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

21 Jul 2014

      Hi,

superficially¹ +1

Joachim

¹ I like the idea, but can’t tell what the downsides are.

Am Montag, den 21.07.2014, 16:16 -0400 schrieb David Feuer:
...
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
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Joachim Breitner
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