Using Scheme from R via Guile and Rcpp

This article is originally published at https://gallery.rcpp.org

Introduction

GNU Guile is a programming language “designed to help programmers create flexible applications that can be extended by users or other programmers with plug-ins, modules, or scripts”. It is an extension language platform, and contains an efficient compiler and virtual machine allowing both out of the box use for programs in Scheme, as well as integration with C and C++ programs.

Matías Guzmán Naranjo posed an almost-complete question on StackOverflow about how to combine Guile with R via Rcpp. It turns out that the code by Matías came from this gist showing both a Guile Scheme script, and a C use case example.

The Guile Scheme Script script.scm

This file is posted in this gist.

(define simple-func
  (lambda ()
    (display "Script called, now I can change this") (newline)))

(define quick-test
  (lambda ()
    (display "Adding another function, can modify without recompilation") 
    (newline)
    (adding-without-recompilation)))

(define adding-without-recompilation
  (lambda ()
    (display "Called this, without recompiling the C code") (newline)))

The C wrapper testguile.c

This file is also posted in this gist. We used (as shown below) simpler link instructions which (on Ubuntu) can be ontained via guile-config link and are just -lguile-3.0 -lgc on our Ubuntu 20.04 machine.

/*
 * Compile using: 
 * gcc testguile.c \
 * -I/usr/local/Cellar/guile/1.8.8/include \
 * -D_THREAD_SAFE -L/usr/local/Cellar/guile/1.8.8/lib -lguile -lltdl -lgmp -lm -lltdl -o testguile
 */

#include <stdio.h>
#include <libguile.h>

int main(int argc, char** argv) {
    SCM func, func2;
    scm_init_guile();

    scm_c_primitive_load("script.scm");

    func = scm_variable_ref(scm_c_lookup("simple-func"));
    func2 = scm_variable_ref(scm_c_lookup("quick-test"));

    scm_call_0(func);
    scm_call_0(func2);

    return 0;
}

The Rcpp Caller

This is an edited and updated version of Matías’ initial file.

#include <Rcpp.h>
#include <stdio.h>
#include <libguile.h>

using namespace Rcpp;

// [[Rcpp::export]]
int test_guile(std::string file) {
    SCM func, func2;
    scm_init_guile();

    scm_c_primitive_load(file.c_str());

    func = scm_variable_ref(scm_c_lookup("simple-func"));
    func2 = scm_variable_ref(scm_c_lookup("quick-test"));

    scm_call_0(func);
    scm_call_0(func2);

    return 0;
}

The System Integration

On our Ubuntu 20.04 box, Guile 3.0 is well supported and is easy installable. Header files are in a public location, as are the library files. The configuration helper guile-config (also guile-config-3.0 to permit continued use of 2.2 and 2.0) provides the required compiler and linker flags which we simply hardwired into src/Makevars. A proper package would need to discover these from a script, typically configure, or Makefile invocations. We leave this for another time; if anybody reading this is interested in taking this further please get in touch.

PKG_CXXFLAGS = -I"/usr/include/guile/3.0"
PKG_LIBS = -lguile-3.0 -lgc

Package

The rest of package was created via one call Rcpp.package.skeleton("RcppGuile") after which we

• simply copied in the C++ file above into src/guile.cpp;
• added the src/Makevars snippet above; and
• copied in the example Scheme scripts as inst/guile/script.scm

and that’s it! Then any of the usual steps with R will build and install a working package.

The complete package is available in this directory.

Guile Demo

The package can be used as follows, retrieving the Scheme script from within the package and clearly running it:

> library(RcppGuile)
> test_guile(system.file("guile", "script.scm", package="RcppGuile"))
Script called, now I can change this
Adding another function, can modify without recompilation
Called this, without recompiling the C code
[1] 0
> 

GNU Guile is made for building extensions. This short note showed how easy it is to connect R with Guile via a simple Rcpp package.

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