# DArrays - Haskell Support for Array Computations [OUT OF DATE]

This page has been superseded by the Repa paper in ICFP'10: http://www.cse.unsw.edu.au/~chak/papers/KCLPL10.html

The library provides a layer on top of DPH unlifted arrays to support multi-dimensional arrays, and shape polymorphic operations for fast sequential and parallel execution. The interface for delayed arrays is similar, but in contrast to operations on the former, any operation on a delayed array is only actually evaluated when elements are accessed outside the DArray framework.

The current implementation of the library exposes some implementation details the user shouldn't have to worry about. Once the design of the library is finalised, most of these will be hidden by distinguishing between internal types and representation types.

## Strict Arrays, Delayed Array and Shape Data Type

Both strict and delayed arrays are parametrised with their shape - that is, their dimensionality and size of each dimension. The shape type class has the following interface:

### DArrays

DArrays are collections of values of `primitive' type, which are member of the class Data.Parallel.Array.Unlifted.Elt, which includes all basic types like Float, Double, Bool, Char, Integer, and pairs constructed with Data.Parallel.Array.Unlifted.(:*:), including ().

### The Shape class

Values of class Shape serve two purposes: they can describe the dimensionality and size of an array (in which case we refer to them as 'shape'), and they can also refer to the position of a particular element in the array (in which case we refer to them as an 'index'). It provides the following methods:

class (Show sh, U.Elt sh, Eq sh, Rebox sh) => Shape sh where dim :: sh -> Int -- ^number of dimensions (>= 0) size :: sh -> Int -- ^for a *shape* yield the total number of elements in that array toIndex :: sh -> sh -> Int -- ^corresponding index into a linear representation of -- the array (first argument is the shape) fromIndex:: sh -> Int -> sh -- ^given index into linear row major representation, calculates -- index into array range :: sh -> U.Array sh -- ^all the valid indices in a shape. The following equality should hold: -- map (toIndex sh) (range sh) = [:0..(size sh)-1:] inRange :: sh -> sh -> Bool -- ^Checks if a given index is in the range of an array shape. I.e., -- inRange sh ind == elem ind (range sh) zeroDim :: sh -- ^shape of an array of size zero of the particular dimensionality intersectDim :: sh -> sh -> sh -- ^shape of an array of size zero of the particular dimensionality next:: sh -> sh -> Maybe sh -- ^shape of an array of size zero of the particular dimensionality

Note that a `Shape`

has to be in the type class `Elt`

imported from `Data.Parallel.Array.Unboxed`

so
that it can be an element of `Data.Parallel.Array.Unboxed.Array`

.

The following instances are defined

instance Shape () instance Shape sh => Shape (sh :*: Int)

so we have inductively defined n-tuples of `Int`

values to represent shapes. This somewhat unusual representation
is necessary to be able to inductively define operations on `Shape`

. It should, however, be hidden from the library
user in favour of the common tuple representation.

The multiparameter type class `Subshape sh sh'`

contains all pairs of shapes `sh`

and `sh'`

, for which the dimensionality of `sh'`

is
less than that of `sh`

.

class (Shape sh, Shape sh') => Subshape sh sh' where addDim :: sh -> sh' -> sh modDim :: sh -> sh' -> sh inject :: sh -> sh' -> sh

The method `addDim`

adds the sizes of two shapes (or positions of two indices). If `sh'`

is a strict subshape of
`sh`

, the fields of `sh`

are copied when no corresponding fields of `sh'`

exist, accordingly for `modDim`

### Representation, Order of Elements, and Lifted Values

As mentioned when introducing the functions `toIndex`

and `range`

, the following relationship should hold:

map (toIndex sh) (range sh) = [:0..(size sh)-1:]

this means that, for example

range (() :*: 2 :*: 3) = [() :*: 0 :*: 0, () :*: 0 :*: 1, ....., () :*: 0 :*: 2, () :*: 1 :*: 0,....

## Operations on Arrays and Delayed Arrays

### Array Creation and Conversion

Strict arrays are simply defined as record containing a flat data array and shape information:

data Array dim e where Array:: { arrayShape :: dim -- ^extend of dimensions , arrayData :: U.Array e -- flat parallel array } -> Array dim e deriving Show toArray :: (U.Elt e, Shape dim) => dim -> U.Array e -> Array dim e fromArray:: (U.Elt e, Shape dim) => Array dim e -> U.Array e

Delayed arrays, in contrast, in addition to the shape, only contain a function which, given an index, yields the corresponding element.

data DArray dim e where DArray :: {dArrayShape::dim -> dArrayFn:: (dim -> e)} -> DArray dim e

Delayed arrays can be converted to and from strict arrays:

toDArray:: (U.Elt e, Array.Shape dim) => Array.Array dim e -> DArray dim e fromDArray:: (U.Elt e, Array.Shape dim) => DArray dim e -> Array dim e

the result of `toDArray`

is a DArray which contains an indexing function into
an array. In general, the function `dArrayFn`

can be much more complex. The function
`forceDArray`

(should this be called `normalizeDArray`

?) forces the evaluation `dArrayFn`

on
every index of the range, and replaces `dArrayFn`

by a simple indexing function into an array
of the result values.

forceDArray:: (U.Elt e, A.Shape dim) => DArray dim e -> DArray dim e

Singular (zero-dimensional) arrays are isomorphic to scalar values and can be converted to one using the following function:

toScalar:: U.Elt e => DArray () e -> e

Note that in contrast to all the previous operations, `toScalar`

requires the array to be of a particular
dimensionality.

## Collection Oriented Operations

### Elementwise Application of Functions

The `map`

operation takes a function over element types and applies it to every
data element of the DArray, which can have arbitrary dimensionality.

map:: (U.Elt a, U.Elt b, A.Shape dim) => (a -> b) -> DArray dim a -> DArray dim b

Similarily, `zip`

and `zipWith`

apply to every data element in the array as well. Both arguments
have to have the same dimensionality (which is enforced by the type system). If they have a different
shape, the result will have the intersection of both shapes. For example, zipping an array of shape
`(() :*: 4 :*: 6)`

and `(() :*: 2 :*: 8)`

results in an array of shape `(() :*: 2 :*: 6)`

.

zipWith:: (U.Elt a, U.Elt b, U.Elt c, A.Shape dim) => (a -> b -> c) -> DArray dim a -> DArray dim b-> DArray dim c zip:: (U.Elt a, U.Elt b, A.Shape dim) => DArray dim a -> DArray dim b-> DArray dim (a :*: b)

The function `fold`

collapses the values of the innermost rows of an array of at least dimensionality 1.

fold :: (U.Elt e, A.Shape dim) => (e -> e-> e) -> e -> DArray (dim :*: Int) e -> DArray dim e

Again, it's not possible to use `fold`

directly to collapse an array along any other axis, but, as
we will see shortly, this can be easily done using other functions in combination with `fold`

.

Related to `fold`

, we have `scan`

:

scan :: (U.Elt e, A.Shape dim) => (e -> e-> e) -> e -> DArray dim e -> Array dim e

Note that `scan`

returns a value of type `Array`

, not `DArray`

: this means that, if we apply scan to an array and access one of its elements, the whole array will be created.

### Support for Parallel Execution

Since the implementation of DArrays builds on the DPH library, all the array operations can be executed in parallel. That is, we compiling a DArray program, the compiler generates a sequential as well as a parallel executable. All collective operations, like `map`

, `fold`

, and so on are executed in parallel.

### Shape Polymorphic Computations on Arrays

The library provides a range of operation where the dimensionality of the result depends on the dimensionality of the argument in a non-trivial manner, which we want to be reflected in the type system. Examples of such functions are generalised selection, which allows for extraction of subarrays of arbitrary dimension, and generalised replicate, which allows replication of an array in any dimension (or dimensions). For example, given a three dimensional matrix, we can use select to extract scalar element values, rows, columns, as well as two dimensional matrices along any of the three axes.

For selection, we can informally state the relationship between dimensionality of the argument, the selector, and the result as follows:

select:: Array dim e -> <select dim' of dim array> -> Array dim' e

Another example for such a generalised function would be a generalised
`map`

, which can apply a function to all elements, all rows, all
columns, or submatrices of different orientation of a multidimensional
array.

For the former example, we need a way to express the relationship between the
shape of the argument and the shape and orientation of the result, as well as
the numerical position of the structure (i.e., first, second, third element).
In case of the generalised `map`

, we don't need the numerical information, since
the operation will be applied to all elements, rows, columns etc.

To express this dependency between input and output shape and orientation,
as well as possibly a concrete position, the library provides the `Index`

GADT,
which expresses a relationship between the source and the projected dimension.
It is defined as follows:

data Index a initialDim projectedDim where IndexNil :: Index a initialDim initialDim IndexAll :: (Shape init, Shape proj) => Index a init proj -> Index a (init :*: Int) (proj :*: Int) IndexFixed :: (Shape init, Shape proj) => a -> Index a init proj -> Index a (init :*: Int) proj

To refer to a specific element, the type parameter `a`

is instantiated with the type `Int`

, otherwise
with the unit type:

type SelectIndex = Index Int type MapIndex = Index ()

Given this definition, the type of `select`

now becomes

select:: (U.Elt e, Shape dim, Shape dim') => Array dim e -> SelectIndex dim dim' -> Array dim' e

Even though the index type is well suited to express the relationship between the selector/multiplicator and the dimensionality of the argument and the result array, it is inconvenient to use, as the examples demonstrate. We therefore need some syntactic sugar to improve the usability of the library. In the following, the use a SAC-like notation for values of Index-type in comments to improve the readability of the examples.

Example:

arr:: Array (() :*: Int :*: Int :*: Int) Double arr' :: () :*: Int :*: Int arr' = select arr (IndexFixed 3 (IndexAll (IndexAll IndexNil))) -- (3,.,.)

We could generalise this further, to extract from any array `arr`

which is at least one dimensional
the third element:

arr:: Shape dim => Array (dim :*: Int) Double arr' :: Array dim Double arr' = select arr (IndexFixed 3 IndexNil) -- (3,*)

The index type is also used to express the type of generalised replicate

replicate:: Array dim' e -> SelectIndex dim dim' -> Array dim e

which, given an array, can be used to expand it along any dimension. For example,

simpleReplicate:: (U.Elt e, Shape dim) => Array dim e -> Int -> Array (dim :*: Int) e simpleReplicate arr n = replicate arr (IndexFixed n IndexNil) -- (*,n)

replicates the argument array (which can of any dimensionality) `n`

times and behaves
thus similarly to list replicate, whereas

elementwiseReplicate:: (U.Elt e, Shape dim) => Array (dim :*: Int) e -> Int -> Array (dim :*: Int :*: Int) e elementwiseReplicate arr n = replicate arr (IndexAll (IndexFixed n IndexNil)) -- (*,n,.)

replicates each element of an array `n`

times (similarly to `map (replicate n)`

on lists).

Note that the library provides no way to statically check the pre- and postconditions on the actual size of arguments. This has to be done during run time using assertions.

## `Nesting' Array Functions

We already introduced the `map`

function, which applies a given function to all data elements
of an array:

map:: (U.Elt a, U.Elt b, A.Shape dim) => (a -> b) -> DArray dim a -> DArray dim b

We can't use this function, however, to apply a function to all columns, rows, or other sub-arrays of
a multidimensional array, and generalising `map`

to be able to handle this wouldn't make sense
in this framework. Consider, for example, a function `filter`

, which takes a one-dimensional
array and creates a new array containing only the even elements of the argument array. If we mapped
this function over all the rows of a two-dimensional array, the resulting structure would, in general,
not be a two dimensional array anymore, since each row might potentially have a different length.
Therefore, we restrict the class of functions that can be mapped over sub-arrays to functions where
the shape of the argument determines the shape of the result. All `mappable`

(for the lack of a better term)
functions can be implemented such that they abstract over the exact dimensionality of their argument, and have the type

f::(A.Shape dim, U.Elt e, U.Elt e') => DArray (dim :*: Int ..... :*: Int) e -> DArray (dim :*: Int :*: .... :*: Int) e'

and those functions can be trivially mapped since

map f = f

The function `fold`

, which we introduced earlier, is an example of a mappable library function. Applied to a matrix, `fold (+) 0`

will calculate the sum of all rows. If run in parallel, `fold`

itself is run in parallel, and all the rows are processed in parallel.

fold :: (U.Elt e, A.Shape dim) => (e -> e-> e) -> e -> DArray (dim :*: Int) e -> DArray dim e

So, for example, we can write a mappable function which takes an array and selects every data element with an even index:

selectEvenInd:: (A.Shape dim, U.Elt e) => DArray (dim :*: Int) e -> DArray (dim :*: Int) e selectEvenInd (DArray (sh :*: n ) f = DArray (sh :*: n `div` 2) (\(sh :*: n) -> f (sh :*: 2*n)

In this case `dim`

could simply be unit, if and `selectEven`

extracts all elements with an even index, or it could
be any other shape, and thus

map selectEvenInd = selectEvenInd

where `selectEvenInd`

on the left and right-hand side of the equation are two different instances of the function. Now, lets
try and write function `selectEvenElems`

, which selects all even elements from an array. To determine the shape of the
result, it is not sufficient to look at the shape of the argument. Instead, we have to calculate the new size by counting the
even elements in the array using `fold`

. The function `fold`

is mappable, and returns an array. If the argument is a one-dimensional
array, the result is a singular array, which then can be converted to a scalar using `toScalar`

. The necessary application of `toScalar`

,
however, also restricts `sh`

, which can now only be unit, and therefore the whole operation `selectEvenElems`

is restricted to one-dimensional
arrays, and not mappable.

selectEvenElems (DArray (sh :*: n) f) = DArray (sh :*: newSize) <......> where newSize = toScalar $ fold (+) 0 $ map (\x -> if (even x) then 1 else 0) arr

## Example 1: Matrix multiplication

As a simple example, consider matrix-matrix multiplication. We can either implement it by directly manipulating the array function, or use the operations provided by the DArray library. Let as start with the former, which is more fairly similar to what we would write using loops over array indices:

mmMult1:: DArray (() :*: Int :*: Int) Double -> DArray (() :*: Int :*: Int) Double -> DArray (() :*: Int :*: Int) Double mmMult1 arr1@(DArray (() :*: m1 :*: n1) _) arr2@(DArray (() :*: m2 :*: n2) _) = fold (+) 0 arrDP where arrDP = DArray (():*: m1 :*: n2 :*:n1) (\(() :*: i :*: j :*: k) -> (index arr1 (() :*: i :*: k)) * (index arr2 (() :*: k :*: j)))

In the first step, we create the intermediate three dimensional array which contains the products of all
sums and rows, and in the second step, we collapse each of the rows to it's sum, to obtain the two dimensional
result matrix. It is important to note that the elements of `arrDP`

are never all in memory (otherwise, the memory
consumption would be cubic), but each value is consumed immediately by `mapfold`

.

This implementation suffers from the same problem a corresponding C implementation would - since we access one
array row-major, the other column major, the locality is poor. Therefore, first transposing `arr2`

and adjusting the
access will actually improve the performance significantly:

mmMult1:: DArray (() :*: Int :*: Int) Double -> DArray (() :*: Int :*: Int) Double -> DArray (() :*: Int :*: Int) Double mmMult1 arr1@(DArray (() :*: m1 :*: n1) _) arr2@(DArray (() :*: m2 :*: n2) _) = fold (+) 0 arrDP where arr2T = forceDArray $ transpose arr2 arrDP = DArray (():*: m1 :*: n2 :*:n1) (\(() :*: i :*: j :*: k) -> (index arr1 (() :*: i :*: k)) * (index arr2T (() :*: j:*: k))) transpose:: DArray (() :*: Int :*: Int) Double -> DArray (() :*: Int :*: Int) Double transpose (DArray (() :*: m :*: n) f) = DArray (() :*: n :*: m) (\(() :*: i :*: j) -> f (() :*: j :*: i))

However, we do need to force the actual creation of the transposed array, otherwise, the change would have no effect at all. We therefore
use `forceDArray`

, which converts it into an array whose array function is a simple indexing operation (see description of `forceDArray`

above). This means that the second version requires more memory, but this is offset by improving the locality for each of the multiplications.

As it is, `mmMult`

can only take two-dimensional arrays as arguments, and is not mappable. If we look at the implementation closely, we can see that the restriction to two-dimensional arrays is unnecessary. All we have to do to generalise it is to adjust the type signatures and replace `()`

with an arbitrary shape variable:

mmMult1:: Shape dim => DArray (dim :*: Int :*: Int) Double-> DArray (dim :*: Int :*: Int) Double-> DArray (dim :*: Int :*: Int) Double mmMult1 arr1@(DArray (sh :*: m1 :*: n1) _) arr2@(DArray (sh' :*: m2 :*: n2) _) = fold (+) 0 arrDP where arr2T = forceDArray $ transpose arr2 arrDP = DArray (sh:*: m1 :*: n2 :*:n1) (\(sh :*: i :*: j :*: k) -> (index arr1 (sh :*: i :*: k)) * (index arr2T (sh :*: j:*: k))) transpose:: Shape dim => DArray (dim :*: Int :*: Int) Double -> DArray (dim :*: Int :*: Int) Double transpose (DArray (sh:*: m :*: n) f) = DArray (sh :*: n :*: m) (\(sh :*: i :*: j) -> f (sh :*: j :*: i))

An alternative way to define matrix-matrix multiplication is in terms of the collective library functions provided. First, we
expand both arrays and, in case of `arr2`

transpose it such that the elements which have to be multiplied match up. Then,
we calculate the products using `zipWith`

, and then use `fold`

to compute the sums:

mmMult2:: (Array.RepFun dim, Array.InitShape dim, Array.Shape dim) => DArray (dim :*: Int :*: Int) Double -> DArray (dim :*: Int :*: Int) Double -> DArray (dim :*: Int :*: Int) Double mmMult2 arr1@(DArray (sh :*: m1 :*: n1) fn1) arr2@(DArray (sh' :*: m2 :*: n2) fn2) = fold (+) 0 (arr1Ext * arr2Ext) where arr2T = forceDArray $ transpose arr2 arr1Ext = replicate arr1 (Array.IndexAll (Array.IndexFixed m2 (Array.IndexAll Array.IndexNil))) -- (*,.,m2,.) arr2Ext = replicate arr2T (Array.IndexAll (Array.IndexAll (Array.IndexFixed n1 Array.IndexNil))) -- (*,n1,.,.)

In this implementation, `transpose`

is necessary to place the elements at the right position for `zipWith`

, and we call `forceDArray`

for
the same reason as in the previous implementation, to improve locality. Also, `mmMult2' outperforms `

mmMult1`, as the use of `

replicate`
exposes the structure of the communication, whereas the general index calculations in `mmMult1`

hide this structure, and thus are less efficient.

### Performance of Matrix-Matrix Multiplication

The following table contains the running times in milliseconds of `mmMult1`

and `mmMult2', applied to two matrices of with `

size * size` elements. As mentioned before, `

mmMult2` is faster than `

mmMult1`, as `

replicate` can be implemented more efficiently than the general permutation which is the result of the element-wise index computation in `

mmMult1`. This is the case for most problems: if it is possible to use collection oriented operations, than it will lead to more efficient code. We can also see that using `

forceDArray` for improved locality has a big impact on performance (we have O (size*size*size) memory accesses, and creating the transposed matrix has only a memory overhead of O(size*size)). `

mmMult1` without the
transposed matrix is about as fast as `mmMult2`

without `forceDArray`

(times omitted). We can also see that the speedup on two processors is close to the optimal speedup of 2.

To get an idea about the absolute performance of DArrays, we compared it to two C implementations. The first (handwritten) is a straight forward C implementation with three nested loops, iterations re-arranged to get better performance, which has a similar effect on the performance than the `forceDArray`

/`transpose`

step. The second implementation uses the matrix-matrix multiplication operation provided by MacOS accelerate library. We can see that, for reasonably large arrays, DArrays is about a factor of 3 slower than the C implementation if run sequentially.

---------------------------------------------------------------------- size | 256 | 512 | 1024 | ---------------------------------------------------------------------- mmMult1 | 675 | 5323 | 42674 | ---------------------------------------------------------------------- mmMult2 | 345 | 2683 | 21442 | ---------------------------------------------------------------------- mmMult2 (2PE) | 190 | 1463 | 11992 | ---------------------------------------------------------------------- mmMult2 (without forceDArray) | 974 | 8376 | 73368 | ---------------------------------------------------------------------- mmMult2 (without forceDArray, 2PE) | 508 | 4368 | 37677 | ---------------------------------------------------------------------- C (hand written) | 34 | 514 | 7143 | ---------------------------------------------------------------------- C (MacOS Accelerated Vector ops) | 33 | 510 | 6949 | ----------------------------------------------------------------------

## Example 2: Red-Black Relaxation

(example taken from SAC web page)

redBlack:: Array.Shape dim => Double -> Double -> DArray (dim :*: Int :*: Int :*: Int) Double -> DArray (dim :*: Int :*: Int :*: Int) Double -> DArray (dim :*: Int :*: Int :*: Int) Double redBlack factor hsq f arr@(DArray (d :*: l :*: n :*:m) fn) = applyFactor $ insert odd arr' $ sumD $ getBlack $ stencil arr' where arr' = applyFactor $ insert even arr $ sumD $ getRed $ stencil arr applyFactor = zipWith (\fi -> \si -> factor * (hsq * fi + si)) f sumD arr = fold (+) 0 arr getRed (DArray (sh :*: l :*: m :*: n :*: c) f ) = DArray (sh :*: l-2 :*: m-2 :*: (n-1)`div` 2 :*: c) (\(sh :*: h :*: i :*: j :*: c) -> f(sh :*: h+1 :*: i+1 :*: 2*j+1 :*: c)) getBlack (DArray (sh :*: l :*: m :*: n :*: c) f) = DArray (sh :*: l-2 :*: m-2 :*: (n-2) `div` 2:*: c) (\(sh :*: h :*: i :*: j:*:c) -> f (sh :*: h+1 :*: i+1 :*: 2*j+2:*:c)) isBorder (d :*: h :*: i :*: j) = ((h * i * j) == 0) || (h >= (l-1)) || (i >= (m-1)) || (j >= (n-1)) insert p (DArray sh f) (DArray sh' f') = DArray sh (\d@(sh :*: h :*: i :*: j) -> if ((isBorder d) || p j) then f d else (f' (sh :*: h-1 :*: i-1 :*: (j-1)`div` 2))) stencil (DArray sh f) = DArray (sh :*: 6) f' where f' (sh :*: n :*: m :*: 0) = f (sh :*: n :*: m+1) f' (sh :*: n :*: m :*: 1) = f (sh :*: n :*: m-1) f' (sh :*: n :*: m :*: 2) = f (sh :*: n+1 :*: m) f' (sh :*: n :*: m :*: 3) = f (sh :*: n-1 :*: m) f' (sh :*: k :*: n :*: m :*: 4) = f (sh :*: k+1 :*: n :*: m) f' (sh :*: k :*: n :*: m :*: 5) = f (sh :*: k-1 :*: n :*: m)

## Example 3: 3D Fast Fourier Transformation

Applied FFT to each vector in a three dimensional matrix, once along each of the three axes, iterate a given number of times (example taken from SAC web page)

fft3d:: Int -> DArray Array.DIM3 Complex -> DArray Array.DIM3 Complex -> DArray Array.DIM3 Complex fft3d it rofu m | it < 1 = m | otherwise = fft3d (it-1) rofu $ fftTrans $ fftTrans $ fftTrans m where fftTrans = forceDArray . (fft rofu) . transpose' transpose' darr@(DArray (() :*: k :*: l :*: m) _) = backpermute darr (() :*: m :*: k :*: l) (\(() :*: m' :*: k' :*: l') -> (() :*: k' :*: l' :*: m')) fft:: Array.Subshape dim dim => DArray (dim :*: Int) Complex -> DArray (dim :*: Int) Complex -> DArray (dim :*: Int) Complex fft rofu@(DArray ( _ :*: s) _ ) v@(DArray sh@(_ :*: n) f) | n > 2 = assert (2 * s == n) $ append (fft_left + fft_right) (fft_left - fft_right) sh | n == 2 = assert (2 * s == n) $ DArray sh f' where f' (sh :*: 0) = f (sh :*: 0) + f (sh :*: 1) f' (sh :*: 1) = f (sh :*: 0) - f (sh :*: 1) f' (sh :*: x) = error ("error in fft - f:" ++ (show x) ++ "/" ++ (show sh)) rofu' = split rofu (\(sh :*: i) -> (sh :*: 2*i)) fft_left = forceDArray $ rofu * (fft rofu' (split v (\(sh:*: i) -> (sh :*: 2*i)))) fft_right = forceDArray $ fft rofu' (split v (\(sh:*: i) -> (sh :*: 2*i+1))) split:: Array.Shape dim => DArray (dim :*: Int) Complex -> ((dim :*: Int) -> (dim :*: Int)) -> DArray (dim :*: Int) Complex split arr@(DArray (sh :*: n) fn) sel = (DArray (sh :*: (n `div` 2)) (fn . sel))