The .fun extension is commonly used for Standard ML.

Some projects written in Standard ML use the .fun extension.  For example:
 * [RedBlackTree.fun](https://github.com/clf/celf/blob/master/RedBlackTree.fun)
 * [main.fun](https://github.com/MLton/mlton/blob/master/mlton/main/main.fun)

Also, the Pygments Standard ML lexer recognizes the .fun extension; see:
 * [functional.py](https://bitbucket.org/birkenfeld/pygments-main/src/0710694cba81a1544e6f789953ef164c7cfc4251/pygments/lexers/functional.py?at=default#cl-1082)