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doc/manual/source/language/evaluation.md

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+# Lazy Evaluation
+
+Decodal evaluates values when they are needed.
+
+## Lazily evaluated values
+
+The following values are not evaluated until they are referenced or materialized:
+
+- Module roots and top-level fields
+- Object fields
+- `let` bindings
+- Function arguments
+- `default` expressions
+- Imported values
+
+When the same binding is referenced more than once, its evaluation result is reused.
+Consequently, a failure in an unreferenced field or function argument does not occur until that value is needed.
+
+```dcdl
+let
+    safe = 42;
+    unused = missing_name;
+in
+    safe
+```
+
+This expression evaluates to `42` because it never references `unused`.
+
+## Cycle detection
+
+A cycle diagnostic is produced when a value being evaluated depends on itself again.
+
+```dcdl
+{
+    a = b + 1;
+    b = a + 1;
+}
+```
+
+Modules and imports may refer to one another cyclically as long as the dependency graph of the fields actually evaluated is not cyclic.
+
+## Evaluation and materialization
+
+A normal evaluation result may retain abstract values such as constraints, `Unknown`, `default`, and functions.
+Materialization converts that result to concrete data for use outside Decodal.
+
+This separation allows schemas to be composed as values, only required fields to be evaluated, and selection of defaults to be deferred until output.
+See [Materialization and Errors](./materialization-and-errors.md) for details.