Is Haskell truly pure (is any language that deals with input and output outside the system)?

Question

After touching on Monads in respect to functional programming, does the feature actually make a language pure, or is it just another "get out of jail free card" for reasoning of computer systems in the real world, outside of blackboard maths?

EDIT:

This is not flame bait as someone as said in this post, but a genuine question that I am hoping that someone can shoot me down with and say, proof, it is pure.

Also I am looking at the question with respect to other not so pure Functional Languages and some OO languages that use good design and comparing the purity. So far in my very limited world of FP, I have still not groked the Monads purity, you will be pleased to know however I do like the idea of immutability which is far more important in the purity stakes.

Answer 1

I'm very new to functional programming, but here's how I understand it:

In haskell, you define a bunch of functions. These functions don't get executed. They might get evaluated.

There's one function in particular that gets evaluated. This is a constant function that produces a set of "actions." The actions include the evaluation of functions and performing of IO and other "real-world" stuff. You can have functions that create and pass around these actions and they will never be executed unless a function is evaluated with unsafePerformIO or they are returned by the main function.

So in short, a Haskell program is a function, composed of other functions, that returns an imperative program. The Haskell program itself is pure. Obviously, that imperative program itself can't be. Real-world computers are by definition impure.

There's a lot more to this question and a lot of it is a question of semantics (human, not programming language). Monads are also a bit more abstract than what I've described here. But I think this is a useful way of thinking about it in general.

Answer 2

I think of it like this: Programs have to do something with the outside world to be useful. What's happening (or should be happening) when you write code (in any language) is that you strive to write as much pure, side-effect-free code as possible and corral the IO into specific places.

What we have in Haskell is that you're pushed more in this direction of writing to tightly control effects. In the core and in many libraries there is an enormous amount of pure code as well. Haskell is really all about this. Monads in Haskell are useful for a lot of things. And one thing they've been used for is containment around code that deals with impurity.

This way of designing together with a language that greatly facilitates it has an overall effect of helping us to produce more reliable work, requiring less unit testing to be clear on how it behaves, and allowing more re-use through composition.

If I understand what you're saying correctly, I don't see this as a something fake or only in our minds, like a "get out of jail free card." The benefits here are very real.

Answer 3

Take the following mini-language:

data Action = Get (Char -> Action) | Put Char Action | End

Get f means: read a character c, and perform action f c.

Put c a means: write character c, and perform action a.

Here's a program that prints "xy", then asks for two letters and prints them in reverse order:

Put 'x' (Put 'y' (Get (\a -> Get (\b -> Put b (Put a End)))))

You can manipulate such programs. For example:

conditionally p = Get (\a -> if a == 'Y' then p else End)

This is has type Action -> Action - it takes a program and gives another program that asks for confirmation first. Here's another:

printString = foldr Put End

This has type String -> Action - it takes a string and returns a program that writes the string, like

Put 'h' (Put 'e' (Put 'l' (Put 'l' (Put 'o' End)))).

IO in Haskell works similarily. Although executing it requires performing side effects, you can build complex programs without executing them, in a pure way. You're computing on descriptions of programs (IO actions), and not actually performing them.

In language like C you can write a function void execute(Action a) that actually executed the program. In Haskell you specify that action by writing main = a. The compiler creates a program that executes the action, but you have no other way to execute an action (aside dirty tricks).

Obviously Get and Put are not only options, you can add many other API calls to the Action data type, like operating on files or concurrency.

I didn't say anything about monads. Action is not a monad. Monads are only some combinators, like conditionally above, and are not important to purity.

The essential thing about purity is referential transparency. In Haskell, if you have f x+f x you can replace that with 2*f x. In C, f(x)+f(x) in general is not the same as 2*f(x), since f could print something on the screen, or modify x. The advantages are mentioned here: basically it allows better optimalizations, since the compiler has much more freedom.

Answer 4

For an expanded version of sdcwc's sort of construction of IO, one can look at the IOSpec package on Hackage: http://hackage.haskell.org/package/IOSpec

Answer 5

It is important to understand that there is nothing inherently special about monads – so they definitely don’t represent a “get out of jail” card in this regard. There is no compiler (or other) magic necessary to implement or use monads, they are defined in the purely functional environment of Haskell. In particular, sdcvvc has shown how to define monads in purely functional manner, without any recourses to implementation backdoors.

Answer 6

What does it mean to reason about computer systems "outside of blackboard maths"? What kind of reasoning would that be? Dead reckoning?

Side-effects and pure functions are a matter of point of view. If we view a nominally side-effecting function as a function taking us from one state of the world to another, it's pure again.

We can make every side-effecting function pure by giving it a second argument, a world, and requiring that it pass us a new world when it is done. I don't know C++ at all anymore but say read has a signature like this:

vector<char> read(filepath_t)

In our new "pure style", we handle it like this:

pair<vector<char>, world_t> read(world_t, filepath_t)

This is in fact how every Haskell IO action works.

So now we've got a pure model of IO. Thank goodness. If we couldn't do that then maybe Lambda Calculus and Turing Machines are not equivalent formalisms and then we'd have some explaining to do. We're not quite done but the two problems left to us are easy:

• What goes in the world_t structure? A description of every grain of sand, blade of grass, broken heart and golden sunset?

• We have an informal rule that we use a world only once -- after every IO operation, we throw away the world we used with it. With all these worlds floating around, though, we are bound to get them mixed up.

The first problem is easy enough. As long as we do not allow inspection of the world, it turns out we needn't trouble ourselves about storing anything in it. We just need to ensure that a new world is not equal to any previous world (lest the compiler deviously optimize some world-producing operations away, like it sometimes does in C++). There are many ways to handle this.

As for the worlds getting mixed up, we'd like to hide the world passing inside a library so that there's no way to get at the worlds and thus no way to mix them up. Turns out, monads are a great way to hide a "side-channel" in a computation. Enter the IO monad.

Some time ago, a question like yours was asked on the Haskell mailing list and there I went in to the "side-channel" in more detail. Here's the Reddit thread (which links to my original email):

http://www.reddit.com/r/haskell/comments/8bhir/why_the_io_monad_isnt_a_dirty_hack/