The I/O monad modifies the Universe that contains the set of all functions that comprise your program (if I even understand the concept correctly).
The right half is usually empty, except when you use the `>>=` operator (or flatMap) on an existing action, e.g.
let c = a >>= f
That operator chains the function `f` can take the value produced by the first IO action and produce another IO action. After we apply that operator, the result `c` is a cons cell where the left part is the original action `a` and the right part is the function f which takes the value produced by a and produces the next action. Its sort of like a cons cell in regular lists, except the next value is provided by a function. A lazy cons cell, perhaps :)
The final result is a lazy chain of I/O cons cells (called "main", of course :P). Its passed to the Haskell runtime, which executes that chain as a recipe, alternating between doing I/O actions and evaluating the function to decide what to do next.
So what does this buy us? Mostly just referential transparency. What does referential transparency buy us? Easy refactoring. We can replace any expression with its value, even stuff like `putStrLn "test"`. We can say `let writeTest = putStrLn "test"` at the top of the file then write `do writeTest; writeTest` in main.
Another neat thing is that do syntax isn't limited to just IO, but works with anything that implements `flatMap` (and `unit`, which I forgot to mention). That means we can build our own imperative DSLs that produce IO-like monads which are then interpreted by our own interpreter, and the users of those DSLs can use the same do syntax. Which is pretty awesome. Here is a simple example: https://gist.github.com/tonymorris/b5dba9d7d877051d0164 and a much more complex one http://augustss.blogspot.com/2009/02/more-basic-not-that-any... :)
You can handle every single error explicitly (as in Go) using pattern matching
eitherResultOrError = operation1 arg
case eitherResultOrError of
Left error -> handle error
Right result -> handle' result
let eitherResultOrError = do
x <- operation1 arg
y <- operation2 x + 1
z <- operation3 x y
case eitherResultOrError of
Left error -> handle error
Right result -> handle' result
For IO operations and other monadic actions, its best to use EitherT, ErrorT or MaybeT, which are monads that can add error handling to any other monad. To understand how these work, its probably best to implement MaybeT. Basically, they add another wrapper to other monads to redefine what the bind operator (`>>=`) does
That sounds rather complicated. My way of thinking of it is simpler than that. a -> IO b is just a function from a to b that can do some I/O. Nothing more complicated than that.
No, you have a tiny, very simple type that allows for type safe IO. It also happens to make haskell a more powerful imperative language than most imperative languages, as IO actions are first class and can be passed around and manipulated like anything else.
>So, basically, they acknowledge that their theoretical model has a huge impedance mismatch with what we write programs to do (I/O, eventually, somewhere)
No, they acknowledge that doing IO is so important that it should be done correctly. You are going to some pretty extreme mental gymnastics to misrepresent a language you want to hate.