Simon PJ's "Tackling the awkward squad" has an excellent (and highly readable) account of IO, if you want a more precise treatment. (It also covers concurrency, exceptions, and FFI to a degree.)

http://research.microsoft.com/en-us/um/people/simonpj/papers/marktoberdorf/

It's hard to choose a favorite amongst Simon's writings, but this one stands out in my opinion in its lucidity, and how clear it makes these "awkward" parts of Haskell, without giving up any rigor.

-Levent.

On Thu, Apr 16, 2015 at 6:06 PM, David Feuer <david.feuer@gmail.com> wrote:

Others have already discussed this in terms of GHC's model of IO, but as Tom Ellis indicates, this model is a bit screwy, and not really the best way to think about it. I think it is much more useful to think of it in terms of a "free monad". That is, think about the `IO` type as a *data structure*. An `IO a` value is a sort of recipe for producing a value of type `a`. That is,

data IO :: * -> * where
ReturnIO :: a -> IO a
BindIO :: IO a -> (a -> IO b) -> IO b
HPutStr :: Handle -> String -> IO ()
HGetStr :: Handle -> IO String
....

And then think about the runtime system as an interpreter whose job is to run the programs represented by these IO values.

Perhaps I need to be more specific.

main = do
a <- getLine
b <- getLine

Can we say "a" absolutely always receives the first line of input and if
so what makes this the case rather than "b" receiving it ? Or do things
need to be slightly more complicated to achieve this ?

Sorry it's just the engineer in me. I think once I've got this clear I'll
be happy to move on.

Jon

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