Question

hello I am very beginer of haskell

I am making GUI program that

1. open file selection dialog

2. take word

3. search the word in selected txt file

4. print number of found to label

but I stuck with error which I can’t solve it

I paste error and code here

could somebody please help me?

thank you

the full code is here

--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 <- buttonNew
         st <- labelNew $ Just "Found : 0      "

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

         buttonSetLabel btn "Click to search"

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

the error is here

gui-word-search.hs:33:49:
    Couldn't match expected type `FilePath'
       against inferred type `Maybe FilePath'
    In the first argument of `readFile', namely `targetFile'
    In a 'do' expression: fileData <- readFile targetFile

Answer 1

fileChooserGetFilename can’t always return a filename (the user might click on “cancel” for example). For that reason its return type is Maybe FilePath, not FilePath. So if a file was chosen, it returns a Just containing the FilePath. If no file was chosen it returns Nothing.

However readFile takes a FilePath as an argument, not a Maybe FilePath (calling readFile with Nothing makes no sense).

So what you need to do is you need to pattern match on targetFile. If it is Nothing, you need to handle that somehow (you could print an error message, or just keep asking the user for a file until he picks one), and if it’s a Just, you take the FilePath it contains and feed that to readFile.