Are side-effects possible in pure functional programming

Question

I have been trying to wrap my head around functional programming for a while now? I have looked up lambda calculus, LISP, OCML, F# and even combinatorial logic but the main

Answer 1

Most real-world functional programming is not "pure" in most senses, so half of the answer to your question is "you do it by giving up on purity". That said, there are alternatives.

In the "purest" sense of pure, the entire program represents a single function of one or more arguments, returning a value. If you squint your eyes and wave your hands a bit, you can declare that all user input is part of the function's "arguments" and that all output is part of the "return value" and then fudge things a bit so that it only does the actual I/O "on demand".

A similar perspective is to declare that the input to the function is "the entire state of the outside world" and that evaluating the function returns a new, modified "state of the world". In that case, any function in the program that uses the world state is obviously freed from being "deterministic" since no two evaluations of the program will have exactly the same outside world.

If you wanted to write an interactive program in the pure lambda calculus (or something equivalent, such as the esoteric language Lazy K), that's conceptually how you'd do it.

In more practical terms, the problem comes down to making sure that I/O occurs in the correct order when input is being used as an argument to a function. The general structure of the "pure" solution to this problem is function composition. For instance, say you have three functions that do I/O and you want to call them in a certain order. If you do something like RunThreeFunctions(f1, f2, f3) there's nothing to determine the order they'll be evaluated in. On the other hand, if you let each function take another function as an argument, you can chain them like this: f1( f2( f3())), in which case you know that f3 will be evaluated first because the evaluation of f2 depends on its value. [Edit: See also the comment below about lazy vs. eager evaluation. This is important, because lazy evaluation is actually quite common in very pure contexts; e.g., the standard implementation of recursion in the pure lambda calculus is nonterminating under eager evaluation.]

Again, to write an interactive program in the lambda calculus, this is how you'd probably do it. If you wanted something actually usable for programming in, you'd probably want to combine the function composition part with the conceptual structure of functions taking and returning values representing the state of the world, and create some higher-order abstraction to handling pipelining the "world state" values between I/O functions, ideally also keeping the "world state" contained in order to enforce strict linearity--at which point you've all but reinvented Haskell's IO Monad.

Hopefully that didn't just make you even more confused.

Answer 2

Given that most programs have some effects on the outside world (writing to files, modifying data in a database...) programs as whole are rarely side-effect free. Outside of academic exercises, there is no point in even trying.

But programs are assembled out of building blocks (subroutine, function, method, call it what you want), and pure functions make for very well-behaved building blocks.

Most functional programming languages do not require functions to be pure, although good functional programmers will try to make as many of their functions pure as is feasible and practical, in order to reap the benefits of referential transparency.

Haskell goes further. Every part of a Haskell Programm is pure (at least in the absence of sins such as "unsafePerformIO"). All functions that you write in Haskell are pure.

Side-effects are introduced through monads. They can be used to introduce a sort of "shopping-list -- shopper"-separation. Essentially your program writes a shopping list (which is just data and can be manipulated in a pure fashion), while the language runtime interprets the shopping list and does the effectful shopping. All your code is pure and friendly to equational reasoning and such, whereas the impure code is provided by the compiler-writers.

Answer 3

Haskell is a pure functional programming language. In Haskell all functions are pure (i.e. they always give the same output for the same inputs). But how do you handle side-effects in Haskell? Well, this problem is beautifully solved through the use of monads.

Taking I/O as an example. In Haskell every function that does I/O returns an IO computation, i.e. a computation in the IO monad. So, for instance, a function that reads an int from the keyboard, instead of returning an int, returns an IO computation that yields an int when it is run:

askForInt :: String -> IO Int

Because it returns an I/O computation instead of an Int, you cannot use this result directly in a sum, for example. In order to access the Int value you need to "unwrap" the computation. The only way to do this is to use the bind function (>>=):

(>>=) :: IO a -> (a -> IO b) -> IO b

Because this also returns an IO computation, you always end up with an I/O computation. This is how Haskell isolates side-effects. The IO monad acts as an abstraction of the state of the real world (in fact under the covers it is usually implemented with a type named RealWorld for the state part).