No kidding. This is very nice. I might be taking some liberties here, but the point about the shape of the output driving the shape of the program reminds me

No kidding. This is very nice. I might be taking some liberties here, but the point about the shape of the output driving the shape of the program reminds me of using schema information in SQL (think of declared foreign keys) to generate queries: the schema tells you the shape of the data in more ways than just tables and columns, and this in turn tells you about the queries you might want. If you enrich a relational schema with more metadata, then you can generate even more queries. Think of declaring the set of all relations that define authorization data, not just group memberships, but access controls that refer to groups, define ownership, and so on. So, for example, 'give me all the authorization-relevant entities "to the right" of this entity' might be a query that, given enough schema metadata, can generate a much more complex SQL query. But that abstract query itself becomes an obvious query given the shape of the data (i.e., the schema). A more concrete example is "what can this user do?" and "who can do what to this resource?", "what groups is this user a member of?", "what are all the members of this group?" (note: these are not trivial if groups nest) which are instantiations of "what are all the entities to the {left, right} of this entity, using the set {name} of relations?". In terms of SQL these are all JOINs or LEFT JOINs (depending on declared relation NULLability, or relation cardinalities), with RECURSIVE queries using UNION (not UNION ALL, to avoid infinite recursion in the presence of circular relations) for "nesting" relations (from a table to itself, perhaps crossing relations on other tables in the process). SQL is extremely powerful and expressive, but we can make these queries pithier and more obvious with a more abstract language that takes advantage of schema metadata. Indeed, graph databases are such a language. SQL needs a richer schema definition language (extensions will do) by which to formally describe more of the shape of the data.