GP talks about getting encoding and decoding together by implementing the spec once. In Erlang/Elixir you can only get one at a time because you can't do the equivalent of this:

  some_predicate(<concrete values>, Decoded). % decoder
  some_predicate(Encoded, <concrete values>). % encoder
  some_predicate(<concrete values>, <concrete values>). % validator

But in Prolog, that's what many predicates permit as a baseline. In Erlang, yes, a validator is, or can be, a parser/decoder. But it's not an encoder which still needs to be a separate function (or set of functions) to describe movement in the other direction.