Case matching on read function in Haskell [duplicate]

Question

I am wondering how to check success or failure (resulting in Prelude.read: no parse) of the read function in Haskell. In my case i run "(read formatted :: Int)" in code formatting a record structure, where the fields might be a single Int in String form but might also contain something else. I want to apply my function only to the fields where the read returns an Int. Thanks.

Answer 1

You should consider readMaybe from Text.Read. Once the value is returned within the Maybe monad then you can use case to decide what to do.

import Text.Read

add1 :: String -> Maybe Int
add1 str = case intval of  
   Just x -> Just (x + 1)
   Nothing -> Nothing

   where
      intval = readMaybe str


main = do
   print $ add1 "7"
   print $ add1 "7.0"

If you want to be more adventurous, now that the data is in the Maybe monad, we can treat the Maybe as a functor and use applicative functors to process them.