[Haskell-cafe] What is a Boxed Array?

[Cale Gibbard]

A "box" is a cell representing some value in a program. It generally
contains a pointer to code (a thunk), or to a proper value. When
evaluation of that box is forced for the first time, the code
executes, and when it is done, it updates the pointer with a pointer
to the result value. There are a number of slight variations on this
scheme, but essentially the idea is that there is a layer of
indirection which keeps track of whether the value has been evaluated
yet or not. This sort of thing forms a major role in the
implementation of a lazy language like Haskell.

Ordinary Arrays in Haskell, say, Array i e for some index type i, and
element type e, are arrays of boxed values. That is, each element of
the array is a pointer to either code, or a value, and each may be
evaluated separately and lazily.

By contrast, unboxed arrays, like those of type UArray i e, are arrays
of unboxed values. The layer of indirection which normally allows for
separate lazy evaluation of the elements is missing. Consequently,
such arrays consume much less space in overhead, however, there is a
penalty to be paid in ease of use. The array's box now has code which
determines the value of every element of the array as soon as it is
forced. Thus, even a single lookup in the array will cause the entire
array to be computed. If you were going to compute it anyway, you
don't lose much. What you do lose however, is the ability to define
the array elements recursively in terms of one another.

The following definition works for boxed array types, but not unboxed ones:
a = array (0,20000) $ (0,0):(1,1):[(i, a ! (i-1) + a ! (i-2)) | i <- [2..20000]]

Further, you restrict yourself to only a small collection of possible
element types which have  a compact unboxed representation. A list of
these in the case of UArray is available in the documentation for
Data.Array.Unboxed.

One paper which might be of interest is the one located at
http://citeseer.ist.psu.edu/peytonjones92implementing.html
which describes the Spineless Tagless G-machine, a low-level mechanism
used to implement Haskell code by GHC.

Hope this helps,
 - Cale


On 09/12/05, John Velman <velman at cox.net> wrote:
> I've tried google and google scholar, wikipedia, and planetMath.  Can't
> find a description.  Can someone point me to a freely available reference?
>
> Thanks,
>
> John Velman
>
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