Day 10, Part 2

1 files changed, 9 insertions, 0 deletions

diff --git a/Common/NonEmptyList.lean b/Common/NonEmptyList.lean
index 786e192..6e76f89 100644
--- a/Common/NonEmptyList.lean
+++ b/Common/NonEmptyList.lean
@@ -9,3 +9,12 @@ def toList (l : NonEmptyList α) : List α :=
 def ofList (l : List α) (_ : ¬l.isEmpty) : NonEmptyList α :=
   match l with
   | head :: tail => {head, tail}
+
+def ofList? (l : List α) : Option $ NonEmptyList α :=
+  if h : l.isEmpty then
+    none
+  else
+    ofList l h
+
+instance [ToString α] : ToString (NonEmptyList α) where
+  toString := List.toString ∘ toList