ES4 draft: Vector

Jon Zeppieri jaz at bu.edu
Wed Mar 5 15:52:01 PST 2008


On 3/5/08, Lars Hansen <lhansen at adobe.com> wrote:
>
>  > So, why a read/write, rather than a read-only, 'fixed' property?
>
>
> It was my hunch when I designed that feature that code that uses
>  fixed-length vectors wants to be able to catch errors most of the time,
>  but also does not want to be stuck in the straightjacket that a
>  settable-but-not-resettable fixedness property creates, leaving it
>  unable to extend the vector in place after making it fixed.

I don't think I've ever encountered this feature in another
language/library.  I've seen the following:

- Languages like Java that have a distinction between fixed-length
arrays and variable length ones

- Dependently typed languages that allow you to express the type
"Array with length N"

- Languages that provide immutable arrays (which goes beyond the
fixed-length guarantee) in addition to mutable ones


But I haven't seen vectors that can change from fixed- to
variable-length and vice versa.  So I guess I'm a bit skeptical of the
demand for this feature.  I'm trying to think of tasks where I need a
vector to be a certain length for part of the computation and a
different length for a different part -- and where I would also be
upset to be forced to create a new vector (and copy into it) in order
to accomplish that.  (Of course, the "new vector + copy" is a simple
invocation of the Vector constructor as a function.)

Obviously, we can save the cost of the copy if we have the 'fixed'
flag, but we don't get the benefit of having a nice invariant.


Incidentally, since I brought up immutable arrays, above: intrinsic
methods make it impossible to enforce immutability via subclassing,
right?  You could create an immutable sequence type through
delegation, but not simply by overriding the mutators.  Or is there a
way to make the intrinsics unavailable publicly?

-Jon



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