A substrate is a kind of self-sufficient model or material for the construction of software. Jonathan Edwards’ definition is a great starting point comprising the following points:
A complete and self-sufficient programming system Persistent code & data store Direct-manipulation UI on that state Live programming Programming & using are on a spectrum, not distinct Conceptually unified — not a “stack” Summarized as a slogan: “A PL, DB, & WYSIWYG document unified together.
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An unavailable value is a special payload to a reactive system unifying handling of errors and asynchrony under a common mechanism responding to “design incompletion for any reason”. I named a special reactive payload as an “unavailable value” indicating that the design was incomplete either as a result of depending on pending I/O or else on a design/syntax error in the system.
More docs on unavailable values in the modern fluid.
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