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doc/manual/source/language/expression/object.md

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+# Object Expression
+
+An object expression is a collection of named fields.
+
+```dcdl
+{
+    host = "127.0.0.1";
+    port = 8000;
+}
+```
+
+Objects can represent both configuration values and schemas.
+
+```dcdl
+MyConfig = {
+    host = String;
+    port = Int default 8080;
+};
+```
+
+## Dot-path fields
+
+A nested field can be defined with a dot path.
+
+```dcdl
+{
+    feature_hoge.enable = false;
+}
+```
+
+This represents the same structure as:
+
+```dcdl
+{
+    feature_hoge = {
+        enable = false;
+    };
+}
+```
+
+## Map constraint
+
+An associative-array constraint applies the same schema to every field value in an object.
+
+```dcdl
+Services = {...{
+    port = Int;
+    enabled = Bool default true;
+}};
+```
+
+`{...T}` represents an object whose keys are not fixed and whose values all satisfy `T`.
+An empty object is allowed. After materialization, it becomes the same object data as an object with named fields.
+
+```dcdl
+services = {
+    api = { port = 8080; };
+    worker = { port = 8081; enabled = false; };
+} as Services;
+```
+
+Each entry is refined by `as` into the right-hand field domain, so a field other than `port` or `enabled` on the left produces an error.
+A field found only on the right remains abstract whether or not it has a default.
+An object supplied by the host can preserve string keys that are not valid identifiers, although field names written in DCDL source are limited to ordinary identifiers.
+
+## Object rest constraint
+
+Fixed fields and a value range for arbitrary keys can coexist in one object.
+
+```dcdl
+OpenConfig = {
+    enabled = Bool default true;
+    ...Unknown
+};
+```
+
+`...T` applies only to remaining fields that were not named explicitly.
+Each named field uses its own range; the rest range is not additionally applied to it.
+A rest constraint does not generate fields. Materialization outputs only additional fields that actually exist.
+
+```dcdl
+{
+    enabled = false;
+    plugin = { name = "cache"; };
+} as OpenConfig
+```
+
+An object can contain one `...T`, at its end.
+An object without it is closed, and an undeclared left-hand field under `as` produces an error.
+`{...T}` with no named fields is an abstract map constraint; materializing it by itself requires a default or a concrete value.