Zulip Chat Archive
Stream: new members
Topic: Typeclasses in category_theory/limits
Markus Himmel (Mar 05 2020 at 07:32):
@Scott Morrison, while I was thinking about your comment on #2083 I noticed that I don't understand how the typeclasses in the limits library work. Say I have some concrete category where I can instantiate has_finite_products
and has_equalizers
explicitly. Now assume that there is some theorem that tells me something about an equalizer, but it requires has_finite_limits
for some reason and infers the existence of equalizers from that (for example using #2083). If I use this theorem with finite_limits_from_equalizers_and_finite_products
, do I get a statement about the nice equalizers I instantiated manually or about those constructed by finite_limits_from_equalizers_and_finite_products
?
Scott Morrison (Mar 05 2020 at 07:34):
I'm not exactly sure what you mean, but I suspect a pessimistic answer is appropriate: "it depends". Certainly we could end up with non-definitionally equal instances of these typeclasses.
Johan Commelin (Mar 05 2020 at 07:53):
At the moment, all these typeclasses carry data. To what extent could we actually make them Prop
s?
Johan Commelin (Mar 05 2020 at 07:54):
I guess that would mean an ugly amount of classical.choice
all over the place. Which probably means that for all practical purposes the answer to my question is "No. Don't".
Markus Himmel (Mar 05 2020 at 07:55):
Maybe one could make has_equalizers
a synonym for \forall f g, has_limit (parallel_pair f g)
?
Scott Morrison (Mar 05 2020 at 07:56):
I'm not sure what you mean. parallel_pair f g
isn't a shape
Markus Himmel (Mar 05 2020 at 07:57):
Sorry, that was a typo
Last updated: Dec 20 2023 at 11:08 UTC