[R] R and Scheme

[Gabor Grothendieck]

A few comments interspersed.

On Mon, Dec 8, 2008 at 5:59 PM, Stavros Macrakis <macrakis at alum.mit.edu> wrote:
> I've read in many places that R semantics are based on Scheme semantics.  As
> a long-time Lisp user and implementor, I've tried to make this more precise,
> and this is what I've found so far.  I've excluded trivial things that
> aren't basic semantic issues: support for arbitrary-precision integers;
> subscripting; general style; etc. I would appreciate corrections or
> additions from more experienced users of R -- I'm sure that some of the
> points below simply reflect my ignorance.
>
> ==Similarities to Scheme==
>
> R has first-class function closures. (i.e. correctly supports upward and
> downward funarg).
>
> R has a single namespace for functions and variables (Lisp-1).

Environments can be used to create separate name spaces.

R packages can use the NAMESPACE file to set up their own namespace.

>
> ==Important dissimilarities to Scheme (as opposed to other Lisps)==
>
> R is not properly tail-recursive.
>
> R does not have continuations or call-with-current-continuation or other
> mechanisms for implementing coroutines, general iterators, and the like.
>

True although there is callCC but it just lets you jump right of a nested
sequence of calls.

> R supports keyword arguments.
>
> ==Similarities to Lisp and other dynamic languages, including Scheme==
>
> R is runtime-typed and garbage-collected.
>
> R supports nested read-eval-print loops for debugging etc.
>
> R expressions are represented as user-manipulable data structures.
>
> ==Dissimilarities to all (modern) Lisps, including Scheme==
>
> R has call-by-need, not call-by-object-value.

Call by need?

>
> R does not have macros.

You can create them. See:

Programmer's Niche: Macros in R
http://cran.r-project.org/doc/Rnews/Rnews_2001-3.pdf

>
> R objects are values, not pointers, so a<-1:10; b<-a; b[1]<-999; a[1] =>
> 999.  Similarly, functions cannot modify the contents of their arguments.
>

a[1] is not 999 after the above completes (if that is what is meant by
a[1] => 999):

> a<-1:10; b<-a; b[1]<-999
> a
 [1]  1  2  3  4  5  6  7  8  9 10

> There is no equivalent to set-car!/rplaca (not even pairlists and
> expressions).  For example, r<-pairlist(1,2); r[[1]]<-r does not create a
> circular list. And in general there doesn't seem to be substructure sharing
> at the semantic level (though there may be in the implementation).
>
> R does not have multiple value return in the Lisp sense.

You can do this:

http://finzi.psych.upenn.edu/R/Rhelp02a/archive/36820.html

> R assignment creates a new local variable on first assignment, dynamically.
> So static analysis is not enough to determine variable reference (R is not
> referentially transparent). Example: ff <- function(a){if (a) x<-1; x} ;
> x<-99; ff(T) -> 1; ff(F) -> 99.
>
> In R, most data types (including numeric vectors) do not have a standard
> external representation which can be read back in without evaluation.

???

>
> R coerces logicals to numbers and numbers to strings. Lisps are stricter
> about automatic type conversion -- except that false a.k.a. NIL == () in
> Lisps other than Scheme.