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

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+# Default Expression
+
+A `default` expression specifies a fallback selected during materialization when no explicit value exists.
+
+```dcdl
+port = Int default 8080;
+```
+
+`default` is not a constraint.
+See [Constraints and Defaults](../constraints-and-defaults.md) for details.
+
+## Evaluation
+
+The fallback expression is evaluated only if materialization requires it.
+When an explicit value exists, the `default` is neither evaluated nor selected.