Builtins and Closures in R

Question

I don't know , if this is the correct place to ask subjective questions and I am fairly new to R, but i am really confused at this time.I was going through R-Language reference and found two objects by running typeof(is.na) and typeof(mean) which returned "builtin" and "closure" respectively on R prompt. I don't know what does it mean, I went to this site after searching, http://www.r-bloggers.com/closures-in-r-a-useful-abstraction/ , but unable to understand , Can anyone help me in understanding "closure" and "builtin" in some layman terms?

Answer 1

From help("closure"):

This type of function is not the only type in R: they are called closures (a name with origins in LISP) to distinguish them from primitive functions.

From help(".Primitive"):

a ‘primitive’ (internally implemented) function.

"internally implemented" is just a different term for "builtin".

Both (closures and primitives) are functions in R. A primitive function is implemented exclusively in C (with a special calling mechanism), a closure (mostly) in R.

Another distinction between them is, that a closure always has an environment associated with it (see the language definition). That's actually pretty much the definition of "closure".