Question

Say I have a code like this:

tmp <- switch("b",
              a = print("foo"),
              b = function() paste("I want to evaluate this one!"),
              stop("say what now?")
)

Now, if I type tmp I get an unevaluated function, so I have to do add a pair of brackets afterwards in order to evaluate it:

tmp
## function() paste("I want to evaluate this one!")
tmp()
## [1] "I want to evaluate this one!"

Of course, I can predefine this function and pass it within switch (in that case it’s not anonymous), but I want to know if it’s possible and/or reasonable to evaluate anonymous function within switch statement.

Answer 1

I suppose one could arrange for do.call() to call the anonymous function:

tmp <- switch("b",
              a = print("foo"),
              b = do.call(function() paste("I want to evaluate this one!"), 
                          list()),
              stop("say what now?")
)

e.g.:

> tmp
[1] "I want to evaluate this one!"

---

Edit
A simpler version of the above is:

tmp <- switch("b",
              a = print("foo"),
              b = (function() paste("I want to evaluate this one!"))(),
              stop("say what now?")
)

So the anonymous function is created in the first set of parentheses and the resulting function is called by appending the second set of ().

---

But it seems cleaner to me to turn the anonymous function into a named function and call it:

foo <- function() paste("I want to evaluate this one!")
tmp <- switch("b",
              a = print("foo"),
              b = foo(),
              stop("say what now?")
)

Which has the same end result:

> tmp
[1] "I want to evaluate this one!"

If this is all within a function, foo() could be defined inline so it only exists during the execution of the outer function call.