functions not in type classes
Fri, 18 Jan 2002 16:59:29 -0800
At 2002-01-18 03:06, Cagdas Ozgenc wrote:
>Same story, you shouldn't have had a Container of Char only in the first
Well, what about a file-handle? Not necessarily exactly a Container
(though it might be), but it's always "of Char", or better yet, "of
Word8". You could make a generalised file-handle to be of anything, but
your read and write functions would necessarily have to be restricted.
For instance, given this:
class Stream1 c where
read :: c a -> Integer -> IO [a]
write :: c a -> [a] -> IO ()
data FileHandle1 a = ...
fileRead :: FileHandle1 Word8 -> Integer -> IO [Word8]
fileWrite :: FileHandle1 Word8 -> [Word8] -> IO ()
...you cannot make FileHandle1 an instance of Stream1.
However, you could do this:
class Stream2 s a | s -> a where
read :: s -> Integer -> IO [a]
write :: s -> [a] -> IO ()
data FileHandle2 = ...
fileRead :: FileHandle2 -> Integer -> IO [Word8]
fileWrite :: FileHandle2 -> [Word8] -> IO ()
instance Stream2 FileHandle2 Word8 where
read = fileRead
write = fileWrite
Ashley Yakeley, Seattle WA