[Haskell-cafe] Re: Lazy IO and closing of file handles

[Ferenc Wagner]

"Matthew Brecknell" <haskell at brecknell.org> writes:

>> enumLines :: (a -> String -> Either a a) -> a -> FilePath -> IO a
>> enumLines iter accum filename = do
>>   h <- openFile filename ReadMode
>>   flip fix accum $
>>     \iterate accum -> do
>>       try_line <- try (hGetLine h)
>>       case try_line of
>>         Left e -> hClose h >> return accum
>>         Right line -> do
>>           case iter accum line of
>>             Left accum -> hClose h >> return accum
>>             Right accum -> iterate accum

Another variation, enabling multiple iteratees (like a state machine),
exception propagation, and no flip fix :)

> newtype Iterator a = Iterator (a -> String -> (a,Maybe (Iterator a)))
> 
> enumLines :: Iterator a -> a -> FilePath -> IO (a,Maybe Exception)
> enumLines iterator start filename = do
>   h <- openFile filename ReadMode
>   let f (Iterator iter) accum = do
>         try_line <- try (hGetLine h)
>         case try_line of
>           Left e -> hClose h >> return (accum,Just e)
>           Right line -> do
>             case iter accum line of
>               (acc',Nothing)   -> hClose h >> return (acc',Nothing)
>               (acc',Just cont) -> f cont acc'
>   f iterator start
-- 
Feri.