Why in Declarative Programming variables are not reassigned?

Question

I've read that a Declarative language is called "stateless". This means that we can imagine that internally every variable is a constant variable, and it never get reassigned ( in contrast with what usually happens in procedural programming). How can this work?

Answer 1

This isn't as exotic as it sounds, and you can do it in a procedural language fairly easily. The basic idea is just that instead of changing a value you just have a function return a new value. For instance, instead of something like

string read()
{
  string s = '';

  while((string c = getchar()) != '\n')
  {
    s += c;
  }

  return s;
}

you could write something like

string read()
{
  string c = getchar();

  if(c == '\n')
  {
    return '';
  }
  else
  {
    return c + read();
  }
}

See how no variable ever gets reassigned here?

There are some subtleties here that I'm not going into, but I just wanted to illustrate that the idea is not that farfetched.