    {-# LANGUAGE BangPatterns #-}
    module Data.Array.Repa.Operators.Selection
            (selectP)
    where
    import Data.Array.Repa.Index
    import Data.Array.Repa.Base
    import Data.Array.Repa.Eval.Selection
    import Data.Array.Repa.Repr.Unboxed             as U
    import qualified Data.Vector.Unboxed            as V
    import System.IO.Unsafe

    -- | Produce an array by applying a predicate to a range of integers.
    --   If the predicate matches, then use the second function to generate
    --   the element.
    --
    --   * This is a low-level function helpful for writing filtering
    --     operations on arrays.
    --
    --   * Use the integer as the index into the array you're filtering.
    --
    selectP :: (Unbox a, Monad m)
            => (Int -> Bool)        -- ^ If the Int matches this predicate,
            -> (Int -> a)           -- ^  ... then pass it to this fn to produce a value
            -> Int                  -- ^ Range between 0 and this maximum.
            -> m (Array U DIM1 a)   -- ^ Array containing produced values.

    selectP match produce len
     = return
     $ unsafePerformIO
     $ do   (sh, vec)       <- selectIO
            return $ sh `seq` vec `seq`
                     fromUnboxed sh vec

     where  {-# INLINE selectIO #-}
            selectIO
             = do   vecs            <- selectChunkedP match produce len
                    vecs'           <- mapM V.unsafeFreeze vecs

                    -- TODO: avoid copy somehow.
                    let result      = V.concat vecs'

                    return  (Z :. V.length result, result)
    {-# INLINE [1] selectP #-}