Hi Pavel,
>>>>> "pavel" == pavel <[EMAIL PROTECTED]> writes:
pavel> I'm having a problem with writing a function dealing with
pavel> I/O. Maybe it's just a lack of experience or simple Haskell
pavel> knowledge because I'm just a beginer. The problem: I want to
pavel> write a function that converts an IO String into String lala
pavel> :: IO String -> String
pavel> Is it possible? If yes, how?
you cannot use (except from a dirty trick) an IO operation that
returns a string (IO String) as a string. What you can do
--and, I assume, that's what you want to do-- is
to apply a function inside a composition of IO operations that uses the
result of a previous IO operation, e.g., function "f" in the
following program text:
main = do
string1 <- readFile "input" <<or something else of type IO String>>
let result = f string1
print result <<or something else>>
return ()
Note that this kind of "let" does not have a corresponding "in".
The reason that a function like unIO :: IO a -> a
does not legally exist is that it would break a nice
theoretical property of Haskell called referential transparency
which makes reasoning about programs extremely productive.
Hope that helps.
--
Christoph Herrmann
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