Type Aliases vs. Type Definitions in Go: When Should You Use Each?

Type Aliases vs. Type Definitions in Go: Clarifying the Distinctions

In Go, type aliases and type definitions are two different ways to create new types. While they may appear similar syntactically, there are fundamental differences between them.

Type Aliases: Syntactic Convenience

type A = string creates an alias for the existing string type. This means that A is functionally equivalent to string, occupying the same memory and supporting the same operations. For all practical purposes, A can be used anywhere string is valid.

Type Definitions: Creating New Types

On the other hand, type A string defines a new distinct type called A. This type shares the same underlying representation as string, allowing for seamless conversions. However, it is an independent type with the ability to support its own method definitions.

Key Differences

The primary distinctions lie in these areas:

• Method Definitions: Type definitions can have methods attached to them, while type aliases cannot.
• Reflection: The reflect package can distinguish between type definitions and type aliases, recognizing the former as separate types.
• Interface Compatibility: Type definitions can implement interfaces, while type aliases inherit the interface compatibility of their underlying type.

Example

Consider the following code:

package main

import (
    "fmt"
)

type A = string
type B string

func main() {
    var a A = "hello"
    var b B = "hello"
    fmt.Printf("a is %T\nb is %T\n", a, b)
}

The output demonstrates the difference:

a is string
b is main.B

While a is recognized as a string, b is identified as an instance of the new type B.

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