A comment of the form (*#showBasis "<file>"*) is recognized as a directive to save the current basis (i.e., environment) to <file> (in the same format as the -show-basis <file> compile-time option). The <file> is interpreted relative to the source file in which it appears. The comment is lexed as a distinct token and is parsed as a structure-level declaration. [Note that treating the directive as a top-level declaration would prohibit using it inside a functor body, which would make the feature significantly less useful in the context of the MLton compiler sources (with its nearly fully functorial style).]
This feature is meant to facilitate auto-completion via company-mlton and similar tools.