Yet another weakly defined bug report

[Simon Marlow]

> The following code runs out of file handles, so I am seeking
> enlightenment as to why.
>=20
> > -- | add data from a file to the histogram
> > addFile :: FiniteMap String Int -> String -> IO (FiniteMap=20
> String Int)
> > addFile fm name =3D do
> > 		  x <- readFile name
> > 		  return (addHist fm x)
> >=20
> > -- | add data from all files in a directory to the histogram
> > addDir :: FiniteMap String Int -> String -> IO (FiniteMap=20
> String Int)
> > addDir fm dir =3D do
> > 		dc <- getDirectoryContents dir
> > 		fs <- filterM doesFileExist (map ((dir++"/")++) dc)
> > 		foldM addFile fm fs
>=20
> Running addDir on a directory with few hundred files trigs it.
> Shouldn't all of each file's contents be consumed by each addFile, and
> thus the handle be released?=20

It's not possible to tell from this code whether the readFiles will be
fully evaluated or not: it depends on how much evaluation addHist does,
and to what extend the result FiniteMap is demanded, amongst other
things.

These things are always tricky to understand, which is why I recommend
not using lazy I/O.  File reading is not a pure operation: running out
of file descriptors is a good counter-example.

Cheers,
	Simon