Formal core, natural language shell

A fundamental architecture pattern of functional software architecture is Functional Core, Imperative Shell. The gist of this pattern is that you design the core of your application (your domain logic, or: business logic) in a pure, immutable, algebraic (i.e. functional) language, and there's a surrounding shell that deals with the messy outside world of side effects and mutable state.

While working on an application that was mandated to include an "AI component" -- by which the sponsor of course meant: large language models --, I got the epiphany that LLMs only ever make sense as part of the user interface. This leads to a software design pattern similar to Functional Core, Imperative Shell, which I clunkily call ‘Formal Core, Natural Language Shell’.

The rationale behind Formal Core, Natural Language Shell is similar to the rationale behind Functional Core, Imperative Shell. In both cases, the former allows for clear, concise reasoning, which is vital for the application logic to work as intended. The latter is either a neccessity for which we have not yet found better abstractions (imperative shell) or a nice add-on that makes life easier (natural language shell).

In the case of the aforementioned software I built, which follows this pattern, the formal core consists of an RDF knowledge graph along with a simple modification logic based on git. The natural language shell allows users to specify a set of facts in English prose. An LLM then tries to translate this prose into RDF and presents a graphical representation thereof to the user. When the user confirms, the corresponding RDF data is committed to the formal core.