Document justification for null

docs/null.md

@@ -0,0 +1,15 @@
+# Null
+
+> I call it my billion-dollar mistake. It was the invention of the null
+> reference in 1965. — Tony Hoare
+
+One of the intended use cases of RCL is to be an abstraction layer for formats
+with no or poor abstraction abilities, such as json and yaml. Many tools take
+input in such format, and it is not up to RCL to make demands about the schema
+of that data. RCL _has_ to provide a way to emit `null`.
+
+Fortunately, the problems commonly attributed to null are not problems with
+having a null value per se, they are problems with implicit nullability, which
+RCL does not have. `null` is the sole value of type `Null`, which is distinct
+from and does not unify with other types. In this sense, null behaves like the
+unit type in languages such as Haskell and Rust (written `()` there).

docs/theme

@@ -1 +1 @@
-Subproject commit 3fe234a38b38ad6823370cc11c781516cb4de94c
+Subproject commit 7d3f15554d5184d24eabc0beadc045dfc431de58

mkdocs.yml

@@ -14,9 +14,10 @@ markdown_extensions:
       permalink_title: null
 
 nav:
-  - Overview: index.md
-  - User guide:
-    - Terminology: terminology.md
-    - Syntax: syntax.md
-    - Strings: strings.md
-  - Development: development.md
+  - "Overview": "index.md"
+  - "User guide":
+    - "Terminology": "terminology.md"
+    - "Syntax": "syntax.md"
+    - "Strings": "strings.md"
+    - "Null": "null.md"
+  - "Development": "development.md"