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Does anyone know any work about the interaction of operads and logic? In particular, it seems weird nobody ever defined a "syntactic operad" for type theories, or viceversa, extracted type theories from operads.
This is in fact already common. Lawvere theories, which represent universal algebras, are equivalent to cartesian operads. It's quite common to see people working with cartesian multicategories, representing sorted algebraic structures.
Correspondingly, different kinds of operads correspond to different kinds of logic. Symmetric operads correspond to linear logic, for instance.
Multicategories work better than categories in some ways, as you don't have to internalise the notion of context, or context extension.
I think Mike Shulman's Categorical logic from a categorical point of view takes a multicategory-oriented approach to introducing the connection between logic and category theory.
There is at least one recent PhD thesis on linear logic that systematically takes an operadic view on syntax: https://lipn.univ-paris13.fr/~pellissier/soutenance/manuscript.pdf
@Nguyễn Lê Thành Dũng (This is excellently written, so far...)