CODEWITHSUNDEEP
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Ashish Negi
Ashish Negi

Reputation: 5301

haskell write to file it if it does not exists before reading

I am learning haskell and am stumbled upon the parse error on input contents' compilation error.

What i want to do :

I store my previous session state state in a file. I read this file before starting the program. However, during the first run of program, file may not exist. In this case, i want to first create file first with default value and then proceed ahead.

main :: IO()
main = do
    -- Take input
   let fileName = "ashish-temp.txt"

   let dummyBoard = take 5 $ repeat "-----"

   fileExist <- doesFileExist fileName

   if False == fileExist
   then writeFile fileName $ unlines dummyBoard

   -- getting an error on this line
   contents <- readFile fileName

   -- do processing () :)

   --  i want the value in contents
   putStrLn "Done"

Also, i think that rather than writing dummyBoard to the file i can just initialize the contents with dummyBoard. But i also failed in doing it. And i guess the way should be the same for both.

Please help. Thanks.

Edit Solution:

else is required for every if in haskell.

Also another problem that you would face after this problem is : *** Exception: ashish-temp.txt: openFile: resource busy (file is locked)

use import qualified System.IO.Strict as S and S.redFile for reading file.

Upvotes: 4

Views: 3714

Answers (2)

&#216;rjan Johansen
&#216;rjan Johansen

Reputation: 18189

You had a second part of the question which hasn't been answered yet.

Also, i think that rather than writing dummyBoard to the file i can just initialize the contents with dummyBoard. But i also failed in doing it. And i guess the way should be the same for both.

Indeed you can, as follows:

contents <- if fileExist
    then readFile fileName
    else return $ unlines dummyBoard

Upvotes: 1

Sibi
Sibi

Reputation: 48734

There are some problem with your codebase:

  • You are missing the else part of your if expression. In Haskell, since if is an expression , it would need the else part as opposed to other languages where the if-else are statements and the else part is not mandatory.
  • What exactly is dim ? You have to define it.

A working program which shows similar in concept of what you want to do will look like this:

main :: IO()
main = do

   let fileName = "somefile.txt"

   fileExist <- doesFileExist fileName

   if not fileExist
   then writeFile fileName "something"
   else return ()

   contents <- readFile fileName
   -- do stuff with contents here

   putStrLn "Done"

Upvotes: 6

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