Question

What are the exact differences between underscore variables and a named variable that starts with underscore from the Erlang compiler point of view (apart from adding readability to the code)?

For example are _ and _Var different?

Answer 1

The don’t care variable _ is a VERY SPECIAL variable which matches anything and is NEVER bound to a value. It is used when I know there is something there but I don’t care what the value is and I will never use. Seeing _ is never bound it can not be used in an expression and the compiler flags it as an error.

Variables like _Var are perfectly normal variables which you can match against and will be bound to values which means they can be used in expressions. Prefixing a variable with _ is about intent. The compiler normally warns you about a variable which is bound in a pattern but is never used, often a sign of an error. But the compiler does not warn for variables prefixed with _ like in _Var. The intent being that I want to give the variable a name, naming things is good, but that I know I will never use it.

Remember that _ is really the only special variable and that _Var are normal variables and behave as such if used. If you are feeling perverse then you could prefix all your variables with _ and everything will still work.