Question

I am trying to convert a Haskell program to a Haskell GUI program,
but since I am very very new at Haskell,
every time I try something I get lots of errors.
I asked on Stack Overflow many time for this program,
but whenever an error disappears, two errors arise.

Sorry for asking similar question, but
the program’s ability what I intend to convert is
very simple word searching.
Receive input string, search the word, print on window.

Any advice, hint or example would be very helpful for me.

I am on Windows XP. Sorry for very poor code.

--GUI routine
import Graphics.UI.Gtk
import Text.Regex.Posix ((=~))
import Control.Monad (when)
--core routine
matchWord :: String -> String -> Int
matchWord file word = length . filter (== word) . concat $ file =~ "[^- \".,\n]+"

--main start
main :: IO ()
main =
      do initGUI
         win <- windowNew
         windowSetTitle win "WORD SEARCHER"
         win `onDestroy` mainQuit

         fch <- fileChooserWidgetNew FileChooserActionOpen
         containerAdd win fch 

         targetFile <- fileChooserGetFilename fch --wrong?

         ent <- entryNew
         btn <- buttonNewWithLabel "Click to search"
         st <- labelNew $ Just "Found : 0      "

         col <- vBoxNew False 5
         containerAdd col ent
         containerAdd col btn
         containerAdd col st    

         btn `onClicked` do targetWord <- entryGetText ent
                            fileData <- readFile Just targetFile
                            found <- matchWord fileData targetWord
                            labelSetText st found
         containerAdd win col
         widgetShowAll win
         mainGUI

thank you for reading

Answer 1

This will get you started.

targetFile <- fileChooserGetFilename fch

At this point, targetFile has type Maybe String; that is, it will return either Just "somestring" or Nothing. You want the "somestring" part, if it’s available. You can get it by pattern matching:

Just targetFile <- fileChooserGetFilename fch

This will fail with an opaque error message if the result of fileChooserGetFilename returned Nothing. For more robustness you can case analyse the result:

maybeTargetFile <- fileChooserGetFilename fch
targetFile <- case maybeTargetFile of
                  Nothing -> fail "I need a filename!"
                  Just file -> return file

The other problem is in this line:

found <- matchWord fileData targetWord

x <- m is used to bind the result of an action m into the variable x, but matchWord returns an Int, not an action (eg. IO a for some a).