9

I'm currently trying to master Erlang. It's the first functional programming language that I look into and I noticed that in Erlang, each assignments that you do is a single assignment. And apparently, not just in Erlang, but in many other functional programming languages, assignments are done through single assignment.

I'm really confused about why they made it like that. What exactly is the purpose of single assignment? What benefits can we get from it?

2240
  • 1,547
  • 2
  • 12
  • 30
Kemal Fadillah
  • 9,760
  • 3
  • 45
  • 63
  • I suggest checking [this answer](http://stackoverflow.com/a/9764997/1229023). In short, it's not an assignment as such; it's a name binding process. – raina77ow Jun 29 '12 at 03:30
  • Probably a better fit for [programmers.se]. –  Jun 29 '12 at 03:49
  • The answers to [this question](http://stackoverflow.com/questions/9494601/why-is-functional-programming-good) may be more to the point. – Andreas Rossberg Jun 29 '12 at 09:32

3 Answers3

10

Immutability (what you call single assignment), simplifies a lot of things because it takes out the "time" variable from your programs.

For example, in mathematics if you say

  x = y

You can replace x for y, everywhere. In operational programming languages you can't ensure that this equality holds: there is a "time" (state) associated with each line of code. This time state also leaves the door open to undesired side effects which is the enemy number one of modularity and concurrency.

For more information see this.

2240
  • 1,547
  • 2
  • 12
  • 30
Diego
  • 4,353
  • 1
  • 24
  • 22
3

Because of Single Assignment, Side effects are so minimal. Infact, its so hard to write code with race conditions or any side effects in Erlang. This is because, the Compiler easilly tells un-used variables, created terms which are not used, shadowed variables (especially inside funs ) e.t.c.

Another advantage that Erlang gained in this is Referential Transparency. A function in Erlang will depend only on the variables passed to it and NOT on global variables, except MACROS (and macros cannot be changed at run-time, they are constants.).

Lastly, if you watched the Erlang Movie, the Sophisticated Error Detection Mechanism which was built into Erlang depends so much on the fact that in Erlang, variables are assigned Once.

Muzaaya Joshua
  • 7,736
  • 3
  • 47
  • 86
  • 2
    Note, however, that functions in Erlang aren't completely referentially transparent: sending a message to another process may result in side-effects which change the value of future calls to the function. As for global variables, there's the process dictionary, ETS tables and the like. – Martin Törnwall Jun 29 '12 at 17:31
  • 1
    Yes, Erlang is not at all referentially transparent, and immutability removes only one side effect (namely editing memory that might change the state of the world), but does nothing to limit side effects in general. – Magnus Kronqvist Jul 02 '12 at 21:59
2

Having variables keep their values makes it much easier to understand and debug the code. With concurrent processes you get the same kind of problem anyway, so there is enough complication anyway without having just any variable potentially change its value at any time. Think of it as encapsulating side effects by only allowing them when explicit.

Koistinen
  • 21
  • 4