What is the difference between == and =:= in Erlang when used with terms in general?

Question

Apart from the fact that =:= prevents unwanted integer casts:

1> 1=:=1.0. false 

What is the advantage of using =:= with terms in general?

Better performance?

Answer 1

The biggest advantage of =:= is it returns true only for same terms in the same way as pattern matching. So you can be sure they are same. 1 and 1 are same terms and 1 with 1.0 are not. That's it. If you write function like foo(A, B) when A =:= B -> A. and bar(A, B) when A =:= B -> B. they will behave same. If you use == it will not be same functions. It simply prevents surprise. For example, if you make some key/value storage it would not be right if you store value with key 1 and then get this value if ask for key 1.0. And yes, there is a little bit performance penalty with == but least astonishment is far more important. Just use =:= and =/= when it is your intent to compare same terms. Use == and /= only if it is your intent to compare numbers.

Answer 2

*Eshell V5.9.3.1  (abort with ^G)    1> 1.0==1.    true    2> 1.0=:=1.   false   

When we go with == it will transfer both elements into the same format to match. With =:= it does not happen – when the two elements are same-type and same-value it will return true.