[Haskell-beginners] Re: Re: testing and the culture of Haskell

[Adrian Adshead]

Hi Alex,

That does help, but raises a further question.

If  (>>=) :: m a -> (a -> m b) -> m b

Then whenever the IO monad is entered it can not be escaped,
so in the example you gave:-

f :: IO a -> Int
f _ = 10


while f (putStrLn "foo") is a pure function evaluating to 10,
neither the IO Action, nor the a (from 'IO a') can be used without
using the (>>=) or (>>). Therefore since the parameter can not
be used in the function f without entering the IO monad, your
function would be equivalent to :-

f :: Int
f = 10

I have two questions to try and clarify my understanding.

1) Is it possible to have a function with an IO Action as an input
parameter (and not just ignore the parameter) and have a non-IO
return value?
(ie is     f :: IO a -> b    possible without ignoring the IO a)

2) Is it possible to have a function which evaluates any IO Action
within the body and have a non-IO return value?
(ie is    f :: a -> b     possible while evaluating any IO Action
within the body.)



My understanding of Haskell was that both of these situations
would not be possible, and the return value would always be
of the form IO b. (For these questions I am excluding use of
unsafePerformIO)

Many thanks for your time.

Adrian.


--- On Sat, 23/1/10, Alexander Dunlap <alexander.dunlap at gmail.com> wrote:

From: Alexander Dunlap <alexander.dunlap at gmail.com>
Subject: Re: [Haskell-beginners] Re: Re: testing and the culture of Haskell
To: "Adrian Adshead" <adrianadshead at yahoo.co.uk>
Cc: beginners at haskell.org
Date: Saturday, 23 January, 2010, 5:22

When you put actions in a "do" block, what you are really doing under
the hood is using the ">>=" function to string actions together. (>>=)
:: m a -> (a -> m b) -> m b, so, when m ~ IO, (>>=) :: IO a -> (a ->
IO b) -> IO b. Thus, a do block of IO actions will itself be an IO
action. However, consider the function:

f :: IO a -> Int
f _ = 10

Now the value f (putStrLn "foo") is totally pure, and has the value of
10. A similar thing is going on with the greetAdrian function.

Hope that helps.

Alex

On Fri, Jan 22, 2010 at 7:34 PM, Adrian Adshead
<adrianadshead at yahoo.co.uk> wrote:
>
> OK, so I get the idea that all functions are pure because they
> just return actions.
>
> So as I understand it now...
>
> All functions are pure.
> The evaluation of all functions not declared as IO results in no side effects.
> The evaluation of all functions declared as IO may or may not have side effects.
>
> But how can the function greetAdrian from the example below not be an IO
> operation?
>
>
>
>
> From: Maciej Piechotka <uzytkownik2 <at> gmail.com>
> Subject: Re: Re: testing and the culture of Haskell
> Newsgroups: gmane.comp.lang.haskell.beginners
> Date: 2010-01-23 00:31:42 GMT (2 hours and 52 minutes ago)
>
> On Fri, 2010-01-22 at 20:59 +0000, Adrian Adshead wrote:
> > >All Haskell functions are pure without exception.  For example:
> > >
> > >greet :: String -> IO ()
> > >greet name = putStrLn $ "Hello, "++name
> > >
> > >This is a pure function from String to IO ().  This function (like all
> > >Haskell functions) has no side effects.  Its return value of type IO ()
> > >merely _represents_ an IO action.  The runtime system knows how to act
> > >on this representation.
> > >
> > >This also means that there is no such thing in Haskell as marking a
> > >function as side-effecting.
> > >
> > >This distinction may be subtle, but it's important.
> > >
> > >
> > >Steve
> >
> > Steve,
> >
> > Please could you clarify this for me since you are making exactly
> > the opposite assertion than I have
>  understood.
> >
> > I am confused by you stating "All Haskell functions are pure
> > without
> >  exception.".
> >
> > Pure functions have no impact on 'anything'. They take input
> > parameters (which they don't change) and return exactly the
> > same result whenever the same input parameters are given.
> >
> > >greet :: String -> IO ()
> > >greet name = putStrLn $ "Hello, "++name
> >
> > This example you gave is not a pure function since it does have
> > the side effect that the screen is changed by outputting the string
> > "Hello, " and the name passed in.
> >
> >
>
> greatAdrian :: String
> greetAdrian = let x = greet "Adrian"
>               in x `seq` f x
>
> greet can be consider a pure function and value IO () is evaluated by
> seq. IO () represents an action(s) not execution of action(s). If f of x
> does not use any tricks nothing will be
>  printed.
>
> IO a value it can be:
> - cast unsafely into a. However I guess we omit this shame for a moment
> - binded with other action. But the resultant type is now again IO b. So
> we still get a something
> - returned as main. Then we might consider whole Haskell program as
> metalanguage which returns single thing - other program. In similar way
> as:
>
> type PythonProgram = String
> main :: PythonProgram
> main = "print \"Hello World\""
>
> is pure add_impure is pure. What we do with the result is other thing.
>
> Regards
>
>
>
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