These aren't strange rules at all. They are pretty common in functional languages (of which Erlang is one) - less so in procedural, imperative and object-orient

These aren't strange rules at all. They are pretty common in functional languages (of which Erlang is one) - less so in procedural, imperative and object-orientated languages. The reason is simple, with immutability you know what the value is, and you can pattern match on it with confidence. Variables that can change value are a mini-version of global state with all the reasoning problems that 'globality' gives: "what is the value at this point in the code?" "when does the value change" "what range of values can this have depending on which code path was executed?" ie sometimes the value is changed and sometimes not... Trust me, once you have gone immutable, you don't want to go back.