Commits on Mar 15, 2026

1. Add higher-kinded types (HKTs) — v0 proof of concept

   Introduce first-class support for abstracting over type constructors via
   kind annotations in type parameter syntax: `<F : * -> *>`.
   
   This enables writing generic abstractions like Functor and Monad that
   work across Array, Promise, user-defined types, and any type constructor
   of the right kind — eliminating the need for defunctionalization hacks.
   
   What works:
   - Kind annotation syntax parsed from `<F : * -> *>` (no new tokens)
   - `Functor<Array>`, `Monad<Box>` — concrete and user-defined constructors
   - `F<A>` type application resolves to `Array<A>` when F = Array
   - Monad<F> extends Functor<F> — HKT params forwarded through inheritance
   - Generic functions: lift, when, sequence all typecheck
   - Type inference: `boxMonad.pure(42)` infers `Box<number>`
   - Invariant assignability for abstract constructor applications
   - Arity checking: `F<A, B>` errors when `F : * -> *`
   - Full backward compatibility with existing TypeScript code
   - 12 test cases passing in the standard test suite
   
   Compiler changes:
   - parser.ts: parseKindAnnotation() for `* -> *` syntax
   - types.ts: kindArity on TypeParameterDeclaration and TypeParameter;
     hktConstructorSymbol and hktTypeArguments on Type
   - checker.ts: TypeConstructorRef and HKTApplicationType representations
     (both piggyback on TypeFlags.Substitution); HKT-aware type argument
     resolution, instantiation, assignability, and inference
   
   Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

   

   and claude committed

   Mar 15, 2026
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2. Add HKT playground — interactive browser demo

   Self-contained HTML page that loads the custom TypeScript compiler and
   provides a live type-checking editor with example buttons for Functor,
   Monad, Sequence, and error cases.
   
   Serve from repo root: python3 -m http.server 8080
   Open: http://localhost:8080/hkt-playground.html
   
   Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

   

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3. Upgrade HKT playground to Monaco editor

   Replace textarea with Monaco editor for full IDE experience: syntax
   highlighting, bracket matching, error squigglies, line numbers, and
   click-to-navigate on diagnostic locations.
   
   Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

   

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4. Add custom hover provider using HKT-aware compiler

   Replace Monaco's built-in TypeScript hover (which shows `any` for HKT
   types) with a custom hover provider that queries our compiler's type
   checker. Hovering over `fa: F<A>` now shows the correct type from our
   HKT-aware checker, and type parameters with kind annotations display
   their kind (e.g., `F : * -> *`).
   
   Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

   

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5. Change kind syntax to uncurried form with tuple parameters

   Replace Haskell-style curried kind arrows with explicit tuple syntax:
   - `* -> *` for one-arg constructors (unchanged)
   - `(*, *) -> *` for two-arg constructors (was `* -> * -> *`)
   - `(*) -> *` is sugar for `* -> *`
   - `(* -> *) -> *` parses but errors: "Higher-order kinds are not yet
     supported"
   
   The parser now handles a full kind grammar with parens, commas, and
   right-associative arrows. Internally, a KindNode tree is stored on
   the AST alongside the simple kindArity count. The checker validates
   that all kind parameters and return are `*`, rejecting higher-order
   kinds with a clear diagnostic (TS2900).
   
   Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

   

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6. Support higher-order kinds: (* -> *) -> * and beyond

   Remove the "not yet supported" restriction on higher-order kinds.
   Type constructors can now take other type constructors as parameters:
   
     type ApplyToArray<F : (* -> *) -> *> = F<Array>
     interface Wrapper<F : * -> *> { wrap<A>(a: A): F<A> }
   
   Changes:
   - Store full KindNode tree on TypeParameter (alongside kindArity)
   - Add kind utilities: kindsMatch(), getKindOfType(), getKindOfSymbol(),
     kindToString() for recursive kind comparison
   - resolveTypeConstructorArgument now uses full kind matching instead
     of simple arity equality
   - isHKTTypeArgumentContext handles TypeParameter parents (F<Array>
     where F is itself a higher-kinded parameter)
   - Force-resolve type alias symbols in isHKTTypeArgumentContext to
     ensure typeParameters are populated
   
   Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

   

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8. Add Bifunctor and Higher-Order examples to playground

   New example tabs showing (*, *) -> * with Pair/Bifunctor and
   (* -> *) -> * with ApplyToNumber, NaturalTransformation, and
   nested kind aliases.
   
   Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

   

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9. Improve Higher-Order example with Nat composition

   Show natural transformations (Nat<F, G> where F, G : * -> *) and
   composition of natural transformations (composeNat with three * -> *
   params), demonstrating genuine higher-kinded programming.
   
   Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

   

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