Standard ML of New Jersey
Release Notes

Version 2021.1
December 31, 2021


This release contains significant changes to the SML/NJ system. As such, it should be viewed as beta software. The major changes are

  • The version numbering scheme has changed to a YYYY.NN scheme, where YYYY is the year of the release and NN is the release number for the year.

  • The MLRISC code generator that we have used for over 25 years has been replaced with one based on the LLVM Libraries

  • We have dropped support for 32-bit systems. Since we do not have a 64-bit Windows port yet, this version only supports "Unix" systems on the AMD64 processor (we have tested Linux and macOS). For other systems, we recommend using the legacy 110.99.2 version of *SML/NJ. A 64-bit Windows version is a high priority and we hope to include it in the next release.

  • The pattern-match compiler has been completely rewritten.

While we hope that people will experiment with this version (and report any problems), we recommend sticking with the "legacy" releases of the system (currently 110.99.2) for production work.

Using the new backend, we have added code generation support for the 64-bit ARM processor (currently for macOS only). Unfortunately, we have not been able to finish debugging this port, but we hope to have a release early in 2022 that adds both native ARM support and 64-bit Windows support.



The compiler has a new implementation of pattern-match compilation (see the change log for details).

The CPS to MLRISC code generator has been replaced by a new backend with a new IR (CFG). After optimization and closure-conversion, we now translate the first-order CPS IR to CFG. The CFG IR is then serialized and passed to the runtime system, where a LLVM-based code generator translates it to machine code. This new backend is described in the paper

A New Backend for Standard ML of New Jersey, by Kavon Farvardin and John Reppy.
Proceedings of the 32nd Symposium on Implementation and Application of Functional Languages (IFL 2020), September 2020.


The MLRISC library has been removed from the core part of SML/NJ and is no longer an enabled target in the build. We plan to restore it as a target in the next release (there is some installation-script issues that have to be resolved first).

Runtime System

The runtime system now includes much of the LLVM source tree in the directory base/runtime/llvm. If you are building from source code, the installation script will compile LLVM before building the runtime system. Be warned that compiling LLVM can take significant time on slower systems.

SML/NJ Library

Several improvements to the JSON library:

  • Added convenience function int to the JSON_STREAM_OUTPUT interface.

  • Refactored the output modules in the JSON library. These changes should not affect current clients of the library, but allow the addition of printing JSON to a CharBuffer.buf.

  • Bug fix to JSONUtil module; the FIND path arc was not getting handled for the update functions (i.e., replace, insert, and append).


The config/ script continues to work as before. We have added a -dev command-line option to enable cross compilation (i.e., it compiles LLVM for all of the supported target architectures).


Here is a list of tracked bugs fixed (or closed) with this release, please see the bug tracker for more details. Note that this list is relative to Version 110.99 and includes fixes that were part of the 110.99.* patch releases.


ListMergeSort is documented as stable, but is not (esp. since 110.78 rewrite!)


Real.toLargeInt returns zero for anything in range [-512,512]


110.99 config/ -64 fails on macOS 10.15.7


Redundant error messages for when a constructor name is misspelled


Installation of SML/NJ v110.99.1 fails in fresh Ubuntu 20.04/macOS 10.15


Install errors with request asdl


SMLNJ Incompatibility with macOS 12 Beta


Compiler bug: Recover

The following unnumbered bug was also fixed:

  • Floating-point spill records were twice as large as necessary on 64-bit systems.

  • Fixed a bug in the printing of char values in the REPL.

Supported systems

We believe that SML/NJ will build and run on the following systems, but have only tested some of them:

Architecture Operating System Status



macOS 11 (Big Sur)


macOS 12 (Monterey)


Ubuntu 20.04