Isabelle is usually advertized as environment for interactive and automated theorem proving, but its Prover IDE (PIDE) may be used for regular program development as well. Standard ML is particularly important here, since it is the bootstrap language of Isabelle/ML (i.e. SML with many add-ons) and Isabelle/Pure (i.e. the logical framework).

Using Isabelle/PIDE for bootstrapping Isabelle itself is now possible in recent repository versions, e.g. Isabelle/1c1f8531ca37 – see also README_REPOSITORY for general explanations how to build and run that. Here is the relevant NEWS entry from that version:

IDE support for the Isabelle/Pure bootstrap process. The initial files src/Pure/ROOT0.ML or src/Pure/ROOT.ML may be opened with Isabelle/jEdit: they act like independent quasi-theories in the context of theory ML_Bootstrap. This allows continuous checking of ML files as usual, but results are isolated from the actual Isabelle/Pure that runs the IDE
itself.

The ML project consists of a sequence of ML_file commands in ROOT.ML. Projects other than Isabelle can do the same with SML_file for official Standard ML. Afterwards, the following Poly/ML command line is able to build the project without the IDE: poly --eval "val SML_file = PolyML.use" --use ROOT.ML

What is also notable in the Isabelle/Pure bootstrap environment is the structure Thread_Data for global state variables within the current thread. There are two implementations: (1) physical and (2) virtual. The virtual version is used when Isabelle/Pure is loaded into itself: it allows to manage many versions of the load process with different intermediate states in a value-oriented manner.