readMyFile.hs

import System.Directory

data FileError = NoSuchFile deriving (Show)

readMyFile :: FilePath -> IO (Either FileError String)
readMyFile pathToFile = do
    fileIsHere <- doesFileExist pathToFile
    if fileIsHere then do
        content <- readFile pathToFile
        return $ Right content
    else
        return $ Left NoSuchFile

main :: IO ()
main = do
    result <- readMyFile $ "/home/denis/Code/cardano-sl/nixpkgs-src.json"
    case result of
        Left problem  -> print problem
        Right content -> print content