CODEWITHSUNDEEP
haskell
probably at the beach
probably at the beach

Reputation: 15217

Basic haskell program to find a string inside a file

This is my first Haskell attempt so my understanding is fairly limited. I wanted to write a very basic program to check if a file contains the word ERROR. I came up with the following which of course doesn't even compile. 

 import Text.Regex.Posix

 containsErrorString :: String -> Bool
 containsErrorString x = x =~ "ERROR" :: Bool

 fileContainsErrorString fileName = do
    s <- readFile fileName
    containsErrorString s

Is this the right approach and if not what would be the correct way to accomplish this?

Thanks

Upvotes: 1

Views: 523

Answers (2)

viorior
viorior

Reputation: 1803

Main problem is in fact that fileContainsErrorString :: IO Bool,

but containsErrorString (s:: String) :: Bool.

So, you need to wrap containsErrorString into IO, for example use return :: Monad m => a -> m a

fileContainsErrorString fileName = do
    s <- readFile fileName
    return $ containsErrorString s

Upvotes: 1

Thomas M. DuBuisson
Thomas M. DuBuisson

Reputation: 64740

A light rewrite

I suggest:

  1. Use Text instead of String for any "serious" work.
  2. If you don't need regex, don't use regex.

In code:

import Data.Text as T

containsErrorString :: Text -> Bool
containsErrorString = ("ERROR" `T.isInfixOf`)

fileContainsErrorString :: FilePath -> IO Bool
fileContainsErrorString = containsErrorString `fmap` T.readFile

-- Warning, code typed and not tested.

Your Code

There isn't anything wrong with your approach. The use of String remains common and perfectly acceptable for most uses. Your error is just that you forgot to return the result (you are in a monad, IO, and containsErrorString is a pure function).

Instead of:

containsErrorString s

you should have had:

return (containsErrorString s)

Upvotes: 5

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