# Library/ArrayRef

### From HaskellWiki

Arrays&References library supports Hugs2003, Hugs2005 and GHC. It includes the following features:

## Contents |

## 1 Unboxed references

This substitutes the numerous "fast mutable Ints", "fast mutable Bools", "fast mutable Ptrs" ghc-specific modules that are used in almost any large project. In contrast to them, this library mimics well-known interface of IORef/STRef:

import Data.Ref main = do x <- newIOURef (0::Int)

writeIOURef x 1 readIOURef x >>= print

Unboxed references for IO monad has the type "IOURef a" and operations newIOURef, readIOURef, writeIOURef. Unboxed references for ST monad has the type "STURef s a" and operations newSTURef, readSTURef, writeSTURef.

Unboxed references can contain only values of following types: Bool, Char, Int, Int8..Int64, Word, Word8..Word64, Float, Double, Ptr a, FunPtr a, StablePtr a. These types are members of Unboxed class and you can implement new instances of this class by converting values of some new type (say, CChar) to values of already supported type.

Despite all these improvements, operations with unboxed references are compiled to the same code as for any "fast mutable variables". Moreover, unboxed references are available even for Hugs what allows to simplify debugging of programs that uses them. Please note that unboxed references always hold computed values, in contrast to boxed references, that can contain unevaluated thunk.

I wish to thank Simon Marlow and especially Oleg Kiselyov who supposed idea of these references and its implementation (in particular, see http://www.haskell.org/pipermail/haskell-cafe/2006-February/014324.html)

You can find examples of using unboxed references in "Examples/URef.hs"

## 2 Monad-independent references

Sometimes you need to write code that will be compatible both with IO and ST monads, and even better with any monad that has lifted from one of these two. This is especially useful for writing library code that should be as generic as possible. Operations for arrays, for example, are ready for such type of usage - readArray and writeArray can work in any monad. But it's not true for references - you need to use readIORef for IO monad, but readSTRef for ST monad, so if you need to implement monad-independent algorithm that uses references, you will be in trouble. This module solves this problem by providing monad-independent operations on boxed and unboxed references. So, the following routine:

test_Ref = do x <- newRef (0::Int)

writeRef x 1 readRef x

can be executed both in IO and ST monads:

main = do print =<< test_Ref

print $ runST test_Ref

This example uses the boxed references, unboxed references can be used in similar way with operations newURef, readURef, writeURef.

You can find examples of writing monad-independent routines in "Examples/Universal.hs". Another my library, Streams, widely use this facility to implement common functionality for streams working in different monads.

## 3 Syntax sugar for mutable types

Haskell don't support convenient syntax for using mutable vars, such as references, arrays and hash tables. The library includes module that partially simplifies their usage. For example:

main = do -- syntax sugar used for reference:

x <- ref (0::Int) x += 1 x .= (*2) val x >>= print -- syntax sugar used for array: arr <- newArray (0,9) 0 :: IO Array Int Int (arr,0) =: 1 val (arr,0) >>= print

Basically, the module supports syntax sugar for using the following data types: all types of references, arrays and hash tables. Also, it includes two operations to creating references - ref (=newRef) and uref (=newURef). Other operations include

=: assign += increase -= decrease .= apply pure function to contents .<- apply monadic computation to contents val return current value

Left part of these operations may be one of the following:

reference x += 1 (array,index) (arr,0) =: 1 (hash,key) (hash,"str") .= (*2)

You can also omit extra parentheses when indexing 2d or 3d array:

(arr,0,1) =: 1

is equivalent to

(arr,(0,1)) =: 1

Let's pay attention that this module supports arrays implementation included in the library, not standard Data.Array.* modules, Module "Examples/SyntaxSugar.hs" should contain further examples.

## 4 Reimplemented Arrays library

The library also includes modified implementation of Data.Array.* modules. The main benefit of these modifications is simplified internal library structure

Nevertheless, it also includes a few user-visible changes:

- Unboxed arrays now can be used in polymorphic functions, they are defined for every element type that belongs to classes Unboxed and HasDefaultValue (again, look at http://www.haskell.org/pipermail/haskell-cafe/2004-July/006400.html). You can add new instances to these classes

- MArray class now supports arrays with dynamic bounds. It includes monadic operation getBounds, and if you will change your code to use this operation with mutable arrays instead of `bounds`, your code also will be ready to work with dynamic (resizable) arrays

- Support for dynamic (resizable) arrays included. Their bounds can be changed either explicitly (by `resizeDynamicArray`) or implicitly (by writing to non-existing position). Policy of automatic array expansion is selected (or disabled) on array creation. See "Examples/Array/Dynamic.hs" for further reference on using these arrays

- Unboxed arrays of Bool values occupy one byte per element (in old implementation they are used one bit per element)

- castUArray/castIOUArray/castSTUArray operations are non-monadic, require "Enum ix" and recalculates upper bound of array according to size of elements: UArray (1,2) Word32 -> UArray (1,8) Word8

- Some operations can be slower with new implementation because i'm not sure that i discovered all the clever tricks used in original lib. Please test speed and report me about any problems

In other aspects, using of new arrays are equivalent to the old ones. Just change "Array" to the "ArrayBZ" in your import statements and enjoy! :) Directory "Examples/Array" contains demonstrations of using each array type

### 4.1 Changes in MArray usage

Old Arrays library contained the following definitions:

class HasBounds a where

bounds :: Ix i => a i e -> (i,i)

class (Monad m, HasBounds a) => MArray a e m where ...

In new library, MArray class defined as:

class (Monad m) => HasMutableBounds a m where

getBounds :: Ix i => a i e -> m (i,i)

class (Monad m, HasMutableBounds a m) => MArray a e m where ...

This means that definitions like this will no more work:

arrayHead :: (MArray a e m, Ix i) => a i e -> m e arrayHead marr = case bounds marr of

(l,_) -> readArray marr l

because `bounds` operation is part of HasBounds class that is no more base class for MArray. That you can do to fix this problem? Either:

- Add HasBounds restriction to the operation type:

arrayHead :: (MArray a e m, HasBounds a, Ix i) => a i e -> m e

This way, your code will become compatible with both old and new versions of Arrays library, but it will work only with "old" mutable arrays and don't support dynamic arrays.

- Replace using of `bounds` operation with calls to `getBounds`. This way, your function will become compatible with any instance of MArray class, including dynamic arrays:

arrayHead marr = do (l,_) <- getBounds marr

readArray marr l

I should mention that despite MArray now don't based on the HasBounds class, all the old mutable array types (IOArray..StorableArray) still implements this interface. Only the new dynamic arrays don't implement it because this is impossible. So, you can use the `bounds` operation in code that works with one of "old" array constructors:

arrayHead :: IOArray i e -> IO e arrayHead marr = case bounds marr of

(l,_) -> readArray marr l

### 4.2 Using dynamic (resizable) arrays

Just to let you know - current implementation of dynamic arrays is very trivial: it just saves reference (IORef or STRef) to the mutable array. When dynamic array resized, new mutable array is allocated and contents copied. New elements are filled with the value that was supported as default if array was created with the newArray or newDynamicArray operation. If dynamic array was created with newArray_ or newDynamicArray_ operation then new elements will be left undefined.

Dynamic array can be resized explicitly by resizeDynamicArray operation:

resizeDynamicArray array (l,u)

where (l,u) are new array bounds. If the dynamic array was created by newArray or newArray_ operation, it is the only way to resize it - attempts to write beyond current bounds will raise exception:

arr <- newArray (0,-1) 99 :: IO (DynamicIOArray Int Int) resizeDynamicArray arr (0,0) writeArray arr 1 1 -- this operation raises error

To create array that will be automatically resized on attempt to write
beyond current bounds, you should use newDynamicArray or
newDynamicArray_ operation (former initialize array to given initial
value while later leave array uninitialized). Their first argument
determine array expansion policy:

arr <- newDynamicArray_ growTwoTimes (0,-1) :: IO (DynamicIOArray Int Int)

This array will grow at least two times each time when automatic expansion occurs, what is determined by using the `growTwoTimes` parameter. This parameter is just the ordinary function what has the following type:

type GrowBoundsF i = (i,i) -> i -> (i,i)

This function accepts old array bounds and offending index and returns new array bounds. You can write new functions for expansion policy himself, or use one of premastered ones:

growTwoTimes - expand array at least two times growMinimally - minimal growth that ensures inclusion of new index noGrow - disable automatic growth. This policy used for arrays created by newArray or newArray_

Please note that not every array can work with any expansion policy and it is why i supported freedom of selection this policy. Only noGrow policy is compatible with every index type. growMinimally policy by it's type is compatible with any index, but it will not work for partially ordered indexes, in particular for multi-dimensional arrays. Imagine, for example, array with bounds (0,0)..(9,9). When you will try to write to index (15,5), this expansion policy function will be unable to determine what new bounds should be (0,0)..(15,9). So you anyway should provide custom expansion policy function for partially ordered indexes. At last, growTwoTimes policy is compatible only with indexes belonging to class Num, but it is most useful policy from all supported because it ensures that the program will not spend all it's time expanding the array. On the other side, you can provide your own policy function that will, for example, expand array only 1.5 times.

Dynamic array supports the same MArray and HasMutableBounds interfaces as other mutable arrays, but they don't support HasBounds interface.

And now about types of dynamic arrays. These types reflect all the
types you can use for mutable arrays, and includes DynamicIOArray,
DynamicIOUArray, DynamicSTArray, DynamicSTUArray, that has just the
same parameters as corresponding arrays without "Dynamic" prefix.
Some examples are:

DynamicIOArray Int Double DynamicSTUArray s (Int,Int) Bool

You can also create dynamic arrays from other mutable array types working in IO monad:

DynamicIO StorableArray Int Double

or ST monad:

DynamicST s (STUArray s) (Int,Int) Bool

or any other monad (ask me if you need this). Btw, implementation of dynamic arrays use the monad-independent references class mentioned above.