R and Scheme
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.