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doc/manual/source/language/materialization-and-errors.md

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+# Materialization and Errors
+
+A normal evaluation result may retain constraints, `default` values, functions, and other abstract values.
+Materialization converts an evaluation result into concrete data that can be passed outside Decodal.
+
+## Materialization rules
+
+Materialization performs the following work:
+
+- Evaluate required fields.
+- Apply `default` to abstract ranges that have no explicit value.
+- Verify that concrete values and selected defaults satisfy their constraints.
+- Reject `Unknown` and other unresolved ranges without a `default`.
+- Reject unapplied functions and other values that cannot be represented as data.
+
+```dcdl
+Service = {
+    host = String;
+    port = Int default 8080;
+};
+```
+
+Materializing `Service` directly fails because `host` has neither a concrete value nor a `default`.
+
+```dcdl
+Config = {
+    host = "localhost";
+} as Service;
+```
+
+Materializing `Config` produces the following data:
+
+```dcdl
+{
+    host = "localhost";
+    port = 8080;
+}
+```
+
+A `default` is not selected for a field that has an explicit value.
+
+## Diagnostics
+
+Errors are returned as diagnostics, not ordinary values.
+Expressions cannot branch on a diagnostic's kind or contents.
+
+Representative diagnostics include:
+
+- Syntax errors
+- Unresolved identifiers or fields
+- Type mismatches and constraint violations
+- Conflicts in `&` or `default`
+- Dependency cycles
+- Import failures
+- Match failures
+- Materialization failures
+
+A diagnostic identifies the relevant DCDL source span.
+When several expressions conflict, it also points to the related fields, constraints, values, and defaults.
+If a structured import value has no source span, the diagnostic identifies the stable key supplied by the host and the logical value path.
+
+## Fallback
+
+A `match` with no matching arm and no fallback arm produces a diagnostic.
+
+```dcdl
+match value {
+    >= 10: "large";
+}
+```
+
+Decodal does not provide general-purpose `try / catch`, optional imports, or optional field access for catching diagnostics.
+Use `default` for a missing-value fallback and `match` for branching over a finite set of values.