Decodal/doc/manual/source/language/evaluation.md

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.

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.

{
    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 for details.