How to Define Custom Types with type in Go

Use the type keyword to define a new type alias or a new named type based on an existing one. Aliases are interchangeable with the original type, while named types create a distinct type that requires explicit conversion.

// Type alias: interchangeable with string
type MyString = string

// Named type: distinct from string, requires conversion
type MyStringType string

var s string = "hello"
var a MyString = s          // OK: alias
var b MyStringType = s      // Error: cannot use string as MyStringType
var c MyStringType = "hi"   // OK: literal
var d string = string(b)    // OK: explicit conversion

Defining a custom type lets you create a specific name for a data structure to make your code clearer and safer. Think of it like creating a specific label for a box; you can either make a sticker that means the same thing as the original box (alias) or create a brand new, unique box that only holds that specific kind of item (named type). This prevents you from accidentally mixing up similar data, like confusing a user ID with a product ID.