Can you explain what type constraints are in programming?

Type constraints in programming are rules that define which data types can be used in certain contexts, such as in generic functions or classes.

Their goal is to enhance code safety and prevent errors by ensuring that operations are applicable to the passed types.

For example, in Swift, you can constrain a generic type to conform to a specific protocol:

generic func printDescription<T: CustomStringConvertible>(item: T) {
    print(item.description)
}

Here, T must implement the CustomStringConvertible protocol, otherwise the function will not compile.

Thus, type constraints help create more versatile yet safe code.