Golang : When to use public and private identifier(variable) and how to make the identifier public or private?

Got a Golang related question by my friend today, who is learning Golang for the first time. Her question is :

When to use public or private identifier(a.k.a variable or function name) and how to make the identifier public or private?

In order to answer this question, first we need to explore Golang's rule of declaring an identifier.

From http://golang.org/ref/spec#identifier :

Identifiers name program entities such as variables and types. An identifier is a sequence of one or more letters and digits. The first character in an identifier must be a letter.

identifier = letter { letter | unicode_digit } .

and from (http://golang.org/ref/spec#Exported_identifiers):

An identifier may be exported to permit access to it from another package. An identifier is exported if both:

1. the first character of the identifier's name is a Unicode upper case letter (Unicode class "Lu"); and
2. the identifier is declared in the package block or it is a field name or method name.

All other identifiers are not exported.

In other words, Golang definition of exported means public type.

So, when to use public or private identifier?

• use public(exported) to make that variable/function available from anywhere, other classes and instances of the object.

• use private to make your variable/function available in its own class only.

and how to make the identifier public or private ?

See examples :

 var aName // private

 var BigBro // public (exported)

 var 123abc // illegal

 func (p *Person) SetEmail(email string) {  // public because SetEmail() function starts with upper case
 p.email = email
 }

 func (p Person) email() string { // private because email() function starts with lower case
 return p.email
 }

if you use Unicode such as Chinese, Korean, Cyrillic or Japanese characters as your identifier - the identifier will be in public mode.

References :

http://golang.org/ref/spec#identifier

http://golang.org/ref/spec#Exported_identifiers