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

Maciej Piechotka uzytkownik2 at gmail.com
Fri Jan 22 19:31:42 EST 2010

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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