Re: Proposal: end lazy IO results with errors/exceptions

+1 from me personally. Even as a "breaking change" it has semantics that are much more sane. Any code that can see the exception has a deeply flawed model of how withFile works and has behavior that will vary fairly wildly across platforms, give silently wrong answers, etc. This is just my personal stamp of approval, though, not necessarily the informed opinion of the core libraries committee. On Tuesday, July 22, 2014, Petr Pudlák wrote:

+1

Petr Dne 21. 7. 2014 22:16 "David Feuer" javascript:_e(%7B%7D,'cvml','david.feuer@gmail.com');> napsal(a):

...

Currently,

withFile "foo" hGetContents >>= putStrLn

prints out an empty line, the better to confuse newbies.

I propose modifying the lazyRead function in GHC.IO.Handle.Text that currently reads

lazyRead :: Handle -> IO String lazyRead handle = unsafeInterleaveIO $ withHandle "hGetContents" handle $ \ handle_ -> do case haType handle_ of ClosedHandle -> return (handle_, "") SemiClosedHandle -> lazyReadBuffered handle handle_ _ -> ioException (IOError (Just handle) IllegalOperation "hGetContents" "illegal handle type" Nothing Nothing)

to something like

lazyRead :: Handle -> IO String lazyRead handle = unsafeInterleaveIO $ withHandle "hGetContents" handle $ \ handle_ -> do case haType handle_ of ClosedHandle -> return (handle_, error "Forcing the result of a lazy read led to an attempt to read from a closed handle.") SemiClosedHandle -> lazyReadBuffered handle handle_ _ -> ioException (IOError (Just handle) IllegalOperation "hGetContents" "illegal handle type" Nothing Nothing)

Ideally that error should instead be something to throw an imprecise exception, but I don't know how to use those yet. I can't personally see a way for this to break sane, working code, but the folks on #ghc thought it should be discussed and debated on list.

David Feuer _______________________________________________ Libraries mailing list Libraries@haskell.org javascript:_e(%7B%7D,'cvml','Libraries@haskell.org'); http://www.haskell.org/mailman/listinfo/libraries