Strictly speaking the Isabelle environment is for interactive and automated theorem proving, but its SML IDE support is quite sophisticated: source files are statically checked and semantically evaluated while the user is editing. The annotated sources contain markup about inferred types, references to defining positions of items etc.

As a quick start, see the Documentation panel, section Examples, entry src/Tools/SML/Examples.thy (as of Isabelle2014).

The time where SML sources need to be edited with vi or emacs are over. See also this related thread on Stackoverflow.