1# Contract Module23The Contract module provides functionality for the runtime to deploy and execute WebAssembly smart-contracts.45- [`Call`](https://docs.rs/pallet-contracts/latest/pallet_contracts/enum.Call.html)6- [`Config`](https://docs.rs/pallet-contracts/latest/pallet_contracts/trait.Config.html)7- [`Error`](https://docs.rs/pallet-contracts/latest/pallet_contracts/enum.Error.html)8- [`Event`](https://docs.rs/pallet-contracts/latest/pallet_contracts/enum.Event.html)910## Overview1112This module extends accounts based on the `Currency` trait to have smart-contract functionality. It can13be used with other modules that implement accounts based on `Currency`. These "smart-contract accounts"14have the ability to instantiate smart-contracts and make calls to other contract and non-contract accounts.1516The smart-contract code is stored once in a `code_cache`, and later retrievable via its `code_hash`.17This means that multiple smart-contracts can be instantiated from the same `code_cache`, without replicating18the code each time.1920When a smart-contract is called, its associated code is retrieved via the code hash and gets executed.21This call can alter the storage entries of the smart-contract account, instantiate new smart-contracts,22or call other smart-contracts.2324Finally, when an account is reaped, its associated code and storage of the smart-contract account25will also be deleted.2627### Gas2829Senders must specify a gas limit with every call, as all instructions invoked by the smart-contract require gas.30Unused gas is refunded after the call, regardless of the execution outcome.3132If the gas limit is reached, then all calls and state changes (including balance transfers) are only33reverted at the current call's contract level. For example, if contract A calls B and B runs out of gas mid-call,34then all of B's calls are reverted. Assuming correct error handling by contract A, A's other calls and state35changes still persist.3637One gas is equivalent to one [weight](https://substrate.dev/docs/en/knowledgebase/learn-substrate/weight)38which is defined as one picosecond of execution time on the runtime's reference machine.3940### Notable Scenarios4142Contract call failures are not always cascading. When failures occur in a sub-call, they do not "bubble up",43and the call will only revert at the specific contract level. For example, if contract A calls contract B, and B44fails, A can decide how to handle that failure, either proceeding or reverting A's changes.4546## Interface4748### Dispatchable functions4950Those are documented in the [reference documentation](https://docs.rs/pallet-contracts/latest/pallet_contracts/#dispatchable-functions).5152## Usage5354This module executes WebAssembly smart contracts. These can potentially be written in any language55that compiles to web assembly. However, using a language that specifically targets this module56will make things a lot easier. One such language is [`ink`](https://github.com/paritytech/ink)57which is an [`eDSL`](https://wiki.haskell.org/Embedded_domain_specific_language) that enables58writing WebAssembly based smart contracts in the Rust programming language.5960## Debugging6162Contracts can emit messages to the node console when run on a development chain through the63`seal_println` API. This is exposed in ink! via64[`ink_env::debug_println()`](https://docs.rs/ink_env/latest/ink_env/fn.debug_println.html).6566In order to see these messages the log level for the `runtime::contracts` target needs to be raised67to at least the `info` level which is the default. However, those messages are easy to overlook68because of the noise generated by block production. A good starting point for contract debugging69could be:7071```bash72cargo run --release -- --dev --tmp -lerror,runtime::contracts73```7475License: Apache-2.0