On 01/05/2014, 4:39 PM, Henrik Bengtsson 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?
"Parse tree" isn't R terminology. Could you give an example of one call
that passes a parse tree, and one that doesn't?
Duncan Murdoch
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
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