Example

Many packages have variables (constants). In the <a href="https://golangr.com/">Go</a> programming language, exported <a href="https://golangr.com/variables/">variable</a> names are a bit different.

Before playing around with Go packages and constants, you should be able to <a href="https://golangr.com/hello-world/">run go programs</a>

Example

For example, if you import the math module you can access some of its variables:

fmt.Println(math.Pi)
fmt.Println(math.E)

These variables reside in the math module. When importing a package, you can refer only to its exported names.

Lets try that in a program

package main

import (
    "fmt"
    "math"
)

func main() {
    fmt.Println(math.Pi)
    fmt.Println(math.E)
    fmt.Println(math.Phi)
    fmt.Println(math.SqrtE)
}

If a variable does not start with a capital letter, it is not exported.

So if you'd try

fmt.Println(math.e)

That wouldn't work, because only variables with a capital letter are exported.

Constants

So how do you know which variables exist in a module?

One way to find out is in the <a href="https://golang.org/pkg/math/">golang package docs</a>. Open the name of the package and search for constants.

You can browse some of the main packages <a href="https://golang.org/pkg/">here</a>