My take would be that this is barking up the wrong tree. If you want to pass
expressions in a way that a function can recognize, use formulas or expression
objects.
One problem is that pretty much every unevaluated argument is a "parse tree".
The only other thing it can be is a constant object, but that is really just
the simplest possible parse tree.
In what situation exactly were you expecting isParseTree to return FALSE?
-pd
On 01 May 2014, at 22:39 , Henrik Bengtsson <h...@biostat.ucsf.edu> wrote:
> This may have been asked before, but is there an elegant way to check
> whether an variable/argument passed to a function is a "parse tree"
> for an (unevaluated) expression or not, *without* evaluating it if
> not?
>
> Currently, I do various rather ad hoc eval()+substitute() tricks for
> this that most likely only work under certain circumstances. Ideally,
> I'm looking for a isParseTree() function such that I can call:
>
> expr0 <- foo({ x <- 1 })
> expr1 <- foo(expr0)
> stopifnot(identical(expr1, expr0))
>
> where foo() is:
>
> foo <- function(expr) {
> if (!isParseTree(expr))
> expr <- substitute(expr)
> expr
> }
>
> I also want to be able to do:
>
> expr2 <- foo(foo(foo(foo(expr0))))
> stopifnot(identical(expr2, expr0))
>
> and calling foo() from within other functions that may use the same
> "tricks". The alternative is of course to do:
>
> foo <- function(expr, substitute=TRUE) {
> if (substitute) expr <- substitute(expr)
> expr
> }
>
> but it would be neat to do this without passing an extra argument. If
> this is not possible to implement in plain R, can this be done
> internally inspecting SEXP:s and so on? Even better would be if
> substitute() could do this for me, e.g.
>
> expr <- substitute(expr, unlessAlreadyDone=TRUE)
>
> Any suggestions?
>
> Thanks,
>
> Henrik
>
> ______________________________________________
> R-devel@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
--
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd....@cbs.dk Priv: pda...@gmail.com
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