Hi, I'm a poor lonesome newbie in Haskell world, and I would like to add a string typed on the prompt to a list of strings which is already defined. It would look like something like : type Path = [String] currentPath::Path currentPath = [] getpiece ::IO String getpiece = do c <-getLine return c putpiece:: String->Path putpiece a = a:currentPath and then I could combine the two functions, but obviously it doesn't work. I dare understand that it's impossible isn't it ? Maybe the only way is to create a new [String] each time I want to add a new string ? No ? Christophe Certain
On Sunday 17 February 2002 08:20, christophe certain wrote:
Hi,
I'm a poor lonesome newbie in Haskell world, and I would like to add a string typed on the prompt to a list of strings which is already defined.
It would look like something like :
type Path = [String]
currentPath::Path currentPath = []
getpiece ::IO String getpiece = do c <-getLine return c
putpiece:: String->Path putpiece a = a:currentPath
and then I could combine the two functions, but obviously it doesn't work. I dare understand that it's impossible isn't it ?
Maybe the only way is to create a new [String] each time I want to add a new string ? No ?
Christophe Certain
You seem to expect currentPath to be updated by putpiece? This won't happen in Haskell. Once you've declared currentPath=[] it will always be []. Values never change. If you want the functional equivalent of accumulator variables they have to be an argument of a recursive function. So try this.. getPath :: Path -> IO Path getPath currentPath = do piece <- getLine if piece == "" then return currentPath else getPath (piece:currentPath) initialCurrentPath::Path initialCurrentPath = [] main :: IO () main = do path <- getPath initialCurrentPath putStrLn (show path) Regards -- Adrian Hey
You seem to expect currentPath to be updated by putpiece? This won't
in Haskell. Once you've declared currentPath=[] it will always be [].
Values never change. If you want the functional equivalent of accumulator variables they have to be an argument of a recursive function. So try
happen this..
getPath :: Path -> IO Path getPath currentPath = do piece <- getLine if piece == "" then return currentPath else getPath (piece:currentPath)
initialCurrentPath::Path initialCurrentPath = []
main :: IO () main = do path <- getPath initialCurrentPath putStrLn (show path)
Regards -- Adrian Hey
Hi Adrian, How can I add a function that sorts this list that I read from the user and accumulate using the function that you described? I am not asking for a sort algorithm of course, I am just wondering how to feed the IO Path as an input to a sort function? Is it suppose to look like this: sort :: IO Path -> IO Path or sort :: IO Path -> Path How do you iterate over IO Path? Thanks for taking time.
On Sun, 17 Feb 2002, Cagdas Ozgenc wrote:
Hi Adrian,
How can I add a function that sorts this list that I read from the user and accumulate using the function that you described? I am not asking for a sort algorithm of course, I am just wondering how to feed the IO Path as an input to a sort function? Is it suppose to look like this:
sort :: IO Path -> IO Path
It could. (to make it simpler you may need a sort' :: Path -> Path function though, as in. sort getpath = do x <- getpath -- x has type Path return (sort' x) where sort' = .... -- sort' is typed as above and produces -- a sorted list list of -- type Path
or
sort :: IO Path -> Path
The point of the IO monad is to thread the state of the "World" (which your program modifies) explicitly through your program. The pure functions are the plumbing. A function of type IO Path -> Path cannot modify the world state since the world state is not the result of function application *.
How do you iterate over IO Path?
Um, I cant think of another good introductory way than that sort :: IO Path -> IO Path I mentioned above. Jay Cox * Minus the obvious facts that haskell implementions obviously do: 1. modify the world state as executing functions create new datastructures that need to be allocated, may cause garbage collection, maybe cause new heap allocation (do they?) or otherwise cause program termination (run out of stack/heap/ ... ), etc. 2. unless it uses sort unsafeIO and/or other "hacks." unsafeIO generally should not be used. PS: Anybody got any other suggestions for IO monad entry-level docs? I suppose Haskell Wiki on haskell.org might be good place to allude to.
On Sunday 17 February 2002 17:04, Cagdas Ozgenc wrote:
getPath :: Path -> IO Path getPath currentPath = do piece <- getLine if piece == "" then return currentPath else getPath (piece:currentPath)
initialCurrentPath::Path initialCurrentPath = []
main :: IO () main = do path <- getPath initialCurrentPath putStrLn (show path)
Hi Adrian,
How can I add a function that sorts this list that I read from the user and accumulate using the function that you described? I am not asking for a sort algorithm of course, I am just wondering how to feed the IO Path as an input to a sort function? Is it suppose to look like this:
sort :: IO Path -> IO Path
or
sort :: IO Path -> Path
How do you iterate over IO Path?
Thanks for taking time.
I don't know what iterating over something means (the opposite of iterating under something I suppose) I think you want something like this.. import List (sort) sortGet :: IO Path -> IO Path sortGet get = do got <- get return (sort got) or maybe even.. import Monad (liftM) sortGet :: IO Path -> IO Path sortGet = liftM sort Regards -- Adrian Hey
participants (5)
-
Adrian Hey -
Cagdas Ozgenc -
christophe certain -
Jay Cox -
Mark Carroll