structure

# Directive

The directive should be placed at the very beginning of the script. Review the example directive:

{-# STDLIB_VERSION 3 #-}
{-# CONTENT_TYPE EXPRESSION #-}
{-# SCRIPT_TYPE ASSET #-}

The given directive provides the compiler with the following information:

the script will use the third version of the library of standard functions
the type of this script is Expression
the script will be attached to the asset (and not to the account)

The directive is the mandatory part of a smart asset script.

# Validation Function

The validation function verifies that outgoing transactions involving a token meet the specified conditions. If the condition is not met, the transaction will not be sent. Possible results of the validation function are

true (transaction allowed)
false (transaction prohibited)
error

An asset script may contain several validation functions.

# Smart Asset Scripts Examples

# Token freeze

Token operations will be prohibited until the blockchain height reaches 100500.

let targetHeight = 100500
height >= targetHeight

# Burn-proof token

For the example smart asset, the token burning transactions are prohibited.

match tx {
  case t : BurnTransaction => false
  case _ => true
}

# Transfer fee only in the specified asset

This script allows the transfer of the smart asset only if the transfer transaction fee is in the sponsored asset oWgJN6YGZFtZrV8BWQ1PGktZikgg7jzGmtm16Ktyvjd.

The sponsor sets up the ratio of WAVES and the sponsored asset. For each transfer of the smart asset, the sponsor receives the fee in the sponsored asset from the sender's account. The equivalent amount of WAVES is deducted from the sponsor's account in favor of block generators. The sponsor can sell the sponsored asset at a higher price and thus make a profit.

Learn more about sponsorship

match tx {
  case t : TransferTransaction =>
    t.feeAssetId == base58'oWgJN6YGZFtZrV8BWQ1PGktZikgg7jzGmtm16Ktyvjd'
  case _ => true
}

# Non-transferrable token

To make token transfers impossible, false should be returned for the transfer transaction, mass transfer transaction and exchange transaction.

match tx {
  case t : TransferTransaction | MassTransferTransaction | ExchangeTransaction => false
  case _ => true
}

# Buying only for BTC

A smart asset with the script below can only be purchased with BTC.

let BTCId = base58'8LQW8f7P5d5PZM7GtZEBgaqRPGSzS3DfPuiXrURJ4AJS'
match tx {
  case t : ExchangeTransaction =>
    t.sellOrder.assetPair.priceAsset == BTCId || t.sellOrder.assetPair.amountAsset == BTCId
  case _ => true
}

# Only a specific matcher is allowed

The script below allows the purchase and sale of a smart asset only on a specific matcher.

match tx {
  case t : ExchangeTransaction =>
    t.sender == addressFromString("3PJaDyprvekvPXPuAtxrapacuDJopgJRaU3")
  case _ => true
}

# Modifying Smart Asset Script

A transaction involving the execution of an asset script has the transaction fee increased by 0.004 WAVES. The fee for its completion is 1 WAVES. Only the account that issued the smart asset can change the script.

# Smart asset fees

The fee for the transaction in which the asset script is executed is increased by 0.004 WAVES. If the account is a smart account, then the size of the fee is increased by an additional 0.004 WAVES.

Let's review the example. The commission for a transfer transaction is 0.001 WAVES. If a user makes a transfer of a smart asset from a smart account, then the amount of the commission will be 0.001 + 0.004 + 0.004 = 0.009 WAVES.

# Buying and selling smart assets

Smart assets can be bought and sold on Waves.Exchange. If the smart asset is one of the bought or sold tokens, then the exchange transaction fee is increased by 0.004 WAVES for each participant of the transaction. If both tokens are smart assets, then the amount of the commission is increased by 0.008 WAVES for each participant in the transaction.

Example 1: one of the tokens is a smart asset. The exchange transaction fee is 0.003 WAVES. Each of the participants in the transaction will pay 0.003 + 0.004 = 0.007 WAVES.

Example 2: both tokens are smart assets. Each of the participants in the transaction will pay 0.003 + 0.004 + 0.004 = 0.011 WAVES.