Re: [Haskell-beginners] 'Simple' function

To be complete, my message actually assumes a function of that type *with the behavior you want*, which would be that of unsafePerformIO. Of course a trivial *pure* function with that type is for instance: asString _ = "Hi" But it does not have the behavior you want, it just ignores its argument. -- Yannis On 10/06/2015 20:22, Yannis Juglaret wrote:

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256

Assuming

asString :: IO String -> String

we have

getLine :: IO String

asString getLine :: String

Yet

asString getLine

could be "Hello" the first time you use it, then "Hi" the second time you use it. Same argument, different result, so this is not a pure function.

- -- Yannis

On 10/06/2015 19:50, Mike Houghton wrote:

...

Thanks for all the replies! It’s become a little clearer. However… (again this is naive begginer stuff.. ) if the signature is

asString :: IO String -> String

why is this not a pure function? The IO string has already been supplied - maybe via keyboard input - and so for the same IO String the function will always return the same value. Surely this behaviour is different to a monadic function that reads the keyboard and its output (rather than the input) could be different. ie if I give asString an input of IO “myString” then it will always return “myString” every time I invoke it with IO “myString”

Many thanks

Mike

...

On 10 Jun 2015, at 18:20, Imants Cekusins wrote:

Mike, if you are trying to run a "hello world" program in ghci, here are 2 working functions.

-- #1 : all it does is prompts for input and sends the value back to IO

module Text where

ioStr :: IO() ioStr = do putStrLn "enter anything" str <- getLine putStrLn str

-- #2 this program prepends the string you pass to it as an arg with "Hello"

str2str:: String -> String str2str s = "Hello " ++ s

-- how to run: -- #1 : ioStr -- #2 : str2str "some text"

hope this helps

On 10 June 2015 at 19:08, aldiyen wrote:

...

And just as a note, you can't really ever get the value inside the IO monad out. IO is not pure / non-deterministic, since it depends on something outside the program, and there's no way to "make it pure", as it were. You have to do all your operations on that String within the context of an IO

-aldiyen

...

On Jun 10, 2015, at 12:47, Steven Williams wrote:

Here is return's type signature:

return :: Monad m => a -> m a

What you are doing with the do notation can also be expressed as ioStr

...

...

...

...

>> = (\str -> return str).

do notation and bind both require you to have a value that has the same monad as before.

Steven Williams My PGP Key: http://pgp.mit.edu/pks/lookup?op=get&search=0xCACA6C74669A54 FA

...

...

...

...

> On 10/06/15 12:35, Mike Houghton wrote: Hi, > > I’ve been tryimg to write a function with signature > > asString :: IO String -> String > > > Does someone please have the patience to explain to me > what the compiler error messages really mean for these > two attempts and exactly what I’m doing (!!!) If I *do > not* give this function any type signature then it works > i.e.. > > asString ioStr = do str <- ioStr return $ str > > and the compiler tells me its signature is > > asString :: forall (m :: * -> *) b. Monad m => m b -> m > b > > which, at this stage of my Haskell progress, is just pure > Voodoo. Why isn’t it’s signature asString :: IO String > -> String ? > > > Another naive attempt is asString ioStr = str where str > <- ioStr > > and then compiler says parse error on input ‘<-’ > > > Many Thanks > > Mike > > _______________________________________________ Beginners > mailing list Beginners@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

...

>

---

...

...

...

...

Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

_______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

- -- Yannis JUGLARET

-- Yannis JUGLARET