Edit: This article contains some errors and wasn't able to transport my intention correctly. So I've created a new version of this article which hopefully contains less errors and is better to read.
What is functional programming? Many people tend to think that having closures in a language makes that language a functional one. But by this definition almost every modern language would qualify as 'functional'. Even Java (because of Javas anonymous inner classes which are closures too). So if this can't really be the qualifying property of a language, what else is it?
Of course it's the "referential transparency": A 'function' is an special kind of relation which assigns values of a domain-set to values of a result-('codomain') set. To qualify as a function this mapping has to be unambiguous: For every element of the domain-set the function always gives the single, same result.
If a 'function' isn't referential transparent, this property isn't fulfilled, so it's not a function anymore. It's something which is often called 'procedure': A block of code which creates a result by applying some algorithm. We can argue if this property really has to be fulfilled rigorously by a language to qualify as 'functional', but I would say 'Yes, it has!'. The reason is that in every language we can use procedures to create functions (just by making sure that there are no side-effects), but those languages aren't still called 'functional' only because the programmer can force himself to use them in a functional way.
If this would be enough, we could also call assembler a functional language, because we can use it to write functional code too. The same would be true for every other paradigm, Assembler would be for example an object-oriented language too (And this is of course true for every Turing-complete language, not only of Assembler).
But with this rigorous definition of the notion 'functional', there aren't many functional languages anymore. Ocaml for example is clearly non-functional now - and even Haskell isn't. So should we maybe lift this requirement a bit? I don't think so.
Why isn't Haskell a functional language? The reason is the IO-monad. To do I/O in Haskell there are functions which aren't referential transparent, because they return a different value at each invocation. If we write
let v = getStr in ...
v is bound to a different value at each invocation. Even if this value is contained in the IO monad, it's still a different value, because we can write code which runs different on each invocation. This creates some kind of avalanche effect which can turn huge parts of the code into simple imperative code - and because of this, we can't talk of Haskell as a functional language. Even if we want to create pure functional code, it's not possible in Haskell in the moment we need I/O (and which program doesn't?).
That's the reason why I consider the IO-monad as 'evil': Haskell relies on it and thus isn't functional anymore.
But is it possible to do I/O in a really functional way? If it's not possible then the idea of functional programming would be deeply flawed and should maybe even abandoned for practical programming. What use is a concept which works only in theory and fails in the moment you want to do something as common as I/O?
But fortunately it is possible. The idea is to put all the external input of a program into a 'world'-value and then call our program with it:
result = app(world)
If 'app' is a command-line application for example, then 'world' would represent all the input the program receives and 'result' would be all the output the program generates. This is quite an easy concept, but how would it work if we need interaction? The idea is to use lazy evaluation: Instead of reading the whole 'world' before calling 'app', 'world' is only evaluated on demand. The same is true for 'result' which is written out in the moment data becomes available and not at the end of the program. So our program could give back a partial result (for example an input prompt), before even requiring any real input.
This would work for GUI-programs too. We can for example put all the mouse/keyboard/etc-events in the 'world' object and 'result' contains all the drawing-commands the applications issues. This approach would create a very distinct structure of the program, because input and output are now clearly separated.
This works well for some problems, but often there is some 'natural' interaction between both. Your simple command-line application may for example open, read and write some data-files, based on commands issued by the user. We can not create a simple function like 'readFile(file_name)' because this 'function' could give different data at each invocation and is thus no function. So how to solve this problem?
The answer is to put all 'external data' into the 'world' object: All files, all system resources etc. Of course this also works with lazy evaluation so those data is not really read and put into a value. The 'world'-value behave just as this is the case. And if we now start our application with all those data in the argument, it would again give the same result as long as the input value is the same too. If we only change a single character in a file which is contained in the 'world', the value isn't the same anymore and so we could get a different result.
The advantage of this approach is that we can limit out 'world-value' to the parts of the system which can be accessed by our 'app'. If we don't want that 'app' can read a file for example, we simply don't put the 'file-system' into the value. This allows very strict security constraints on data access in a very natural way. And our 'app' is of course not a big monolithic function but calls other functions which in turn call again other functions etc.. So we can give those other functions limited subsets of the world-value to limit their access, too.
This approach is truly functional and also very natural: An application simply transforms one 'world-state' into another.
And there is an additional advantage of this approach compared to monads: Higher performance.
To use the plain IO-monad, the language isn't referential transparent anymore. So many of the nice and clever optimization techniques for functional programs can't be used anymore. Using the IO-monad forces a certain order of evaluation and also disallow caching of already calculated values.
But even if you use other monads to simulate state, the result isn't really as fast anymore. The result is that monads are constructors and can only 'give' some value. But how can we create and modify state, if we can only return values and have no 'input'?
With state-monads it works this way: Instead of transforming input-data (which contains the old state) into output-data (which contains the new state), a state monad returns a new program. It doesn't do transformation on the data but on the program itself! In the end this transformation does nothing else then transforming the code into 'transformer-style' code which gets the state as input and creates a new state as output. It does this by putting chunks of code into closures and chaining them together using the 'bind'-function. So if you use a state monad in Haskell, the state monad does nothing more than a code-transformation and then evaluating this code which in turn chains a state-value thru the invocations.
But this transformation and evaluation is done at runtime! Sure, sometimes it can be cached or even unrolled at compile-time, but only in rare cases. The reason is that the 'to transformed' code is dependent on the input values of a function which can (for example with a if-then-else construct) create a different kind of code every time. The Haskell compiler can't resolve this in most non-trivial cases and thus the whole code-transformation and evaluation has do be redone over and over again. This costs time and also often even kills tail recursion. So by using a state monad we often have much less efficient code than by using the transformer based approach directly
A language which does I/O by the transformer approach (but in a different way as proposed here) is Clean, and if you look the benchmarks at the shootout, it seems clear that there is really a performance advantage here.
And whats the disadvantages? Of course there are some, or else nobody had even considered to use monads instead.
The main advantage of monads is that they are in principle code transformers. With monads we can create embedded DSLs, similar to Lisp-macros (But since monads do this code transformation at runtime, it costs time and memory, while Lisps macros are evaluated by the compiler before creating the real code). Having the ability to create new 'embedded languages' has some appeal on many people. With the transformer approach this isn't possible - but of course there are still other ways to do this, if it's really wanted.
The second disadvantage is that the transformer approach requires an additional parameter in each invocation. Instead of writing
a <- getStr
b <- getStr
putStr $ a ++ "," ++ b
we have to write
(a, state) = getStr state
(b, state) = getStr state
state = putStr state (a ++ ", " ++ b)
instead. This look pretty obfuscated compared to the Haskell approach. But the reason is that Haskell has syntactic support for monads but none for the other approach. If we had to write the first example without the syntactic support the 'do' construct provides, it would look even uglier than the second one (just try it, it's also a good practice if you are new to monads).
So why not simply add syntactic support for the transform-approach too? What about this:
with state let
a <- getStr
b <- getStr
putStr a ++ ", " ++ b
This is quite short too, and the 'with' syntax can be used by the compiler to use the function type to chain 'state' it thru all calls which have a parameter with a matching type and also return a value of the same type.
The last disadvantage is that the transformation approach need additional semantic support. Why? Because lazy evaluation alone isn't enough. We often need a certain order of evaluation to read and write data in a certain required order. Often this order is 'automatically correct', but sometimes it isn't.
For example (without the above proposed syntax to make the problem more clear):
state = putStr state "Please enter your name: "
(state, name) = getStr state
state = putStr state ("hello '" ++ name ++ "', please enter your age: ")
(state, age) = getStr state
If we evaluate this statements, the program will write
Please enter your name: hello '
and wait for input. Why? Because the 'getStr state' line isn't evaluated before 'name' is actually required. That's the funny thing with general lasy evaluation: Sometimes the evaluation-order can be quite unexpected.
How so solve this problem? There are multiple solutions. First we can replace the '++' after "hello '" by a strict variant which requires evaluation of both parameters before it continues. This would force the program to the wanted behavior, but it would also require additional thinking by the programmer.
A better way would be to create an internal dependency between data. For example 'putStr' and 'getStr' would create and check a dependency on 'state' which would force evaluation of all previous 'putStr' before a 'getStr' can occur (and vice versa). This would only fore evaluation order into a certain form, but each of the functions would remain referential transparent. But there has to be some compiler support to support this feature.
So I/O is possible, in a totally functional way and with some support from the language even in a relatively simple way. Without monads we lose the possibility to create embedded languages, but I think that this isn't really a big disadvantage. But for I/O monads aren't necessary and even harmful.