Suggestion on Document: Explicit Declaration for Multiple Universe Level Variable
DKXXXL opened this issue · comments
Hello Lean Developer,
I was writing universe-lifting function due to lack of cumulative universe
inductive liftU.{u v} (X: Type u) : Type (max v (u + 1)) :=
| inh : X → liftU X
It doesn't work, and I was stuck on the document for a while, not sure if explicit universe-level variable declaration can be done.
Then my friend told me I have to add a comma between u and v for multiple variables. Problem solved.
I think document it would be of great help.
When I was trying all sorts of syntax, I bump into the following seemingly working.
inductive LiftU {u : Type x} {v : Type y} (X : Type u) : Type (max v (u + 1)) :=
| inh : X → LiftU X
I wonder if this is an intended behaviour (i.e. universe level type is some Type _). Also wonder if the above level x y for the level u v would impact anything.
I believe documenting these would be of great help for the Lean beginner, great thanks!
@david-christiansen, this is a FPIL request, I guess, so I'll let you file it away?
@DKXXXL, if you look up ULift, you will find this already exists, and there is quite a bit of API for it in Mathlib.
I'd say this goes in the reference manual, not FPiL, but I'm the right one to assign!