Ross Paterson <[EMAIL PROTECTED]> wrote,
> On Fri, May 23, 2003 at 07:33:05AM +1000, Manuel M T Chakravarty wrote:
> > Dear Haskell Folks,
> >
> > Release Candidate 10 of the H98 FFI Addendum 1.0 is now
> > available from
> >
> > http://www.cse.unsw.edu.au/~chak/haskell/ffi/
>
> I have an ideological objection. I think that the inclusion of
> unsafePerformIO in an Addendum sends entirely the wrong signal. I know
> it's needed for marshalling for otherwise pure functions that pass their
> data through pointers. Very well, but the inclusion of unsafePerformIO
> allows many more uses. At a stroke it removes many of the trickiest
> design problems of Haskell, and we can't have that.
>
> I propose that the Addendum say that it permits unsafePerformIO for that
> purpose only, i.e. the IO calls it contains are restricted to foreign
> calls and functions from Storable and Marshal*, these may only access
> Ptr's inaccessable outside the unsafePerformIO, and no other visible
> side effects are permitted.
Well, the text already says,
\item[unsafePerformIO ::\ IO a -> a] Execute an \code{IO} action in place of a
pure computations. For the behaviour to be predictable, the IO computation
should be free of side effects and independent of its environment.
If the \code{IO} computation wrapped in \code{unsafePerformIO} performs side
effects, then the relative order in which those side effects take place
(relative to the main \code{IO} trunk, or other calls to
\code{unsafePerformIO}) is indeterminate.
Great care should be exercised in the use of this function. Not only
because of the danger of introducing side effects, but also because
\code{unsafePerformIO} may compromise typing, for example, when it is used
in conjunction with polymorphic references.
I think, the warning sign is clear. Especially in the
context of the FFI, anything more is a waste of paper IMHO.
After all, you can import any C function with a pure type,
which also allows you to wreck arbitrary havoc. We enable
the user to disguise arbitrary machine code as a Haskell
function of essentially arbitrary type. In comparison,
`unsafePerformIO' seems angelic.
Cheers,
Manuel
_______________________________________________
Haskell mailing list
[EMAIL PROTECTED]
http://www.haskell.org/mailman/listinfo/haskell
