Multiple files

Learn that a package can span multiple .go files.

Multiple files

A package can consist of multiple .go files. All files carrying the same package name belong to the same package. This is the basic way to split large packages.

For instance, string_utils.go and number_utils.go both declaring package utils are in the same utils package. A definition in one file is visible in the other.

Compilation happens at the package level with go build; not at the file level. All package files compile together.

Package visibility

In Go, visibility is determined by case. Names starting with an uppercase letter are exported and reachable from other packages. Lowercase names stay package-private.

For example, Println is exported, while print stays inside the fmt package. This mechanism replaces public/private in Java.

Within the same package, lowercase names are visible across files because the package is a single scope.

Exercise Exported names

Try visibility inside a package: write Topla(a, b int) int, starting with an upper-case letter (exported), and ikiKat(n int) int, starting with a lower-case one (package-private). Print fmt.Println(Topla(2, 3), ikiKat(4)).

Show example solution

package main

import "fmt"

func Topla(a, b int) int { return a + b }

func ikiKat(n int) int { return n * 2 }

func main() {
    fmt.Println(Topla(2, 3), ikiKat(4))
}

Test yourself

Can two files in the same package see each other's definitions?
- No, only the exported ones
- Only those in the same file
- Yes, all definitions
- Only the upper-case ones
Show answer

Yes, all definitions — All definitions in the same package are visible across files.

Same package

The string_utils.go and number_utils.go files:
____ utils

Show answer

The string_utils.go and number_utils.go files:
package utils

Visibility matching

Println

Exported
print

Package-private
fmt.Println

External access
hello

Lowercase

Next lesson: go.sum and dependencies

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