[Rd] Give update.formula() an option not to simplify or reorder the result -- request for comments

[Abby Spurdle]

Hi Pavel
(Back On List)

And my two cents...

> At this time, the update.formula() method always performs a number of
> transformations on the results, eliminating redundant variables and
> reordering interactions to be after the main effects.
> This the proposal is to add an option simplify= (defaulting to TRUE,
> for backwards compatibility) that if FALSE will skip the simplification
> step.
> Any thoughts? One particular question that Martin raised is whether the
> UI should be just a single logical argument, or something else.

Firstly, note that the constructor for formula objects behaves differently
to the update method, so I think any changes should be consistent between
the two functions.
> #constructor - doesn't simplify
> y ~ x + x
y ~ x + x
> #update method - does simplify
> update (y ~ x, ~. + x)
y ~ x

Interestingly, this doesn't simplify.
> update (y ~ I (x), ~. + x)
y ~ I(x) + x

I think that simplification could mean different things.
So, there could be something like:
> update (y ~ x, ~. + x, strip=FALSE)
y ~ I (2 * x)

I don't know how easy that would be to implement.
(Symbolic computation on par with computer algebra systems is a discussion
in itself...).
And you could have one argument (say, method="simplify") rather than two or
more logical arguments.

It would also be possible to allow partial forms of simplification, by
specifying which terms should be collapsed, however, I doubt any possible
usefulness of this, would justify the complexity.
However, feel free to disagree.

You made an interesting comment.

> This is not
> always the desired behavior, because formulas are increasingly used
> for purposes other than specifying linear models.

Can I ask what these purposes are?


kind regards
Abs

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