[Haskell-cafe] Re: Question about touchForeignPtr

[Simon Marlow]

Patrick Perry wrote:
> 
> I have the following code:
> 
> IOVector n e = IOVector !ConjEnum !Int (ForeignPtr e)! (Ptr e)! Int!
> newtype Vector n e = IOVector n e
> 
> unsafeAtVector :: Vector n e -> Int -> e
> unsafeAtVector (Vector (IOVector c _ f p inc)) i =
>     let g = if c == Conj then conjugate else id
>     in inlinePerformIO $ do
>            e  <- peekElemOff p (i*inc)
>            io <- touchForeignPtr f
>            let e' = g e
>            e' `seq` io `seq` return e'
> {-# INLINE unsafeAtVector #-}
> 
> 
> The Ptr, 'p' is derived from the ForeignPtr, 'f'. For some offset, 'o', it
> is defined as:
> 
> p = unsafeForeignPtrToPtr f `advancePtr` o
> 
> The "touchForeignPtr" is there to keep the garbage collector from 
> deallocating
> the memory before we have a chance to read 'e'.  My question is the 
> following:
> Is the `seq` on `io` necessary (from a safety standpoint)?  Or am I just 
> being paranoid?

You're just being paranoid - touchForeignPtr returns a (), so seqing it is 
a no-op.

Cheers,
	Simon