MLton provides an option, -show-def-use file, to output precise (giving exact source locations) and accurate (including all uses and no false data) whole-program def-use information to a file. Unlike typical tags facilities, the information includes local variables and distinguishes between different definitions even when they have the same name. The def-use Emacs mode uses the information to provide navigation support, which can be particularly useful while reading SML programs compiled with MLton (such as the MLton compiler itself).

Screen Capture

Note the highlighting and the type displayed in the minibuffer.



  • Highlights definitions and uses. Different colors for definitions, unused definitions, and uses.

  • Shows types (with highlighting) of variable definitions in the minibuffer.

  • Navigation: jump-to-def, jump-to-next, and jump-to-prev. These work precisely (no searching involved).

  • Can list, visit and mark all references to a definition (within a program).

  • Automatically reloads updated def-use files.

  • Automatically loads previously used def-use files at startup.

  • Supports both Gnu Emacs and XEmacs.


There is no separate package for the def-use mode although the mode has been relatively stable for some time already. To install the mode you need to get the Emacs Lisp, *.el, files from MLton’s repository: emacs. The easiest way to get the files is to use Git to access MLton’s sources.


The easiest way to load def-use mode is to first tell Emacs where to find the files. For example, add

(add-to-list 'load-path (file-truename "path-to-the-el-files"))

to your ~/.emacs or ~/.xemacs/init.el. You’ll probably also want to start def-use-mode automatically by adding

(require 'esml-du-mlton)

to your Emacs init file. Once the def-use mode is activated, you should see the DU indicator on the mode line.


To use def-use mode one typically first sets up the program’s makefile or build script so that the def-use information is saved each time the program is compiled. In addition to the -show-def-use file option, the -prefer-abs-paths true expert option is required. Note that the time it takes to save the information is small (compared to type-checking), so it is recommended to simply add the options to the MLton invocation that compiles the program. However, it is only necessary to type check the program (or library), so one can specify the -stop tc option. For example, suppose you have a program defined by an MLB file named, you can save the def-use information to the file my-prg.du by invoking MLton as:

mlton -prefer-abs-paths true -show-def-use my-prg.du -stop tc

Finally, one needs to tell the mode where to find the def-use information. This is done with the esml-du-mlton command. For example, to load the my-prg.du file, one would type:

M-x esml-du-mlton my-prg.du

After doing all of the above, find an SML file covered by the previously saved and loaded def-use information, and place the cursor at some variable (definition or use, it doesn’t matter). You should see the variable being highlighted. (Note that specifications in signatures do not define variables.)

You might also want to setup and use the Bg-Build mode to start builds automatically.


-show-def-use output was extended to include types of variable definitions in revision r6333. To get good type names, the types must be in scope at the end of the program. If you are using the ML Basis system, this means that the root MLB-file for your application should not wrap the libraries used in the application inside local ... in ... end, because that would remove them from the scope before the end of the program.