closing file handles

[Simon Marlow]

> Tom Pledger wrote:
> > [...]
> >     go' f
> >         =3D do h    <- openFile f ReadMode
> >              text <- hGetContents h
> >              case text of
> >                  (c:_) -> do hClose h
> >                              return c
> >
> > This way, the pattern match on `text' happens before the innermost
> > `do', which in turn closes the handle before letting the rest of the
> > computation continue.
>=20
> I think the above code will lead to problems if file input is=20
> done when c is evaluated and not earlier.

Fortunately, in GHC this isn't the case.  Evaluating the (:) is enough
to force the character to be read.  But to be on the safe side you could
also write it like this:

    go' f
        =3D do h    <- openFile f ReadMode
             text <- hGetContents h
		 let c =3D head text
		 c `seq` do { hClose h; return c }

Cheers,
	Simon