On Thu, Oct 26, 2006 at 09:08:42AM -0700, David Roundy wrote:
On Thu, Oct 26, 2006 at 01:15:11AM +0100, Ross Paterson wrote:
hunk ./System/IO.hs 369 +-- | @'withFile' name mode act@ opens a file using 'openFile' and passes +-- the resulting handle to the computation @act@. The handle will be +-- closed on exit from 'withFile', whether by normal termination or by +-- raising an exception. +withFile :: FilePath -> IOMode -> (Handle -> IO r) -> IO r +withFile name mode = bracket (openFile name mode) hClose
I like this function (and use ones like it all the time in darcs), but do wonder whether it might be possible to ensure that the Handle doesn't escape, which would make it ultra-cool. I can't see how to do this, except to put the third argument into another monad and use a phantom type, which would be highly awkward, to say the least. But if there were a pretty approach, it would be very fancy.
In case I've been too vague, what I mean is that I'd like it if one could ensure that code such as
withFile "filename" ReadMode return >>= hGetContents
statically fails, rather than failing at runtime with a file handle closed exception. I know this is a stupid thing for the programmer to do, but it's nice to write library code such that the compiler can help programmers avoid doing stupid things. And I know there are a lot of folks who can figure out cleverer type hackery than I can, so one can always hope (and maybe ask Oleg?)...
He's already been there: http://www.haskell.org/pipermail/haskell/2006-January/017410.html