[Haskell-cafe] Fwd: ANN: vector-buffer package 0.1

[Alexander McPhail]

Hi list,

Could someone explain why the error pointed out by Luke occurred?

From: Luke Palmer <lrpalmer at gmail.com>

I think this would be a very good question for the list.  Don't worry,
they're nice helpful folks.

On Mon, Feb 14, 2011 at 10:10 PM, Alexander McPhail
<haskell.vivian.mcphail at gmail.com> wrote:
> Hi Roman,
>
>
> Can you explain why the following code doesn't work?  'unsafePerformIO' is
> used all the time in hmatrix.  For example, adding two vectors, we create
a
> new Vector then use that as the target for an FFI C call.
>
> Is the difference because I'm using mutable Storable Vectors?  I would
have
> thought that unsafeFreeze (at the end) would make sure the problem
reported
> by Luke wouldn't occur.
>
> Is there a problem with laziness in the let binding of Luke's example?
>
> I note that in Data.Vector.Storable there is a pure 'convert' function,
> which is essentially what I am trying to emulate.
>
> -- | convert to a vector
> toVector :: Storable a => Buffer a -> (V.Vector a)
> toVector (B o n v) = unsafePerformIO $ do
>    w <- M.new n
>    i <- readIORef o
>    M.unsafeWith v $ \p ->
>        M.unsafeWith w $ \q -> do
>          let n' = n-i
>          copyArray q (p `advancePtr` i) n'
>          if i /= 0
>             then copyArray (q `advancePtr` n') p i
>             else return ()
>    V.unsafeFreeze w
> {-# INLINE toVector #-}
>
>
> Vivian
>
> From: Luke Palmer <lrpalmer at gmail.com>
>
> This interface is an outlaw.
>
> main = do
>    buf <- newBuffer 10 :: IO (Buffer Int)
>    pushNextElement buf 1
>    let v1 = V.toList (toVector buf)
>        v2 = V.toList (toVector buf)
>    print v1
>    pushNextElement buf 2
>    print v2
>
> Despite v1 and v2 being defined to equal the exact same thing, this
> program prints two distinct lines.
>
> toVector depends on the current state of the buffer.  If this is to be
> a law-abiding interface, toVector must return a value in IO:
>
>    toVector :: (Storable a) => Buffer a -> IO (Vector a)
>
> Luke
>
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