Sway Program Types

A Sway program is a file ending with the extension .sw, e.g. main.sw, and the first line of the file is a declaration of the type of program.

A Sway program can be one of four types:

• contract
  • Primarily used for protocols or systems that operate within a fixed set of rules e.g. staking contracts, decentralized exchanges, etc.
• library
  • Reusable code for handling common operations
• script
  • Used for complex, multi-step, on-chain interactions that won't persist, such as using a decentralized exchange to create a leveraged position (borrow, swap, re-collateralize)
• predicate
  • A set of preconditions to the construction of a transaction, the result of which must be a Boolean value of true in order for the transaction to be considered valid

Sway Project Types

A project type in Sway refers to which program type is in the main file of the project.

This means that there are four types of projects:

• contracts
• libraries
• scripts
• predicates

All four projects can contain multiple library files in the src directory.

There is a caveat when it comes to contracts, scripts and predicates and it's as follows:

• A project can at most contain any one of a contract, script or predicate.

This means that a project cannot contain more than one contract, one script, one predicate and it cannot mix them together.

Entry Points

An entry point is the starting point of execution for a program.

Since a library is not directly deployable to the blockchain it does not have an entry point and instead its code is exported for use within other programs.

Unlike libraries; contracts, scripts and predicates all have an entry point. Contracts expose an Application Binary Interface (ABI) while scripts and predicates expose a main() function for entry.