Use of underlined variables in Erlang

In Erlang ' _ ' is a special variable (in fact Erlang should not be called "change" amount, take care of the habit, let's call it), it can replace anything, it is very useful in match, for example:

Erlang Code

1. {A, _, [B|_], {B}} = {ABC, +, [+], {] }

This means that _ is treated as a placeholder, indicating that there is a value in match, but I don't care what he is.

In Erlang, a variable name that begins with an underscore ' _ ' has a special meaning, in addition to being used as a normal variable (BINDING,MATCH,COMPUTE,COMPARE,ETC), a special tag that tells the compiler that these variables are ignored, It's okay to declare that you're not using/binding (otherwise, compiling will alert by default)

It is a good idea to mark variables that can be ignored by using a variable that starts with an underscore, such as:

Erlang Code

1. {name,_age}={"Argan",

_age here is actually I do not care about, so use a _ start variable to do match, while reading the code, although the other code does not use the _age, but you can see that this data structure, in addition to name, there is a value of age, very clear expression

However, because a single ' _ ' can match any value, and the match-out value is not available, it is not related to multiple uses in an expression, such as the first example, but if the first example is modified to:

Erlang Code

1. {A, _int, [B|_int], {B}} = {ABC, +, [+], {] }

Will be wrong, the intention is to express more clearly, _int place is an int value, but there is a trap, [head| Tail] Such a match, the result of the Tail is a list, so the expression above the first _int place is an int 23, the second _int is actually a list [23], so the above expression match will fail.

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Use of underlined variables in Erlang