Erlang syntax in function

Question

I have found this code in Ejabberd:

maybe_post_request([$< | _ ] = Data, Host, ClientIp)

I don't understand what [$< | _ ] = Data part do with Data. Could somebody explain?

Answer 1

The construct

[$< | _] = Data

applies a pattern match to Data, expecting it to be a list variable whose first element is the character < and ignoring the rest the elements. Try it in the Erlang shell:

1> Data = "<foo>".
"<foo>"
2> [$<|_] = Data.
"<foo>"

But if Data doesn't match, we get an exception:

3> f(Data), Data = "foo".
"foo"
4> [$<|_] = Data.
** exception error: no match of right hand side value "foo"

Answer 2

I don't understand what [$< | _ ] = Data part do with Data. Could somebody explain?

It binds the variable Data to the entire first argument to the function.

The left hand side pattern matches the first argument so that this function clause only matches when the first argument is a string (list) starting with the character <. The variable Data is assigned the entire string fr use in the function body.