On Mon, May 9, 2016 at 7:12 AM, peter dalgaard <pda...@gmail.com> wrote:
>
> On 09 May 2016, at 02:46 , Bert Gunter <bgunter.4...@gmail.com> wrote:
>
>> ... To be clear, Hadley or anyone else should also feel free to set me
>> straight, preferably publicly, but privately if you prefer.
>
> Not really to "set anyone straight", but there are some subtleties with mode
> call objects versus expression objects and formulas to be aware of.
>
> E.g.,
>
>> a <- 2
>> do.call("print", list(a*pi))
> [1] 6.283185
>> do.call("print", list(quote(a*pi)))
> [1] 6.283185
>> do.call("print", list(expression(a*pi)))
> expression(a * pi)
>> do.call("print", list(~a*pi))
> ~a * pi
>
> Thing is, if you insert a call object into a parse tree, nothing is there to
> preserve its nature as an unevaluated expression. Similarly, in
>
>> call("print", quote(a*pi))
> print(a * pi)
>
> the result is identical to quote(print(a * pi)), so when evaluated, quoting
> is not seen by print().
>
> As far as I understand, this is also the reason that for math in ggplot, you
> may need as.expression(bquote(....)).
>
> In general, I think that a number of things in R had been more cleanly
> implemented using formulas/expression objects than using substitution and
> lazy evaluation, notably subset and offset arguments in lm/glm. It would have
> been so much cleaner to have
>
> lm(math ~ age, data = foo, subset = ~ sex=="1")
>
> than the current situation where lm internally chops its own head off and
> substitutes with model.frame, then evaluates the call to model.frame() which
> in turn does eval(substitute(subset), data, env). Of course, at the time, ~
> was intended specifically for Wilkinson Rogers type formulas; "abusing" it
> for other kinds of expressions is something of an afterthought.
Yeah, to my mind, the cool thing about formulas is that they provide a
concise way to capture an environment and an expression, and then
Wilkinson Rogers are just a special case.
It's obvious impossible to go back and change how lm() etc works now,
but I'm reasonably confident that lazyeval provides a strong
foundation going forward. The quasiquotation stuff is particularly
important - and unquote-splice makes it possible to do things that are
impossible with bquote(). (Of course, unquote-splice could be added
to bquote(), but I think you'll still run into issues with
environments)
Hadley
--
http://hadley.nz
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