feat(reference): Lexical Structure—Identifiers

Author: ospencer

docs_config.yml

@@ -98,7 +98,7 @@ docs_groups:
         - reference/lexical_structure/whitespace
         - reference/lexical_structure/comments
         - reference/lexical_structure/keywords
-  #     - reference/lexical_structure/identifiers
+        - reference/lexical_structure/identifiers
   #     - reference/lexical_structure/literals
   #     - reference/lexical_structure/operators
   #     - reference/lexical_structure/delimiters

src/reference/lexical_structure/identifiers.md

@@ -0,0 +1,35 @@
+---
+title: Identifiers
+---
+
+There are two kinds of identifiers in Grain—those that begin with an uppercase letter, and those that do not.
+
+## Uppercase identifiers
+
+Identifers that start with an uppercase letter are used for the names of modules and concrete types. We call these tokens `UIDENT`s (**U**ppercase **Ident**ifiers).
+
+```ebnf
+UIDENT = ['A'-'Z']['A'-'Z' 'a-z' '0'-'9' '_']* ;
+```
+
+A `UIDENT` starts with an uppercase letter A through Z, followed by any number of:
+
+- Uppercase letters A through Z
+- Lowercase letters a through z
+- Digits 0 through 9
+- Underscores
+
+## Lowercase identifiers
+
+Identifers that don't start with an uppercase letter are used for the names of new bindings, record fields, type variables, and more. We call these tokens `LIDENT`s (**L**owercase **Ident**ifiers) though it should be noted that `LIDENT`s may also start with underscores.
+
+```ebnf
+LIDENT = ['a-z' '_']['A'-'Z' 'a-z' '0'-'9' '_']* ;
+```
+
+An `LIDENT` starts with an underscore or a lowercase letter a through z, followed by any number of:
+
+- Uppercase letters A through Z
+- Lowercase letters a through z
+- Digits 0 through 9
+- Underscores