This is tangentially related to Hadley's question. Suppose I'm building a function programmatically; I have assembled an expression for the body and I know the names of the arguments it wants to take.
Suppose I have some convenience function such that writing make_function(alist(a=, b=), quote(a+b), environment()) is equivalent to writing function(a,b) a+b So how do I make the list that goes in the first argument? Suppose I start with a character vector of argument names. What I need is a list with names and quoted values, but the values are "empty". Say I have argnames <- c("a", "b", "c"). >From that I want to construct the equivalent of alist(a=, b=, c=). "missing" seems different from NULL; if I do arglist <- rep(list(NULL), length(argnames)) names(arglist) <- argnames make_function(arglist, quote(a+b+c)) I get a function where NULL is the default value for each argument, rather than a function with no default values. It seems I can assign a "missing value" to a variable by the_missing_value <- formals(function(x) NULL)[[1]] or by the_missing_value = quote(expr=) However this backfires when I try to use it: > arglist <- rep(list(the_missing_value), length(argnames)) Error in list(the_missing_value) : 'the_missing_value' is missing > arglist <- rep(list(NULL), 3) > arglist[] <- the_missing_value Error: argument "the_missing_value" is missing, with no default Besides which, roxygen2 (and other package tools) choked on encountering a missing value assigned to a name in my package. So, what's going on with missing? Missing and '...' are the two pieces of R's semantics I'm most fuzzy on. (Somewhere along this experimentation I also managed to assign a value to the empty name, and could not figure out how to remove it...) Peter ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel