About the immutability of variables once bounded in Erlang:
Single assignment is like algebra.
When I went to school, my math teacher said, “If there’s an X in several different parts in the same equation, then all the Xs mean the same thing.” That’s how we can solve equations: if we know that X+Y=10 and X-Y=2, then X will be 6 and Y will be 4 in both equations.
But when I learned my first programming language, we were shown stuff like this:
X = X + 1Everyone protested, saying “you can’t do that!”. But the teacher said we were wrong, and we had to unlearn what we learned in math class. X isn’t a math variable: it’s like a pigeon hole/little box…
In Erlang, variables are just like they are in math. When you associate a value with a variable, you’re making an assertion – a statement of fact. This variable has that value. And that’s that.
– Joe Armstrong, in his book Programming Erlang.
