Repa

• Library for parallel array computations on multi-core machines.

• For shared memory.

• Multidimensional unboxed (and boxed) arrays.

Using Repa

The basic array type in Repa is given by

data Array rep sh e

> {-# LANGUAGE TypeOperators #-}
> import Data.Array.Repa as R
> import Data.Array.Repa.Index as I
> import qualified Data.Array.Repa.Repr.Vector as V
> import Data.Array.Repa.Repr.Unboxed
> import Data.Array.Repa.Eval

The type tag rep is a type hint for the array type that is present. The two most important types are U for unboxed data and D for delayed data, i.e. data that has to be calculated (V for boxed values).

We can create a Shape sh with the constructor Z

> shape' :: (Z :. Int) :. Int
> shape' = Z :. 2 :. 3

We can also abbreviate this with

> shape :: DIM2
> shape = ix2 2 3

Using Repa

We create an array from a list with

> m :: Array U DIM2 Int
> m = fromListUnboxed shape [0..5]

The Shape typeclass is not only used to determine the bounds of the arrays but also to access the elements.

• Elements are indexed from 0 :-)
• Elements are in row major order :-/

Element access can be performed with the (!) operator.

m ! ((Z :. 0) :. 0)

Using Repa

The extent of the matrix can be obtained with

> tmp   = extent m

The number of dimensions with

> tmp'  = rank tmp

And the size (number of elements) with

> tmp'' = size tmp

Conversion from and to unboxed Vectors with toUnboxed and fromUnboxed.

Using Repa

> m' :: Array D DIM2 Int
> m' = R.zipWith (+) (R.map (^2) m) m

m' cannot be shown as it is a delayed array. We can turn it into a concrete array using computeS.

> m'' :: Array U DIM2 Int
> m'' = computeS m'

Commands to create a represented array from a calculation

computeS :: (Load r1 sh e, Target r2 e) => Array r1 sh e -> Array r2 sh e
computeP :: (Load r1 sh e, Target r2 e, Source r2 e, Monad m) => Array r1 sh e -> m (Array r2 sh e)

Slices

slice m (Any :. (1::Int) :. All)
slice m (Any :. All :. (1::Int))

slice m (Any :. (1::Int))

is equivalent to

slice m (Any :. All :. (1::Int))

Any cannot be omitted.

Extending

A matrix can be extended along multiple dimensions:

extend (Any :. (4::Int) :. All :. All) m
extend (Any :. All :. (4::Int) :. All) m
extend (Any :. All :. All :. (4::Int)) m

Example: Matrix Matrix Multiplication

$$z_{ik} = \sum_{j} x_{ij} \, y_{jk}$$

$$z_{ik} = \sum_{j} \hat{x}_{ikj} \, \hat{y}_{ikj}$$

where $\hat{x}_{ikj} = x_{ij}$ and $\hat{y}_{ikj} = y_{jk}$

mmult :: Array U DIM2 Double -> Array U DIM2 Double -> Array U DIM2 Double
mmult a b = R.sumS $ R.zipWith (*) a' b''
  where
    a'  = extend (Any :. All :. n :. All) a
    b'  = transpose b
    b'' = extend (Any :. m :. All :. All) b'

Traverse

Generic array traversal with

traverse :: (Source r a, Shape sh', Shape sh) => 
    Array r sh a             -- input array 
 -> (sh -> sh')              -- calculation of new extent
 -> ((sh -> a) -> sh' -> b)  -- function that given a reader of the original array and
                             -- a position in the new array calculates a value
 -> Array D sh' b            -- new array :-)