ActionVisitor

Side‑effecting visitor that maps runtime types to actions, evaluated with first‑match‑wins semantics.

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What It Is

• Type‑to‑action mapping: On<T>(Action<T>) with optional .Default(...).
• First matching action executes; others are skipped.
• Non‑throwing variant: TryVisit(in TBase) returns false when no action/default matched.

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TL;DR Example

var v = ActionVisitor<Tender>
    .Create()
    .On<Cash>(t => CashHandler(t))
    .On<Card>(t => CardHandler(t))
    .Default(t => LogUnhandled(t))
    .Build();

v.Visit(tender); // throws if no match and no default

See src/PatternKit.Examples/VisitorDemo/VisitorDemo.cs:48 for a complete routing demo.

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API

• void Visit(in TBase node) — runs first matching action or the default; throws if neither applies.
• bool TryVisit(in TBase node) — returns false when nothing matches and no default exists.
• Builder.On<T>(Action<T>) — registers a type‑specific action; evaluation order is registration order.
• Builder.Default(ActionHandler) — registers a fallback for unknown types.

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Operational Guidance

• Prefer idempotent and fast actions; guard side effects (I/O, retries) at the edges.
• Put the most specific types first to avoid base‑type shadowing.
• Share built visitors as singletons; builders are not thread‑safe.
• Use .Default(...) to log and continue on unknown types in production.

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Testing

Reference tests: test/PatternKit.Tests/Behavioral/VisitorTests.cs:49

var counters = new int[3];
var v = ActionVisitor<Node>.Create()
    .On<Add>(_ => Interlocked.Increment(ref counters[0]))
    .On<Number>(_ => Interlocked.Increment(ref counters[1]))
    .Default(_ => Interlocked.Increment(ref counters[2]))
    .Build();

v.Visit(new Add(...));
v.Visit(new Number(...));
v.Visit(new Neg(...)); // default

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See Also

• Visitor<TBase, TResult> — result‑producing variant
• AsyncActionVisitor<TBase> — asynchronous actions