Add type-safe handler system with phantom-type layers#5
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Add type-safe handler system with phantom-type layers#5
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Introduces four new types in the capability package that let the compiler verify at build time that every capability an effect requires has a handler: * Empty — phantom type representing an empty (no-capability) environment * With<A,B> — phantom type encoding the union of two capability requirements * HandlerEnv<R> — typed wrapper around CapabilityHandler<?> whose phantom parameter R tracks which capabilities are provided; compose with .and() * Layer<RIn,E,ROut> — recipe for building a HandlerEnv<ROut> from a HandlerEnv<RIn>, possibly performing effects during construction; supports horizontal (.and) and vertical (.andProvide) composition * TypedEffect<R,E,A> — thin wrapper around Effect<E,A> that declares R as the required capability environment; .run(env, runtime) only compiles when the caller holds a HandlerEnv<R> that matches TypeSafeHandlerTest covers: single-cap envs, and() composition, associativity, TypedEffect combinators (map/flatMap), pure effects, Layer.succeed, horizontal layer composition (++), vertical layer composition (>>> / andProvide where a layer reads config during construction), end-to-end layer→env→typedEffect integration, and interop escape hatches (fromHandler / HandlerEnv.fromHandler). https://claude.ai/code/session_01TBuJE3hwUJpjr2avujhXbL
…S doc All factory methods that previously accepted ThrowingFunction<C, ?> now use an F-bound to capture the capability's declared result type R: <C extends Capability<?>> → <R, C extends Capability<R>> ThrowingFunction<C, ?> → ThrowingFunction<C, R> Affected methods: CapabilityHandler.Builder.on() HandlerEnv.of() Layer.succeed() Layer.fromEffect() The compiler now verifies that handler lambdas return the exact type R that the capability family declares, with no wildcards at call sites. Also adds docs/TYPED_LAYERS.md, a comprehensive guide to the typed layer system (HandlerEnv, Layer, TypedEffect, Empty, With) with worked examples, a ZIO comparison table, and notes on Java-specific trade-offs. docs/CAPABILITIES.md updated with a pointer to the new guide. https://claude.ai/code/session_01TBuJE3hwUJpjr2avujhXbL
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Summary
Introduces a ZIO-style layer system for roux that uses phantom types to enforce compile-time verification that every capability an effect requires has a corresponding handler. This eliminates the possibility of runtime
UnsupportedOperationExceptiondue to missing handlers — the program simply won't compile if capabilities are unhandled.Key Changes
HandlerEnv<R>— A typed wrapper aroundCapabilityHandlerwhere the phantom type parameterRtracks which capabilities are provided. Supports composition viaand()to merge multiple capability environments.TypedEffect<R, E, A>— A thin wrapper aroundEffect<E, A>that statically declares capability requirements via phantom typeR. Therun()method only compiles when the caller provides aHandlerEnv<R>with matching phantom type.Layer<RIn, E, ROut>— A recipe for building aHandlerEnv<ROut>from aHandlerEnv<RIn>, with support for both horizontal composition (and()— merge outputs) and vertical composition (andProvide()— feed one layer's output into another's input).Phantom type system —
EmptyandWith<A, B>form a type-level set of capabilities:Emptyrepresents no capabilities (base case)With<A, B>represents bothAandBcapabilitiesWith<A, With<B, C>>F-bounded polymorphism — All factory methods (
HandlerEnv.of,Layer.succeed,Layer.fromEffect) use F-bounds (<R, C extends Capability<R>>) to eliminate wildcards and ensure handler return types match capability declarations at compile time.Comprehensive test suite —
TypeSafeHandlerTestdemonstrates single and combined environments, typed effect execution, layer composition (horizontal and vertical), and full end-to-end integration.Documentation — New
TYPED_LAYERS.mdguide covering motivation, phantom types, all three abstractions, composition patterns, and design notes on how Java's approach differs from ZIO.Implementation Details
HandlerEnvis a functional interface backed by the existingCapabilityHandler— zero runtime overheadTypedEffectis a single-field wrapper aroundEffect— erased at runtime.classfile signatures; no instances are ever createdand,andProvide) thread the phantom types correctly through the type systemCapabilityHandlerAPI remains unchanged; new types layer on top for opt-in type safetyhttps://claude.ai/code/session_01TBuJE3hwUJpjr2avujhXbL