Around p.27 of Monad.Reader#16.
type EnumeratorM el m a = IterV el a -> m (IterV el a)
enumHandle :: Handle -> EnumeratorM Char IO a
-- IterV Char a -> m (IterV Char a)
enumHandle h iter = loop iter where
loop :: IterV Char a -> IO (IterV Char a)
loop i@(Done _ _) = return i
loop i@(Cont k) = do
isEOF <- hIsEOF h
if isEOF
then return i
else do
e <- hGetChar h
loop . k . El $ e
-- k :: (StreamG Char -> IterV Char a)
enumFile :: FilePath -> EnumeratorM Char IO a
-- IterV Char a -> m (IterV Char a)
enumFile fp i = bracket
(openFile fp ReadMode)
(hClose)
(flip enumHandle i)
lengthOfTwoFiles :: FilePath -> FilePath -> IO (Maybe Int)
lengthOfTwoFiles fp1 fp2 =
fmap run $ ((enumFile fp1) >=> (enumFile fp2)) length
-- enumFile fp1 :: IterV Char a -> IO (IterV Char a) -> x -> m x
-- enumFile fp2 :: IterV Char a -> IO (IterV Char a) -> x -> m x
-- length :: IterV el Int -> x
-- (>=>) :: Monad m => (a -> m b) -> (b -> m c) -> a -> m c
The last fmap works over "IO (IterV Char Int)", the right side of "$"
