What are packages in Go and what is special about the 'main' package?

Every Go source file starts with a package declaration. Packages are Go's unit of code organisation, encapsulation, and compilation. A package groups related types, functions, constants, and variables.

The main package is unique: it defines an executable program. The main() function within it is the program's entry point. Any package not named main is a library package — importable by others but not directly runnable.

Exported vs unexported: identifiers starting with an uppercase letter are exported (public). Lowercase identifiers are unexported (package-private). This is Go's entire visibility system — no public, private, or protected keywords.

// main.go — executable program
package main

import (
    "fmt"
    "myproject/mathutil"   // importing a library package
)

func main() {
    result := mathutil.Add(3, 4)  // Add is exported (uppercase)
    fmt.Println(result)           // 7
}

// mathutil/math.go — library package
package mathutil

func Add(a, b int) int { return a + b }    // exported
func helper(x int) int { return x * 2 }    // unexported (private)

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