steve-downey / transpose / 28752701823

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Committed by steve-downey

Commit Message

Applicative: reify invoke as the core; apply becomes derived/alternate

Dual core, GHC-style: an Impl provides pure + (invoke | apply) and the base
derives the missing operation. invoke -- a plain function applied to N
arguments each in a context -- is the conceptual primary: McBride &
Paterson's canonical form pure f <*> u1 <*> ... <*> un IS invoke(f, u1...un),
and GHC's MINIMAL pure, ((<*>) | liftA2) with (<*>) = liftA2 id is the same
contract. apply-from-invoke is invoke(applicative_eval, cf, cx), available
exactly when the context can hold a callable; invoke-from-apply remains the
terminating-partial compatibility derivation.

- apply.hpp: applicative_eval; base no longer requires Impl::apply; derived
  apply dispatches-or-derives with Impl-direct calls (no derivation cycle);
  ap constrained so probing it on an invoke-only context is a clean false;
  neither-core misuse diagnosed by static_assert.
- optional, sender, Identity: migrated to native n-ary invoke; their apply
  is now the base-derived one (flagship (<*>) = liftA2 id proof).
- zip_list, simd_lanes, array: native invoke exposed via Map usings
  (dual-core coherence subjects).
- BinaryTree: deliberately kept apply-primitive as the compatibility-path
  exemplar (documented).
- monad.hpp: n-ary invoke synthesized from bind + pure.
- NEW simd.hpp: registered applicative for std::simd::basic_vec (C++26,
  gated) -- pure = broadcast, invoke = lane-wise generator; apply/ap do not
  exist and cannot be derived (no vec<callable>), verified by
  STATIC_REQUIRE_FALSE. An Applicative that is not Traversable.
- Law tests rewritten invoke-first (identity, generalized homomorphism,
  interchange, functor-composition) with named-concept probes; dual-core
  coherence checks; NEW simd_lanes.test.cpp and gated simd.test.cpp.

Verified: gcc-debug/gcc-release presets (C++23, 55 tests) and gcc-16 C++26
(57 tests incl. simd) all pass; pre-commit clean.

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