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 IO data type, like operating on files or concurrency.
Adding a result value
Now consider the following data type.
data IO a = Get (Char -> Action) | Put Char Action | End a
The previous Action type is equivalent to IO (), i.e. an IO value which always returns "unit", comparable to "void".
This type is very similar to Haskell IO, only in Haskell IO is an abstract data type (you don't have access to the definition, only to some methods).
These are IO actions which can end with some result. A value like this:
Get (\x -> if x == 'A' then Put 'B' (End 3) else End 4)
has type IO Int and is corresponding to a C program:
int f() {
char x;
scanf("%c", &x);
if (x == 'A') {
printf("B");
return 3;
} else return 4;
}
Evaluation and execution
There's a difference between evaluating and executing. You can evaluate any Haskell expression, and get a value; for example, evaluate 2+2 :: Int into 4 :: Int. You can execute Haskell expressions only which have type IO a. This might have side-effects; executing Put 'a' (End 3) puts the letter a to screen. If you evaluate an IO value, like this:
if 2+2 == 4 then Put 'A' (End 0) else Put 'B' (End 2)
you get:
Put 'A' (End 0)
But there are no side-effects - you only performed an evaluation, which is harmless.
How would you translate
bool comp(char x) {
char y;
scanf("%c", &y);
if (x > y) { //Character comparison
printf(">");
return true;
} else {
printf("<");
return false;
}
}
into an IO value?
Fix some character, say 'v'. Now comp('v') is an IO action, which compares given character to 'v'. Similarly, comp('b') is an IO action, which compares given character to 'b'. In general, comp is a function which takes a character and returns an IO action.
As a programmer in C, you might argue that comp('b') is a boolean. In C, evaluation and execution are identical (i.e they mean the same thing, or happens simultaneously). Not in Haskell. comp('b') evaluates into some IO action, which after being executed gives a boolean. (Precisely, it evaluates into code block as above, only with 'b' substituted for x.)
comp :: Char -> IO Bool
comp x = Get (\y -> if x > y then Put '>' (End True) else Put '<' (End False))
Now, comp 'b' evaluates into Get (\y -> if 'b' > y then Put '>' (End True) else Put '<' (End False)).
It also makes sense mathematically. In C, int f() is a function. For a mathematician, this doesn't make sense - a function with no arguments? The point of functions is to take arguments. A function int f() should be equivalent to int f. It isn't, because functions in C blend mathematical functions and IO actions.
First class
These IO values are first-class. Just like you can have a list of lists of tuples of integers [[(0,2),(8,3)],[(2,8)]] you can build complex values with IO.
(Get (\x -> Put (toUpper x) (End 0)), Get (\x -> Put (toLower x) (End 0)))
:: (IO Int, IO Int)
A tuple of IO actions: first reads a character and prints it uppercase, second reads a character and returns it lowercase.
Get (\x -> End (Put x (End 0))) :: IO (IO Int)
An IO value, which reads a character x and ends, returning an IO value which writes x to screen.
Haskell has special functions which allow easy manipulation of IO values. For example:
sequence :: [IO a] -> IO [a]
which takes a list of IO actions, and returns an IO action which executes them in sequence.
Monads
Monads are some combinators (like conditionally above), which allow you to write programs more structurally. There's a function that composes of type
IO a -> (a -> IO b) -> IO b
which given IO a, and a function a -> IO b, returns a value of type IO b. If you write first argument as a C function a f() and second argument as b g(a x) it returns a program for g(f(x)). Given above definition of Action / IO, you can write that function yourself.
Notice monads are not essential to purity - you can always write programs as I did above.
Purity
The essential thing about purity is referential transparency, and distinguishing between evaluation and execution.
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.
Thanks to purity, a compiler has much more freedom and can optimize better. It can rearrange computations, while in C it has to think if that changes meaning of the program.
