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doc/manual/source/language/syntax.md

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+# Lexical Structure and Syntax
+
+This chapter summarizes the surface syntax.
+See the EBNF in [Grammar](./grammar.md) for the precise grammar.
+
+## Language name and extension
+
+The project is named **Decodal**.
+Its full descriptive name is **Deferred Constraint Data Language**, abbreviated **DCDL**.
+Source files use the `.dcdl` extension.
+
+```text
+config.dcdl
+schema.dcdl
+service.dcdl
+```
+
+## Module source
+
+An entire file can contain a single expression.
+A sequence of field definitions at the top level is treated as an implicit object.
+
+```dcdl
+host = String;
+port = Int default 8080;
+```
+
+The source above is equivalent to:
+
+```dcdl
+{
+    host = String;
+    port = Int default 8080;
+}
+```
+
+## Comments
+
+A comment begins with `#` and continues to the end of the line.
+
+```dcdl
+# comment
+host = "127.0.0.1"; # trailing comment
+```
+
+## Semicolons
+
+Semicolons separate object fields, `let` bindings, and `match` arms.
+A trailing semicolon is allowed.
+
+```dcdl
+{
+    host = "127.0.0.1";
+    port = 8000;
+}
+```
+
+## Identifiers
+
+An identifier begins with an ASCII letter and may continue with ASCII letters, digits, or `_`.
+Naming conventions can use `lower_snake`, `lowerCamel`, or `UpperCamel`.
+
+```dcdl
+my_config
+mkConfig
+IPv4Address
+```
+
+## Path references
+
+Use a dot to reference a field.
+
+```dcdl
+config.host
+config.feature_hoge.enable
+```
+
+Fields inside an object can also be defined with a dot path.
+
+```dcdl
+{
+    feature_hoge.enable = false;
+}
+```
+
+This represents the same structure as:
+
+```dcdl
+{
+    feature_hoge = {
+        enable = false;
+    };
+}
+```
+
+## Reserved words
+
+The following words are reserved:
+
+```text
+let
+in
+match
+import
+default
+as
+true
+false
+```
+
+## Operators
+
+The primary operators are:
+
+```text
++ - * /       arithmetic
+++            array concatenation
+! && ||       logical operations
+== != < <= > >= comparisons
+&             constraint-preserving composition
+//            patch composition
+default       fallback specification
+as            range containment check and refinement from left to right
+=>            function
+.             field reference / dot-path definition
+...           value constraint for an array or associative array
+```
+
+Operator precedence is defined in [Operators](./operators.md).