kernel: correct false compiler-constraint claim in Celebration comment

The comment explaining Celebration's single type parameter claimed OCaml
rejects a type variable that appears in no field. That is false: a phantom
season parameter compiles cleanly. The real reason is a design choice, not a
compiler constraint -- a celebration has no season of its own (season is
contextual to the day, and lives in Temporal.t), and a phantom parameter
would carry no information while forcing every consumer to thread a
meaningless variable.

1 files changed, 6 insertions, 2 deletions

diff --git a/lib/kernel/celebration.ml b/lib/kernel/celebration.ml
index 3f7a577..2963366 100644
--- a/lib/kernel/celebration.ml
+++ b/lib/kernel/celebration.ml
@@ -2,8 +2,12 @@ open Sexplib0.Sexp_conv
 
 (* A celebration as computed or as loaded from a layer.
 
-   One type parameter, not two: the record has no season field, and OCaml
-   rejects a type variable that appears in no field. *)
+   One type parameter, not two: a celebration has no season of its own --
+   season is contextual to the day, and lives in Temporal.t, which pairs a
+   celebration with the day it falls on. A phantom 's parameter would compile,
+   but would carry no information while forcing every consumer (Layer,
+   Overlay, and later Precedence and Calendar) to thread a variable that
+   means nothing. *)
 type 'r t = {
   slug : Slug.t;
   names : Names.t;