[m-rev.] for review: arrays and the debugger

[Zoltan Somogyi]

On 19-Jun-2001, Fergus Henderson <fjh at cs.mu.OZ.AU> wrote:
> As I said earlier, certainly we should fix the problem that was
> causing things to be infeasible, the only question is how.
> The efficiency difference between the two different ways of
> fixing it is small.

Which two ways are you referring to? I can see more than two.

1. Don't add limited_deconstruct, and make everyone program their own arity
   test if required.

2. Add limited_reconstruct with the simple "arity test then deconstruct"
   implementation.

3. Add limited_reconstruct with the implementation in my diff.

You said you favoured option 1. This does have a big efficiency downside,
because the chance that someone will omit the arity test when it is required
is significantly higher with option 1 than with the other two options.
Getting people to use the safe version is a lot easier if they don't have to
pay an efficiency penalty for doing so, regardless of the size of the penalty.

My arguments are that (a) there are no valid grounds on which option 1 is
superior to option 2, while there are grounds (e.g. avoidance of code
duplication) on which option 2 is superior to option 1, and (b) option 3,
being more efficient (and therefore more likely to be used) at acceptable cost
in simplicity, is preferable for now to option 2, but there is nothing to
prevent us from switching to option 2 in the future if it becomes warranted.

The only counter-argument you have made to argument (a) was that options
2 & 3 "complicate" the interface, which I believe I have refuted; the
complexity exists, whether it is made explicit or not. The only
counter-argument you have made that is relevant to argument (b) was about
option 3 being possibly hard to modify. I agree with that, but it doesn't
refute argument (b), since argument (b) is not committing us to option 3
forever.

Zoltan.
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