On Fri, 15 Oct 2010 09:07:22 +0100, Duncan Coutts <[email protected]> wrote: > On Thu, 2010-10-14 at 17:45 +0000, Johannes Waldmann wrote: > > Hi. I wonder how to do the following properly. > > > > I have one (large) C type, let's call it T, > > and I want to sell it as an abstract type in Haskell. > > > > I want to use C functions as if they were of type T -> T > > (pure function, returns a modified copy of the input) > > and the question is, how to do the memory allocation for that, > > in particular, how to avoid IO showing up > > in the (visible) types on the Haskell side: > > > > I don't want IO because I don't want to declare some artificial > > order of execution - instead I want lazy evaluation. > > E.g., I might have some Haskell record with a T component > > which may or may not be evaluated (accessed) at all. > > It is exactly for this purpose that the Haskell FFI library includes > unsafePerformIO. This is basically *the* legitimate use case for it, so > you don't need to feel bad about it.
I still feel bad about it. Its so easy to turn unsafePerformIO into unsafeCoerce, that I can well happen by mistake. I would like to have an unsafePerformIO that is only unsafe w.r.t. performing effects, not breaking the type-system. Here is a suggestion, it may be not new but I never seen it on unsafePerformIO: unsafePerformIO :: Typeable a => IO a -> a unsafePerformIO = ... same code ... Provided that Typeable instance are all generated by the compiler this has the desired effect of preventing generalization of mutable data. Best regards, -- Nicolas Pouillard http://nicolaspouillard.fr _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
