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@ncfavier
Last active July 7, 2025 13:16
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A conspiracy board about partiality monads and CPOs.

background

  • constructive HoTT with propositional resizing and HIITs
  • Ω := type of propositions = free DCPPO on 1
  • Σ := Rosolini "dominance" (not constructively) / type of semidecidable propositions = conatural numbers quotiented by bisimilarity = (P : Ω) × ∃[ f : ℕ → 2 ] P = (∃[ i : ℕ ] f i = 0)
  • Sσ := Sierpiński set = initial σ-frame (higher inductive type [1])
  • S∨ := free countably-complete join-semilattice on 1 (higher inductive type [1])
  • Sω := free ω-CPPO on 1 (higher inductive-inductive type [2])
  • 2 := booleans
  • any dominance P induces a mnd-container, hence a monad on sets which I call P-partiality

facts(?)

  • Maybe is the 2-partiality monad
  • Sσ = Sω = S∨ are dominances [1]
  • countable choice → Sσ = Σ → Σ is a dominance [1]
  • LEM → Ω = Σ = Sσ = S∨ = Sω = 2
  • the free DCPPO adjunction induces the Ω-partiality monad [5] and is not monadic
  • the free ω-CPPO adjunction induces the Sω-partiality monad and is not monadic
  • the free countably-complete join-semilattice adjunction does not induce the S∨-partiality monad (is it monadic?)
  • the free pointed partial order adjunction induces the 2-partiality monad and is not monadic
  • the free pointed set adjunction induces the 2-partiality monad and is monadic
  • LEM → the free pointed set adjunction is of Kleisli type (classical equivalence between pointed sets and sets-and-partial-functions)

questions

  • when does an adjunction F ⊣ U : C → Set induce the UF1-partiality monad? binary joins seem like a no-go. when is UF1 a dominance? when is UF a container monad?

references

  1. Quotienting the Delay Monad by Weak Bisimilarity (Chapman et al.)
  2. Partiality, revisited (Altenkirch et al.)
  3. Partiality and Container Monads
  4. https://martinescardo.github.io/TypeTopology/NotionsOfDecidability.SemiDecidable.html
  5. https://1lab.dev/Order.DCPO.Free.html#free-pointed-dcpos
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