Dummies: add exclusion constraints to AnyString (DifferentFrom / Except / OneOf)

[Reefact]

Problem

AnyString is the only scalar builder with no exclusion constraints — no OneOf, no Except, no DifferentFrom. Every other scalar builder (numerics, temporal, AnyChar, AnyGuid, AnyEnum) has the exclusion trio, and DifferentFrom is documented everywhere as carrying the intent "a value different from the one I already hold" — one of the most common dummy-string needs (testing equality/inequality paths with string identifiers).

// Today — no way to express "a name different from an existing one":
string other = Any.String().NonEmpty().Generate();     // might equal existing!
// Proposed:
string other = Any.String().NonEmpty().DifferentFrom(existing).Generate();

Impact

The most-used builder has the largest hole in the constraint matrix, forcing users back to hand-rolled retry loops for a very common scenario — the kind of ad-hoc code the library exists to replace.

Direction

Strings are not ordinal-mapped, so exclusions cannot ride the interval engine (unlike numerics). Two viable implementations, both consistent with existing patterns:

• a bounded redraw after layout that rejects an excluded result (expected collisions ≈ 0 for any non-trivial spec — the same bounded-escape pattern the distinct collections and continuous nudge already use, with a clear AnyGenerationException if the budget is exhausted, e.g. WithLength(1).Alpha() excluding most letters); or
• spec-aware avoidance where cheap.

Add DifferentFrom(string), Except(params string[]), and OneOf(params string[]) to AnyString/StringSpec (the last is partly subsumed by a top-level Any.OneOf but is expected on the builder for symmetry). OneOf combined with other shape constraints should validate eagerly like the other builders.

Acceptance criteria

• Any.String()…DifferentFrom(x).Generate() never returns x; Except(...) excludes each listed value; OneOf(...) draws from the listed set.
• An over-tight exclusion that leaves the spec unsatisfiable fails with a clear, seed-bearing AnyGenerationException (bounded, never an unbounded loop).
• Seeded reproducibility preserved; mirrored behavior documented.
• Tests cover exclusion under length/prefix/charset constraints and the exhaustion path.

Context

Surfaced by the 2026-07-20 architecture & design audit of the standalone Dummies library (doc/handwritten/for-maintainers/audit/2026-07-20-dummies-architecture-and-design-audit.md, §8.2/§10 Must-Have; branch claude/dummies-architecture-audit-cru5aq).

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Filed from the Claude Code session that produced the 2026-07-20 Dummies architecture & design audit: https://claude.ai/code/session_01GvrWFVoG74xiys3oyEft1G

[Reefact]

Addressed in #283.

A note on scope, since this issue's checklist has partly aged out:

• OneOf is already shipped as a terminal generator (Any.String().OneOf(...) → AnyStringOneOf, ADR-0030), so it is not re-added by feat(dummies): add DifferentFrom/Except exclusions to AnyString #283. It stays terminal — the "OneOf combined with other shape constraints should validate eagerly" line no longer matches the delivered design (it conflicts eagerly instead, which is the right call for an unbounded domain). The remaining scope was therefore just the exclusion pair.
• DifferentFrom(string) / Except(params string[]) are added. Since strings are not ordinal-mapped, exclusions cannot ride the interval engine, so they are met by a bounded redraw of the constructive layout (expected collisions ≈ 0 for any non-trivial shape) — the same escape distinct collections use (ADR-0013).
• Unsatisfiability is deferred, not eager. An over-tight exclusion (e.g. WithLength(1).Alpha().Except(<every letter>)) fails at Generate() with a bounded, seed-bearing AnyGenerationException, per the acceptance criteria. A partial eager check was rejected as an inconsistent seam (string satisfiability under exclusion is not cheaply decidable in general); this trades away the "an AnyString that exists can always generate" invariant for exclusions only, documented in the readme and StringSpec.
• The empty string is a valid value to exclude; a null element is not.

This is a public-API contract change on the most-used builder, so it is recorded as ADR-0032 (Proposed) — sibling to ADR-0013. Status left for @Reefact.

Tests: the exclusion pair, shape-preserving exclusion, accumulation, the seeded-reproducibility and exhaustion paths, and the OneOf-stays-terminal conflict are covered; full solution green (1442 tests).

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Generated by Claude Code

[Reefact]

Correction to my note above: the ADR is ADR-0033, not 0032. #223 landed its own ADR-0032 (the top-level Any.OneOf/ElementOf decision) on main first, so this branch's ADR was renumbered to 0033. Everything else stands; the up-to-date change is in #283.

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Generated by Claude Code