🔗 Underscore in Erlang & Elixir - great DX lang feature

In Erlang/Elixir variables can be binded only once. This is typical in many functional languages & it’s the way to express immutability in the language. Once we assigned a variable name to a value it cannot be reassigned anymore.

Bad: This does not compile as we try to rebind an already binded variable

a = 1
a = 2

Good: This compiles as we bind each variable only once

a = 1
b = 2

So this is the general rule for all variables. There is only one exception: the underscore or any variable that starts with “” (ie. “”, “_a”, “_b”, etc.). The underscore is the only variable that :

• can be bound multiple times
• cannot be read

The second property is pretty important. If we would be able to read a variable that starts with “_” then these variables would provide a hack around the language immutablity. As we cannot read the variable it’s not a problem that it can be binded multiple times => we still can bind only once the variables that can be read

Ok great. But this seems a pretty strange exception. Why introduce this feature? The short answer is that the underscore exception is there just as a way to tell the compiler “i don’t care about what is there”. And this is important because it simplifies several core language features:

• extracting pattern matching
• catch all clauses
• ignore function returns
• specific function signatures