How to call a function from another package in Go (Golang)

Overview

The function in another package must start with a capital letter so that it is public outside its package. If the function name starts with lowercase then it will not be visible outside its package. 
To use a function outside its package we need to import the package first which contains that function. 

Program

Here is the code for the same

go.mod

module sample.com/learn
go 1.16

hello/hello.go

package hello

import "fmt"

func SayHello() {
	fmt.Println("Hello")
}

main.go

package main

import "sample.com/learn/hello"

func main() {
    hello.SayHello()
}

Output

Hello

In this program, we first import the hello package from the main package as below. 

import "sample.com/learn/hello"

Then we call the SayHello function as below from the main package

hello.SayHello()

This works because SayHello function is uppercase. 

Change the function to lower case and run this program. It gives the below compilation error

cannot refer to unexported name hello.sayHello

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