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
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
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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