String chomping match expression with bound variable

The following shell sessions shows some behavior I would like to understand:

1> A = "Some text".
"Some text"
2> "Some " ++ R = A.
"Some text"
3> R.
"text"
4> B = "Some ".
"Some "
5> B ++ L = A.
* 1: illegal pattern

Surely statements 2 and 5 are syntacticially identical? I would like to use this idiom to extract some text from a string, where B is being read in from a configuration file. Is this possible, and w开发者_开发技巧hat syntax should I use instead of that shown in 5) above?

Thanks!

---

The LHS ++ RHS pattern is expanded at compiletime to [ lhs0, lhs1, lhs2 | RHS] (where LHS =:= [lhs0, lhs1, lhs2], and the compiler refuses to do this for anything but literal strings/lists. In theory it could do this for variables, but it simply doesn't right now.

I think in your case you need to do:

Prefix = read_from_config(),
TestString = "Some test string",
case lists:prefix(Prefix, TestString) of
    true ->
        %% remove prefix from target string
        lists:nthtail(length(Prefix), TestString);
    false ->
        different_prefix
end.