On Tue, Jan 4, 2011 at 4:27 PM, Ross Paterson
On Tue, Jan 04, 2011 at 04:03:34PM +0100, Henning Thielemann wrote:
Simon Marlow schrieb:
Good plan. I'll make a proposal to add System.IO.binary. A different type for binary handles is the right thing, but it's a larger undertaking so I don't plan to attack it right now (someone else is welcome to do so).
Isn't this also the purpose of the safer-file-handles package?
No, that puts the IOMode and region in the type, not the binary mode.
Indeed. However, I would be glad to add such a safety feature if we can distill a nice API. What would such an API look like? One approach might be something like this: (not type-checked, not even parsed...) -- Types newtype TextHandle = ... newtype ByteHandle = ... -- Opening openFile :: FilePath -> IOMode -> IO TextHandle openBinaryFile :: FilePath -> IOMode -> IO ByteHandle -- Text input/output hGetLine :: TextHandle -> IO String hPutStrLn :: TextHandle -> String -> IO () ... -- Binary input/output hGetBuf :: ByteHandle -> Ptr a -> Int -> IO Int hPutBuf :: ByteHandle -> Ptr a -> Int -> IO () ... -- General operations on handles (text/binary) class Handle h where hClose :: h -> IO () hIsEOF :: h -> IO Bool hSetBuffering :: h -> BufferMode -> IO () ... instance Handle TextHandle where ... instance Handle ByteHandle where ... The disadvantage of this approach is that all general operations on handles (that should both work for Text- and ByteHandles) need to be put in a class. A way to solve this is to have a single Handle type which is parameterized by a phantom type that represents the Text/Byte mode: (I also use this style to encode the IOMode of Handles in safer-file-handles). {-# LANGUAGE EmptyDataDecls #-} newtype Handle mode = ... data Text data Byte type TextHandle = Handle Text type ByteHandle = Handle Byte The types of the open functions and the types of the text I/O and byte I/O functions remain the same. The types of the general operations will change to: hClose :: Handle mode -> IO () hIsEOF :: Handle mode -> IO Bool hSetBuffering :: Handle mode -> BufferMode -> IO () ... Note that these are now polymorphic in the text/byte mode. In both these approaches the following questions remain: * What about the standard handles? What are their types? Maybe we need stdinText :: TextHandle, stdinBytes :: BytesHandle, etc.. But this will make it more difficult to use them at the same time. * What about hSetBinaryMode :: Handle -> Bool -> IO () ? Should we remove it from such an API. What are your thoughts on this? Regards, Bas