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+# FastR Documentation
+
+## Index
+
+[Limitations](Limitations.md)
+
diff --git a/documentation/Limitations.md b/documentation/Limitations.md
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+# Limitations
+
+The long-term goal is for FastR to be a high-performance drop-in replacement for the reference implementation (open source GNU R). However, this is
+a challenging goal for a language and ecosystem as rich as R's. At the time of writing FastR implements almost the entire language, but many packages
+do not install or run correctly. For pure R packages this is generally due to incomplete implementations of the R builtin functions; either they are not
+implemented at all or some argument types are not accepted. In due course we expect to provide a web page listing the status of all the CRAN packages
+on FastR.
+
+## Native Code
+
+GNU R makes extensive use of native code, mostly C, to implement aspects of the default packages and, of course, package developers are encouraged to follow
+the same strategy to finesse performance problems. The native extensions [interface](https://cran.r-project.org/doc/manuals/r-release/R-exts.html#System-and-foreign-language-interfaces)
+is a difficult fit for a Java implementation such as FastR as it is strongly biased towards the existing GNU R implementation. In contrast Java
+has a very abstract and portable native code interface, but it is difficult and inefficent to map this to the R interface. Especially problematic
+are packages that actually depend on the internal details of GNUR, which can be enabled by setting `USE_RINTERNALS` in native code. FastR cannot support this
+at all. Where this is being used by a package simply as a performance speedup (some function calls become C macros), the package will still build, but run more slowly.
+However, where this is genuinely being used to access the GNU R internal data structures, it will fail to build. Note that whereas native C code produces
+a performance speedup with GNU R it will likely run slower on FastR due to the overhead of the JNI interface. We are actively working on a solution to this problem.
+
+In addition many of the default packages, e.g. `stats`, `graphics` contain a lot of C code that may use R internals. FastR has taken a mixed approach to
+implementing these "package" builtins. Some have been translated to Java, some that do not depend on the GNU R internals are accessed through the
+native interface, but many are not implemented. FastR is adopting "as-needed" approach to these functions.
diff --git a/documentation/README.md b/documentation/README.md
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+../README.md
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