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Acquiring & releasing resources

Whole-file IO​

readFile :: FilePath -> IO String
writeFile :: FilePath -> String -> IO ()

Handle-based file IO​

All in System.IO:

hGetLine :: Handle -> IO String
hPutStrLn :: Handle -> String -> IO ()
hIsEOF :: Handle -> IO Bool

withFile ::
FilePath -> IOMode
-> (Handle -> IO r) -- continuation (aka callback)
-> IO r
data IOMode =
ReadMode | WriteMode
| AppendMode | ReadWriteMode

Reading a file line by line​

readFileLineByLine :: FilePath -> IO [String]
readFileLineByLine file =
withFile file ReadMode readFileHandle
readFileHandle :: Handle -> IO [String]
readFileHandle h = do
eof <- hIsEOF h
if eof
then return []
else do
line <- hGetLine h
lines <- readFileHandle h
return (line : lines)

Handle is automatically released at end of continuation.

warning

Both readFile and readFileLineByLine are actually problematic for different reasons. We will (probably) learn about better ways to process (in particular large) files later this week.