[Haskell-cafe] Execution order in IO

[Levent Erkok]

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 at 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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