The fexpr package provides an interface to R's promise objects and dot (...) argument lists.

A "fexpr" is a type of nonstandard-evaluating function present in certain languages of the Lisp family. Fexprs can be used to to implement syntactic abstractions such as new types of control flow, or domain-specific languages. R's implementation of lazy evaluation in terms of promises essentially means that ordinary R functions are fexprs.

R's traditional tools for doing nonstandard evaluation (substitute, parent.frame, match.call) are non-orthogonal and tend to exhibit some scope problems in use. Using fexpr interface, nonstandard evaluating functions can be constructed using a more orthogonal, explicit style.

R functions evaluate their arguments lazily. When R invokes a function, the function arguments are lazily bound to promises. Each promise is a triple of three values: the environment in which the argument appeared, the expression that

The fexpr package allow inspection of each part of a promise.

Here's a problem that happens with traditional R metaprogramming....

Promises can be put into a special type of list.

Variadic arguments (...) and missing values are two of the trickiest spots of R's semantics, and there are very few tools to work with them -- besides missing there's substitute and do.call, both of which are hairy and mostly serve other purposes. Mostly people treat ... as an opaque block to pass along to another function. This package contains a number of functions that let you work explicitly with ... lists, concatenating and subsetting them, while still allowing R's lazy-evaluation semantics to do the right thing. So a function using dots can decide whether and when to evaluate each of its unnamed arguments:

inSomeOrder <- function(...) invisible(list %()% sample(dots(...)))
inSomeOrder(print("Boing!"), print("Boom"), print("Tschack!"), print("Ping"),
            print("Zong"), print("Pssh"))
# [1] "Boing!"
# [1] "Zong"
# [1] "Ping"
# [1] "Boom"
# [1] "Pssh"
# [1] "Tschack!"

For a more pointed example, consider switch. Switch takes its first argument and uses it to decide which if its subsequent arguments to evaluate.

Consider trying to implement an R function that has the behavior of switch properly (not as a C function, and not inspecting the stack using match.call() or parent.frame() which are evil.) This is doable in pure R but wacky and slow -- the only way I can see to selectively evaluate one named argument is to build a function that takes that argument:

switch2 <- function(expr, ...) {
  n <- names(substitute(list(...)))[-1]
  if (!is.null(n))
      arglist <- as.pairlist(structure(
          rep(list(quote(expr=)), length(n)),
          names=n))
  else
      (arglist <- as.pairlist(alist(...=)))

  if (is.numeric(expr))
      body <- as.name(paste0("..", expr))
  else
      body <- as.name(expr)
  f <- eval(substitute(`function`(arglist, body),
                         list(arglist=arglist, body=body)))
  f(...)
}

But with a direct interface to manipulate dotlists, switch is easy:

switch3 <- function(expr, ...) {
  dots(...)[[expr]]
}

You may also use dots_unpack() to inspect the contents of as-yet-unevaluated dots objects, exposing R's promise mechanism:

x <- 1
y <- 2
d <- dots(a=x, b=y, c=x+y)
unpack(d)
#   name         envir  expr value
# a    a <environment>     x  NULL
# b    b <environment>     y  NULL
# c    c <environment> x + y  NULL
# > y <- 3
(function(b, ...) b) %()% d #force the "b" slot to evaluate
# [1] 3
unpack(d)
#   name         envir  expr value
# a    a <environment>     x  NULL
# b    b          NULL     y     3
# c    c <environment> x + y  NULL
c %()% d
# a b c
# 1 3 4
> unpack(d)
#   name envir  expr value
# a    a  NULL     x     1
# b    b  NULL     y     3
# c    c  NULL x + y     4

In R, function arguments may be "missing." When this happens...