What does the V in Pattern<V> mean?

V is the value type parameter. Every position in a Pattern<V> — the root and each element, recursively — holds a value of type V. The type parameter lets you use any value type as the decoration.

Common choices:

Type V	Use case
Subject	Property-graph data (the default; produced by parse_gram)
String	Text annotation or labelling
i32, f64	Numeric computation over structured data
Custom struct	Application-specific domain types

The map operation transforms Pattern<V> into Pattern<U> by applying a function to every value:

let p: Pattern<i32> = Pattern::pattern(1, vec![Pattern::point(2), Pattern::point(3)]);
let p_str: Pattern<String> = p.map(|n| n.to_string());
// p_str has value "1" and elements ["2", "3"]

This is the primary way to change the value type while keeping the structure. The shape (which positions have elements) is never altered by map.

The constraint on V varies by operation: map requires nothing from V; combine requires V: Combinable; storing in a StandardGraph requires V = Subject. This lets the library be general without forcing all users to adopt Subject.