IO in action
IO type serves as a tag for operations (actions) that interact with the outside world. The
IO type is abstract: no constructors are visible to the user. [...]
The Haskell 2010 Report (page 95 of 329).
Instead of that conventional approach:
data IO -- abstract getChar :: IO Char putChar :: Char -> IO () ⋮
IO using other types, ones with no visible constructors:
data (->) a b -- abstract data OI -- also abstract type IO a = OI -> a getChar :: OI -> Char -- an I/O action putChar :: Char -> (OI -> ()) -- a function with one parameter, whose result is an I/O action ⋮
Main.main is also an I/O action:
main :: OI -> ()
Therefore an internal subroutine in the Haskell implementation provides each running program with an initial
OI value. However, most programs will require more than just one:
getLine :: OI -> [Char] getLine u = let !(u1, u2) = partOI u in let !c = getChar u1 in if c == '\n' then  else let !cs = getLine u2 in c:cs putLine :: [Char] -> OI -> () putLine (c:cs) u = let !(u1, u2) = partOI u in let !_ = putChar c u1 in putLine cs u2 putLine  u = putChar '\n' u
So another I/O action is needed in order to access that same internal subroutine from Haskell:
partOI :: OI -> (OI, OI)
If more than two new
OI values are needed:
partsOI :: OI -> [OI] partsOI u = let !(u1, u2) = partOI u in u1 : partsOI u2
But why are these abstract
OI values needed at all - what purpose do they serve?
Actions and functions
Looking more closely at
partsOI reveals an interesting fact:
OIvalue is only used once (if at all).
Why is this important? Because in Haskell, functions have their basis in mathematics. That imposes certain requirements on function, including this one:
- if a function's result changes, it is only because one or more of it's arguments has changed.
If they're always used with different
OI values, then I/O actions can be used like functions:
partsOI :: OI -> [OI] partsOI = unfoldr (Just . partOI)
even if they're defined using subroutines:
foreign import "oi_part" partOI :: OI -> (OI, OI) foreign import "oi_getchar" getChar :: OI -> Char foreign import "oi_putchar" putChar :: Char -> OI -> ()
The need for an
OI value also helps to prevent I/O actions from being used as subroutines:
trace :: [Char] -> a -> a trace msg x = case putLine msg of !_ -> x -- how is this supposed to work?
The monadic interface:
instance Monad ((->) OI) return x = \ u -> let !_ = partOI u in x m >>= k = \ u -> let !(u1, u2) = partOI u in let !x = m u1 in let !y = k x u2 in y
getLine to be defined more compactly:
getLine :: OI -> [Char] getLine = do c <- getChar if c == '\n' then return  else do cs <- getLine return (c:cs) putLine :: [Char] -> OI -> () putLine  = putChar '\n' putLine (c:cs) = putChar c >> putLine cs
and conceals the use of all those
OI values. But not all definitions will benefit from being monadic:
partsOI :: OI -> [OI] partsOI = do (u1, u2) <- partOI; return (u1 : partsOI u2)
- Merely monadic provides more information about Haskell's implementation of the monadic interface.
- For those who prefer it, John Launchbury and Simon Peyton Jones's State in Haskell explains the state-passing approach currently in widespread use.