IO, partiblestyle
IO
is the monad you cannot avoid.
Why Haskell is so HARD? (And how to deal with it); Saurabh Nanda.
...but you kept looking anyway, and here you are!
[...] the input/output story for purelyfunctional languages was weak and unconvincing, let alone error recovery, concurrency, etc. Over the last few years, a surprising solution has emerged: the monad. I say "surprising" because anything with as exotic a name as "monad"  derived from category theory, one of the most abstract branches of mathematics  is unlikely to be very useful to redblooded programmers. But one of the joys of functional programming is the way in which apparentlyexotic theory can have a direct and practical application, and the monadic story is a good example.
Tackling the Awkward Squad: monadic input/output, concurrency, exceptions, and foreignlanguage calls in Haskell, Simon Peyton Jones.
...with that (ahem) "joy" leading to more than a few helpful guides about the topic  that monadic interface: it's abstract alright!
This is hard stuff. Two years ago I spent several hours to write 3 lines invoking IO
computations.
Trying to understand the IO ()
; "belka", Haskell Discourse.
Is that you?
[...] And I still don't believe in monads. :)
Retrospective Thoughts on BitC; Jonathan S. Shapiro, bitcdev mailing list.
You are not alone.
Contents
_{ }
IO
, using OI
Our definition of IO
is a type synonym:
type IO a = OI > a
with OI
being an abstract partible type:
data OI a
primitive primPartOI :: OI > (OI, OI)
instance Partible OI where
part = primPartOI
Like primPartOI
, most other primitives for the OI
type also accept an OI
value as their last (or only) argument e.g:
primitive primGetChar :: OI > Char
primitive primPutChar :: Char > OI > ()
⋮
Borrowing the running example from Philip Wadler's How to Declare an Imperative:
echo :: OI > ()
echo u = let !(u1:u2:u3:_) = parts u
!c = primGetChar u1 in
if c == '\n' then
()
else
let !_ = primPutChar c u2
in echo u3
Wadler also provides an SML version:
val echoML : unit > unit fun echoML () = let val c = getcML () in if c = #"\n" then () else (putcML c; echoML ()) end
in which we replace SML's sequencing operator ;
:
val echoML : unit > unit fun echoML () = let val c = getcML () in if c = #"\n" then () else let val _ = putcML c in echoML () end end
If we compare it to our Haskell version:
echo :: OI > ()
echo u = let !(u1:u2:u3:_) = parts u
!c = primGetChar u1 in
if c == '\n' then
()
else
let !_ = primPutChar c u2
in echo u3


val echoML : unit > unit fun echoML () = let val c = getcML () in if c = #"\n" then () else let val _ = putcML c in echoML () end end 
...we can now see just how similar the two versions of echo
really are: apart from the obvious changes of syntax, the Haskell version replaces all use of unit
values with OI
values, and adds an extra call to parts
to provide them.
So there you have it: for the price of some extra calls and bindings, we can have SMLstyle I/O in Haskell. Furthermore, as the prevailing definition for SML has been available since 1997, there should be plenty of I/O tutorials to choose from...
At this point, you may be tempted to try something like:
type IO a = () > a primitive might_get_Char :: () > Char primitive might_put_Char :: Char > () ⋮
While this might work in some situations, it's unreliable in general. Why?
 Short answer: unlike SML, Haskell's nonstrict evaluation means expressions should be referentially transparent.
 Long answer: read section 2.2 (pages 45) of Wadler's paper.
 Longer answer: read Lennart Augustsson's More points for lazy evaluation.
 Extended answer: read John Hughes's Why Functional Programming Matters.
But  if after all that  you're still not convinced, then perhaps you'll be happier programming in SML...
...you're still here: nice! Now for a small example  here's a basic version of interact
, using those OI
based definitions:
interact :: (String > String) > OI > ()
interact d u = let !(u1, u2) = part u in
putStr (d $ getContents u1) u2
putStr :: String > OI > ()
putStr s u = foldr (\(!_) > id) () $ zipWith primPutChar s $ parts u
getContents :: OI > String
getContents u = case map getChar (parts u) of
l@(!c:_) > l
l > l
_{ }
Some annoyances
 Extra parameters and arguments  As noted by Sigbjørn Finne and Simon Peyton Jones in Programming Reactive Systems in Haskell, passing around all those
OI
values correctly can be tedious for large definitions.
 Polymorphic references  It's been known for a very long time in the SML community that naive declarations for operations using mutable references breaks type safety:
primitive newPolyRef :: a > OI > PolyRef a primitive readPolyRef :: PolyRef a > OI > a primitive writePolyRef :: PolyRef a > a > OI > () kah_BOOM u = let … !vehicle = newPolyRef undefined u1 !_ = writePolyRef ("0" :: [Char]) u2 !crash = readPolyRef vehicle u3 burn = 1 :: Int in crash + burn
 SML's solution is to make all mutable references monomorphic through the use of dedicated syntax:
let val r = ref (…) ⋮
 One alternative for Haskell would be to extend type signatures to support monomorphic typevariables:
primitive newIORef :: monomo a . a > OI > IORef a primitive readIORef :: monomo a . IORef a > OI > a primitive writeIORef :: monomo a . IORef a > a > OI > () { would be rejected by the extended type system: kah_BOOM u = let !(u1:u2:u3:_) = parts u !vehicle = newIORef undefined u1  vehicle :: monomo a . IORef a !_ = writeIORef ("0" :: [Char]) u2  vehicle :: IORef [Char] !crash = readIORef vehicle u3  vehicle :: IORef [Char] ≠ IORef Int burn = 1 :: Int in crash + burn }
 In standard Haskell, one of the few places this already occurs (albeit implicitly) is the parameters of a function:
{ will be rejected by the standard Haskell type system ker_plunk f = (f True, f 'b') }
One solution
 Extra parameters and arguments  What is needed is a succinct interface to "hide the plumbing" used to pass around
OI
values:
instance Monad ((>) OI) where return x = \u > case part u of !_ > x m >>= k = \u > case part u of (u1, u2) > case m u1 of !x > k x u2
 Polymorphic references  we now make
IO
into an abstract data type:
module Abstract.IO ( Monad (..), getChar, putChar, … newIORef, readIORef, writeIORef, ⋮ ) where instance Monad ((>) OI) where return x = \u > case part u of !_ > x m >>= k = \u > case part u of (u1, u2) > case m u1 of !x > k x u2 getChar :: IO Char getChar = primGetChar putChar :: Char > IO () putChar = primPutChar newIORef :: a > IO (IORef a) newIORef = primNewIORef readIORef :: IORef a > IO a readIORef = primReadIORef writeIORef :: IORef a > a > IO () writeIORef = primWriteIORef  these are now local, private entities  type IO a = OI > a data OI a primitive primPartOI :: OI > (OI, OI) primitive primGetChar :: OI > Char primitive primPutChar :: Char > OI > () ⋮ data IORef primitive primNewIORef :: a > OI > IORef a primitive primReadIORef :: IORef a > OI > a primitive primWriteIORef :: IORef a > a > OI > () ⋮
 With the
IO
type now made abstract, the only way to useIO
values is by using:
 the visible
IO
operations:getChar
,putChar
, etc.  the monadic interface 
Monad(return, (>>=), …)
(or via Haskell'sdo
notation).
 the visible
 The key here is the type of
(>>=)
, forIO
values:
(>>=) :: IO a > (a > IO b) > IO b
 in particular, the type of the second argument:
(a > IO b)
 ...it's a function, so the value it receives will be rendered monomorphic in the function's result (of type
IO b
).
 As
(>>=)
is now the onlyIO
operation which can retrieve a result from anIO
value, mutable references (IORef …
) simply cannot be used polymorphically.
GHC's solution
_{ }
newtype IO a = IO (State# RealWorld > (# State# RealWorld, a #))
...you may have noticed that we've already made liberal use of one Haskell extension  bangpatterns  and it would be useful to stay as close as possible to standard Haskell, so we'll simplify matters:
newtype IO a = IO (IOState > (IOState, a))  unboxedtuple replaced by standard one
type IOState = State# RealWorld
Now to make the changes:
 to the type 
IOState
uses anOI
value:
newtype IOState = IOS OI
 to the I/Ospecific operations  each one will use the
OI
value in the initial state to provide two newOI
values: one to make up the final state; the other being used by theOI
primitive:
getChar :: IO Char getChar = IO $ \(IOS u) > let !(u1, u2) = part u !c = primGetChar u1 in (IOS u2, c) putChar :: Char > IO () putChar c = IO $ \(IOS u) > let !(u1, u2) = part u !t = primPutChar c u1 in (IOS u2, t)  etc.
 to the overloaded operations  you've probably seen it all before:
instance Monad IO where return x = IO $ \(!s) > (s, x) IO m >>= k = IO $ \(!s) > let !(s', x) = m s !(IO w) = k x in w s'
 (...if you haven't: it's ye ol'
passtheplanetstatepassing technique.)
One aspect which doesn't change is IO
and its operations being abstract. In fact, the need is even more pressing: in addition to preventing the misuse of certain OI
operations, being an abstract data type prevents IOState
values from being erroneously reused.
Conclusions
 Why is Haskell I/O monadic  to avoid having to use extra arguments and parameters everywhere.
 Why is Haskell I/O abstract  to ensure I/O works as intended, by preventing the misuse of internal data.
 Why is Haskell I/O unusual  because of Haskell's nonstrict evaluation and thus its focus on referential transparency, contrary to most other programming languages.
Further reading
If you've managed to get all the way to here, State in Haskell by John Launchbury and Simon Peyton Jones is also worth reading, if you're interested in how GHC eventually arrived at its current definition of IO
.