Bounds on Array length and Array indexing

[lth]

Rationale:

• Bounded arrays are good for error detection.

Primary proposal:

Arrays shall have a read-write property bound of type uint. The initial value of this property shall be 2³²-1.

The bound property holds the maximum value of the length property: Any operation that would increase length beyond bound shall fail with a !RangeError exception being thrown.

If bound is given a value less than length then a RangeError? is thrown.

Any attempt to access a numbered property on the array object beyond the value of bound shall fail with a !RangeError exception being thrown.

[brendan]

Syntax, we want syntax!

type Pair.<T> = [T, T](2);

or

type Pair.<T> = [T, T, ()];  // works today modulo lack of TyLams

or

type Pair.<T> = [T, T];

where the last case is not what the RI does today, and today's meaning would be written thus:

type TwoOrMore.<T> = [T, T, T...];

or some equivalent Kleene star notation for element types.

/be

[brendan]

See #68, which with the first syntax proposal above might be combined with this proposal:

let intPair = new [int, int](2);

and likewise for the second and third proposals.

/be

[lth]

Can of worms:

We have a minimum length implied by the type syntax [int,String,*], it's not possible to take this below length=2 (or the type system is subverted). But this lower bound is not visible to the program except as an error when trying to set length<2, and it's not an error to access the array beyond the minimum (or add that).

We want to be able to set an upper bound for safe programming, and it implies no access below the bound.

The tuples syntax proposal (#120), if accepted, sort of implies both lower and upper bounds on the length, but says nothing (yet) about accessing below that upper bound.

Allen suggested a "fixedLength" property to cover both, and that's possible.

Conclusion: lth takes this back to the wiki for a real proposal, along with #120, looking to resolutions for #68 and #66.

[lth]

See https://intranet.mozilla.org/ECMA/wiki/doku.php?id=proposals:arrays.

[lth]

Replaced by the array/tuple proposal and the vector proposal.