Question

as we know that variables can’t be variable in erlang. but consider this code,why each value of [1,2,3,4] is sequentially pattern matched to N,and don’t throw exception??

1> [2*N || N <- [1,2,3,4]].
[2,4,6,8]

Answer 1

Saying that a variable can’t be variable isn’t quite true. It’s more that a variable can only be assigned once. So the following psuedo-code is illegal:

N = 4;
foo(N);
N = N + 1;
foo(N);

However, the following is legal:

fact(0) -> 1,
fact(N) -> N * fact(N-1).

When we call fact(4) N will take the value 4 then 3 then 2 then 1 for each different function call. The code you are showing above is similar. For each item in the list N takes on a different value. But you never assigned the value of N more than once.