How does a loop simulation works?

问题

-module(prac).

-export([len/1]).

len([]) -> 

   0;

len([_|T]) ->

   1 + len(T).

So I have this code and it works, but I dont know how to simulate it properly.

回答1:

Okay, if you're looking for an explanation of why the code works the way it does, it goes something like this. Given the following code:

len([])    -> 0;
len([_|T]) -> 1 + len(T).

If you were to call len/1 like len([a,b,c]), then you can think of it executing like:

• call len([a,b,c])
• does [a,b,c] match []? no
• does [a,b,c] match [_|T]? yes, yielding _ = a and T = [b,c]
• call len([b,c])
  • does [b,c] match []? no
  • does [b,c] match [_|T]? yes, yielding _ = b and T = [c]
  • call len([c])
    • does [c] match []? no
    • does [c] match [_|T]? yes, yielding _ = c and T = []
    • call len([])
      • does [] match []? yes
      • len([]) returns 0
    • len([c]) returns 1 + 0
  • len([b,c]) returns 1 + 1
• len([a,b,c]) returns 1 + 2

Does that make sense?

回答2:

Erlang has a debugger call im()

try to use it

来源：https://stackoverflow.com/questions/41008186/how-does-a-loop-simulation-works