Any small complete category is a preorder #
We show that any small category which has all (small) limits is a preorder: In particular, we show
that if a small category C in universe u has products of size u, then for any X Y : C
there is at most one morphism X ⟶ Y.
Note that in Lean, a preorder category is strictly one where the morphisms are in Prop, so
we instead show that the homsets are subsingleton.
References #
- https://ncatlab.org/nlab/show/complete+small+category#in_classical_logic
Tags #
small complete, preorder, Freyd
@[instance 100]
instance
CategoryTheory.instIsThin
{C : Type u}
[CategoryTheory.SmallCategory C]
[CategoryTheory.Limits.HasProducts C]
:
A small category with products is a thin category.
in Lean, a preorder category is one where the morphisms are in Prop, which is weaker than the usual
notion of a preorder/thin category which says that each homset is subsingleton; we show the latter
rather than providing a Preorder C instance.