Why Can't I Use Interface Type Constraints to Initialize a Slice in Go?

Interface Type Constraints: Understanding "Type Constraints" Error

In Go, interfaces can serve as type constraints to restrict the types that can satisfy them. However, certain interfaces, including those containing type constraints, face limitations in usage.

Understanding Type Constraints

Type constraints refer to interfaces that contain type elements, such as unions or embedded types that are comparable. These interfaces are considered non-basic and cannot be used as actual types for values or variables or as part of non-interface types.

Example:

Consider the following interface with a type constraint:

type Number interface {
    int | int64 | float64
}

Restriction on Slice Initialization

An interface with type constraints, such as Number, cannot be used to initialize a slice directly. For instance, the following line would raise an error:

a := []Number{Number(1), Number(2), Number(3), Number(4)}

Reason for Error

The error "interface contains type constraints: cannot use interface in conversion" stems from the non-basic nature of the Number interface. Interfaces with type constraints can only be used in two specific scenarios:

1. As type parameter constraints for generic types and functions.
2. As elements of other interfaces that are also used as constraints.

Example of Allowed Usage

Instead of directly initializing a slice of Number, we can use the following type parameter constraint:

type Coordinates[T Number] struct {
    x, y T
}

Conclusion

Interfaces with type constraints serve a specific purpose in Go. Understanding their limitations and appropriate usage is crucial to avoid errors and ensure correct program behavior.

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