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

Adrian Adshead adrianadshead at yahoo.co.uk
Fri Jan 22 22:34:51 EST 2010

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

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

type PythonProgram = String
main :: PythonProgram
main = "print \"Hello World\""

is pure add_impure is pure. What we do with the result is other thing.


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