Allocation & Marshalling Question (again)

[Adrian Hey]

On Thursday 29 May 2003 13:56, Malcolm Wallace wrote:
> In order to make this work with your definition of <+, you needed to
> change the types.  But perhaps that was because your definition of <+
> was not exactly what you needed.

Nope. <+ as defined is exactly what I need, the only problem with
it is it seems to be broken :-)

> How about the following alternative:
>
>     g s n f = g' <$ withCString s <+ allocaBytes n <+ withForeignPtr f
>
>     infixl 3 <$
>     a <$ b = b a
>
>     infixr 4 <+
>     f <+ g = \h-> f $ \a-> 
>                   g $ \b->
>                   h a b
>
> which preserves all the types exactly as they are in the FFI standard?

That's very neat, though I think a type sig would help users (well me:-)
figure out what to do with this definition of <+
Here's the most appropriate for our purposes I think (not most general)..
 (<+) :: ((a -> IO z)   -> IO z)
      -> ((     b -> c) -> IO z)
      -> ((a -> b -> c) -> IO z)

I don't think this is quite what I wanted, it seems hard to do the
"partial application" trick with this. But even disregarding the issues 
regarding allocation lifetime, the partial application trick seems unsafe
outside the IO monad because of the potential mutability of things that
are pointed to. So maybe that's not really a valid gripe re. your definition
of <+. 

To be honest I still find the whole idea of things that are pointed to
disappearing at the most awkward times quite unsettling. I guess for
the purposes of FFI programming I need to forget my "bad" FP habits
and start thinking like C programmer :-)

Regards
--
Adrian Hey