Problems with Immutable Data in Functional Programming

Question

I am new to functional programming. What I understood is functional programming is writing code using pure functions and without changing value of data.

Instead of changing value of variables we create new variables in functional programming when we need to update a variable.

Suppose we have a variable x which represents the total number of HTTP requests made by the program. If we have two threads then I want the threads to increment x whenever a HTTP request is made by any thread. If both threads make a different copy of the variable x then how can they synchronize the value of x. For example: if thread 1 make 10 HTTP requests and thread 2 made 11 HTTP requests then they will print 10 and 11 respectively but how would I print 21.

Answer 1

I can provide an answer for the clojure case. In clojure, if you need to co-ordinate access to shared state, there are constructs in the language that are designed to deal with these situations.

In this case, you could use an atom to hold the value. Changes made to an atom are atomic, and will be made optimistically by way of clojure's STM. Atoms are one of clojure's reference types. An atom is essentially a reference to a value, which can change over time in a controlled way via the atom's mutation functions.

See the clojure docs for more information on atoms and the other references types.

Answer 2

I will address the Haskell part. MVar is one of the communication mechanism for threads. This is one of the example taken from Simon Marlow's book (the program is self-explanatory):

main = do
  m <- newEmptyMVar
  forkIO $ do putMVar m 'x'; putMVar m 'y'
  r <- takeMVar m
  print r
  r <- takeMVar m
  print r

The output for the above program will be:

'x'
'y'

You can see in the above example how the MVar value in variable m is shared between threads. You can learn more about these techniques in this book.