On 6/20/20 5:04 PM, Duncan Murdoch wrote:
I think you effectively did that in your original post (all but
encapsulating the expression in a function), so yes, it's possible.
However, it's a really bad idea. Why use non-standard evaluation when
standard evaluation is fine? Standard evaluation follows some well
defined rules, and is easy to reason about. NSE follows whatever
rules it wants, so it's really hard for users to follow. For example,
assuming you had the g() you want, what would this give?
z <- 3
f(x = z, y = g(z))
You can't possibly know that without knowing whether there's a local
variable in f named z that is created before y is evaluated.
Duncan Murdoch
Very good point, and I agree it's best to use standard evaluation
whenever possible. The role of g would essentially be to modify one or
more elements of f's formals in-place. For example calling:
f(x = 3, y = g(expr))
would be equivalent to calling a function fm:
fm(x = 3)
where the body of fm is identical to that of f, but:
> formals(fm)
$x
$y
expr
though I expect g would be non-trivial to code up robustly.
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