Hey, I am studying Haskell as a unit at University. I find the concept and design idea's of Haskell interesting but I am finding my self struggling to understand the relationship between the normal state and IO state of Haskell. This is my situation: I have created 12 functions for a program which take in two types: type Catalogue = [Track] type Playlist = [Track] The definition for track is as follows: -- Track = ArtistDetails Title Length PCount data Track = Track ArtistType String Float Int deriving (Show,Eq,Read) -- Popular = Artist | Composor Performer data ArtistType = Popular String | Classical String String deriving (Show,Eq,Read) I have managed to save the data to a file using this code: --Saving Data saveData :: String -> Catalogue -> IO() saveData fileName catalogue = writeFile fileName (show catalogue) Problem now is reading the data back into the program. When I read the data back into the program it comes as IO [Track]. This is the code I have been using to load the data: loadData :: String -> Catalogue loadData fileName = do x <- readFile fileName return (read x :: Catalogue) I think I have missed a trick some where or I am using IO wrong, I really don't know. I believe it is the latter. I have been told that my definition for the function is wrong and that I should use the lazy approach to fix the problem. But this still leaves me with IO [Track]. Could someone inform me on what I am doing wrong? Thank you in advance Smoky
On Thu, 8 May 2008, Mark Wallsgrove wrote:
Problem now is reading the data back into the program. When I read the data back into the program it comes as IO [Track]. This is the code I have been using to load the data:
loadData :: String -> Catalogue loadData fileName = do x <- readFile fileName return (read x :: Catalogue)
type should be loadData :: String -> IO Catalogue But using plainly 'read' may not satisfy you, because the program will abort unrecoverably if the file has corrupt content. You may want to use 'reads' and check manually if parsing was successful: case reads x of [(cat, "")] -> return cat _ -> fail "corrupt file content"
Thank you very much for your fast response! Ok, that is now changed, but everything else in my program is expecting Catalogue without IO. Is there a way to change IO Catalogue into Catalogue? Henning Thielemann wrote:
On Thu, 8 May 2008, Mark Wallsgrove wrote:
Problem now is reading the data back into the program. When I read the data back into the program it comes as IO [Track]. This is the code I have been using to load the data:
loadData :: String -> Catalogue loadData fileName = do x <- readFile fileName return (read x :: Catalogue)
type should be
loadData :: String -> IO Catalogue
But using plainly 'read' may not satisfy you, because the program will abort unrecoverably if the file has corrupt content. You may want to use 'reads' and check manually if parsing was successful:
case reads x of [(cat, "")] -> return cat _ -> fail "corrupt file content"
Am Donnerstag, 8. Mai 2008 14:59 schrieb Mark Wallsgrove:
Thank you very much for your fast response!
Ok, that is now changed, but everything else in my program is expecting Catalogue without IO. Is there a way to change IO Catalogue into Catalogue?
Not a recommendable way. But there's no need to, your programme will probably look like main = do args <- getArgs let realArgs = parseArgs args catalogue <- loadData doSomething realArgs catalogue and doSomething might e.g. construct an updated catalogue and write it to a file.
On Thu, May 8, 2008 at 10:59 PM, Mark Wallsgrove
Thank you very much for your fast response!
Ok, that is now changed, but everything else in my program is expecting Catalogue without IO. Is there a way to change IO Catalogue into Catalogue?
Suppose I have a function "readFile" of type String -> IO String, and a function "read" of type String -> Catalogue. How can I feed the output of the first function into the second function, when that pesky "IO" is in my way? Perhaps I would write something that looks like this: loadData :: String -> IO Catalogue loadData fileName = do x <- readFile fileName return (read x :: Catalogue) Even though the expression "readFile fileName" has type IO String, the variable "x" bound on the third line has type String. This means that I can pass it into "read" without any problems. Eventually you'll learn about things like (>>=) and "liftM", which can be used to write this in a simpler form, but hopefully this will get you going for now. Stuart
On Thu, 8 May 2008, Mark Wallsgrove wrote:
Thank you very much for your fast response!
Ok, that is now changed, but everything else in my program is expecting Catalogue without IO. Is there a way to change IO Catalogue into Catalogue?
Yes, but you do not want that. You will go on writing functions which consume Catalogue and finally you will apply a Catalogue function in the IO monad which does all the work at once.
On Thu, 8 May 2008, Mark Wallsgrove wrote:
Thank you very much for your fast response!
Ok, that is now changed, but everything else in my program is expecting Catalogue without IO. Is there a way to change IO Catalogue into Catalogue?
Btw. there was a nice article precisely about the issue "How to get rid of the IO?" in the old Hawiki. What is the progress in bringing back Hawiki?
Was there? I have been google'ing that problem for ages.. Just one more thing. I have to make a menu system where the user chooses what functionality they want. Because you cannot change a value once it is set I have used recursion so that when something changes it then calls the menu back up. I feel this is way to memory consuming. Is there another way? CODE: main catalogue playlist: x = choose option passtofunction x passtofunction value: function1 1 = main (functionName value) playlist function2 2 = main catalogue (functionName value) Henning Thielemann wrote:
On Thu, 8 May 2008, Mark Wallsgrove wrote:
Thank you very much for your fast response!
Ok, that is now changed, but everything else in my program is expecting Catalogue without IO. Is there a way to change IO Catalogue into Catalogue?
Btw. there was a nice article precisely about the issue "How to get rid of the IO?" in the old Hawiki. What is the progress in bringing back Hawiki?
On 8 May 2008, at 16:31, Mark Wallsgrove wrote:
Was there? I have been google'ing that problem for ages..
Just one more thing. I have to make a menu system where the user chooses what functionality they want. Because you cannot change a value once it is set I have used recursion so that when something changes it then calls the menu back up. I feel this is way to memory consuming. Is there another way?
While this method feels like it should consume lots of memory, it in fact doesn't. Remember that you're dealing with a graph machine, and that no stack is maintained of all the calls you've made. The garbage collector will simply follow you through the menu system clearing up the memory behind you. Bob
Thank you all for your help, you have been invaluable Thomas Davie wrote:
On 8 May 2008, at 16:31, Mark Wallsgrove wrote:
Was there? I have been google'ing that problem for ages..
Just one more thing. I have to make a menu system where the user chooses what functionality they want. Because you cannot change a value once it is set I have used recursion so that when something changes it then calls the menu back up. I feel this is way to memory consuming. Is there another way?
While this method feels like it should consume lots of memory, it in fact doesn't. Remember that you're dealing with a graph machine, and that no stack is maintained of all the calls you've made. The garbage collector will simply follow you through the menu system clearing up the memory behind you.
Bob
participants (5)
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Daniel Fischer -
Henning Thielemann -
Mark Wallsgrove -
Stuart Cook -
Thomas Davie