I mentioned here recently that I was working on updating my Scintilla plugin for OCaml to also support Standard ML - and it is now "done"(*).
Since this coincided fortuitously (and barely) with the release cycle for Scintilla/SciTE 1.79, the new version has been included in the just-released editing package.
If you go to http://www.scintilla.org and look for binaries, you will be pointed to http://www.scintilla.org/SciTEDownload.html. Here you can get compiled versions of the full Scintilla package (supporting a lot of programming languages) as part of either Windows or GTK+/Linux builds of the SciTE editor.
One thing which might be missed were one to jump right in and start editing SML with the SciTE editor + the aforementioned OCaml/SML plugin (called a "lexer" in Scintilla parlance): only ONE language at a time can be automagically associated with a given source file extension (file type).
So, as things come out of the box, OCaml has .ml and .mli files, while SML has .sml files. This may be adjusted two ways: by selecting a specific language from the SciTE Language menu, a default binding may be overridden. Or, by adjusting configuration files, SML can "take over" .ml file support for itself. These issues are discussed in the caml.properties config file in the main SciTE directory - note that you probably don't want to count on .ml and .ML being distinguishable.
Perhaps I am missing something, but it looks like Poly is all about an interactive read-eval-print loop sort of dev environment, as opposed to [also] supporting a non-interactive edit-compile-run model... this makes the efforts underway to have a richer interactive experience even more understandable. ;)
As I have already built OCaml bindings for Scintilla, creating SML bindings should be doable... but would this really be usable, given that Scintilla is Windows or GTK+, while Poly seems to be more Windows or X (including Motif here) oriented. Hmmm. I bring this up, because I think Scintilla can be useful as the "editing and output window" part of an IDE.
(*) - actually, while "done" as much as software projects ever are, some suggestions for updating the 3 [arbitrary] groups of keywords would be welcomed (again, see the caml.properties config file). ;)
Robert