Hi list,
Could someone explain why the error pointed out by Luke occurred?
From: Luke Palmer
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
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