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Everything You Need to Know About Erlang's Magic _ Variable

I had someone ask me what happened to values assigned to the underscore variable in Erlang. I realized I didn’t have a clear understand of the way variables beginning with an underscore handled. I decided to re-read the Erlang documentation on variables.

To summarizing what the documentation says, all variables could be put into three groups:

  • Normal variables
  • Normal variables prefixed with with an underscore
  • The anonymous variable (a single underscore)

Erlang Variables

All types of variables must start with either an uppercase letter or an underscore. They may contain alphanumeric characters, the underscore and the @ symbol. All variables are bound via Pattern Matching. At runtime no variables are special except for the anonymous variable.

Normal variables

Normal variables start with an uppercase letter. Pattern matching and all the compile time warnings work normally. Example:

1> {First, Second} = {foo, bar}.
{foo, bar}
2> First.
3> Second.

Normal variables prefixed with an underscore

When a variable is prefixed with an underscore pattern matching and everything at runtime works like a normally. Values are still bound as they normally would and patterns such as this fail like they normally would. {_Val,_Val} = {foo,bar} fails because the first and second items in the tuple are different. The only difference is at compile time. Normally the compiler generates a warning when a value is bound to a variable and the variable is not later used. Often an important value is returned from a function call but the value is unused. By prefixing the variable with an underscore, the variable retains it’s useful name and does not generate any compile time warnings. Example:

1> {_First, _Second} = {foo, bar}.
{foo, bar}
2> _First.
3> _Second.

The anonymous variable

The anonymous variable is denoted by a single underscore (_). The anonymous variable is used when a variable is required but the value needs to be ignored. When a value is assigned to the anonymous variable the value is never bound to the variable. Since the value is never bound it can be used multiple times in a pattern and each time it is allowed to match a different value. Example:

1> {_, _} = {foo, bar}.
{foo, bar}
2> _.
* 1: variable '_' is unbound 
4> {_, Second} = {foo, bar}.
{foo, bar}
5> _.
* 1: variable '_' is unbound 
6> Second.

For more information check out the Erlang documentation on variables