lux/docs/ROADMAP.md

Lux Development Roadmap

Targeting practical use cases in priority order.

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Feature Status Summary

Schema Evolution

Component	Status
Parser (@v1, @v2, from @v1 = ...)	✅ Complete
AST representation	✅ Complete
Runtime versioned values	✅ Complete
Schema registry & compatibility checking	✅ Complete
Basic migration execution	✅ Complete
Type system integration	⚠️ Partial (versions ignored in typechecker)
Auto-migration generation	❌ Missing
Serialization/codec support	❌ Missing

Behavioral Types

Component	Status
Parser (is pure, is total, etc.)	✅ Complete
AST & PropertySet	✅ Complete
Pure function checking (no effects)	✅ Complete
Total verification	❌ Missing
Idempotent verification	❌ Missing
Deterministic verification	❌ Missing
Where clause enforcement	❌ Missing

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Use Case 1: Backend Services with Complex Business Logic

Value Proposition: Effect tracking shows exactly what each function does. Testing is trivial.

Phase 1.1: Make It Buildable (Current Blockers)

Task	Priority	Effort	Status
Generic type parameters	P0	—	✅ Complete
String interpolation	P1	—	✅ Complete
File effect (read/write)	P1	—	✅ Complete
HTTP effect (client)	P1	—	✅ Complete
JSON parsing/serialization	P1	—	✅ Complete

Phase 1.2: Database & Persistence

Task	Priority	Effort	Status
SQL effect (query, execute)	P1	2 weeks	❌ Missing
Connection pooling	P2	1 week	❌ Missing
Transaction effect	P2	1 week	❌ Missing

Phase 1.3: Web Server Framework

Task	Priority	Effort	Status
HTTP server effect	P1	2 weeks	✅ Complete
Routing DSL	P2	1 week	❌ Missing
Middleware pattern	P2	1 week	❌ Missing
Request/Response types	P1	3 days	✅ Complete (via HttpServer effect)

Phase 1.4: Developer Experience

Task	Priority	Effort	Status
Elm-quality error messages	P1	2 weeks	⚠️ Partial (context shown, suggestions missing)
Full JS compilation	P2	4 weeks	⚠️ JIT for numeric code only
Hot reload / watch mode	P2	—	✅ Complete
Debugger improvements	P3	2 weeks	✅ Basic
JIT CLI integration	P1	—	✅ Complete (lux compile)

Success Criteria for Use Case 1

• Can build a REST API with database access
• Can swap database handler for testing without mocks
• Effect signatures document all side effects
• Compile-time guarantees prevent common bugs

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Use Case 2: High-Reliability Systems

Value Proposition: Behavioral types provide compile-time guarantees (is total, is idempotent).

Phase 2.1: Complete Behavioral Type Verification

Task	Priority	Effort	Status
Total function verification	P1	2 weeks	❌ Missing
Idempotent verification	P1	2 weeks	❌ Missing
Deterministic verification	P1	1 week	❌ Missing
Where clause enforcement	P1	1 week	❌ Missing

Implementation approach:

• Total: Restrict to structural recursion, require termination proof for general recursion
• Idempotent: Pattern-based (setter patterns, specific effect combinations)
• Deterministic: Effect analysis (no Random, Time, or non-deterministic IO)

Phase 2.2: Refinement Types (Stretch Goal)

Task	Priority	Effort	Status
Basic refinements (Int where self > 0)	P2	3 weeks	❌ Missing
SMT solver integration (Z3)	P3	4 weeks	❌ Missing
Contract checking	P3	2 weeks	❌ Missing

Phase 2.3: Fault Tolerance Patterns

Task	Priority	Effort	Status
Retry effect with backoff	P2	1 week	❌ Missing
Circuit breaker pattern	P2	1 week	❌ Missing
Supervision trees (Elixir-style)	P3	3 weeks	❌ Missing

Success Criteria for Use Case 2

• Compiler verifies is total functions always terminate
• Compiler verifies is idempotent functions are safe to retry
• Can build payment processing with idempotency guarantees
• Effect system enables fault isolation

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Use Case 3: Data Pipelines / ETL Systems

Value Proposition: Schema evolution handles changing data formats gracefully.

Phase 3.1: Complete Schema Evolution

Task	Priority	Effort	Status
Type system version tracking	P1	1 week	⚠️ Partial
Auto-migration generation	P1	2 weeks	❌ Missing
Version compatibility errors	P1	1 week	❌ Missing
Migration chain optimization	P2	1 week	⚠️ Basic

Phase 3.2: Serialization Support

Task	Priority	Effort	Status
JSON codec generation	P1	2 weeks	❌ Missing
Version-aware deserialization	P1	1 week	❌ Missing
Binary format support	P3	2 weeks	❌ Missing
Avro/Protobuf interop	P3	3 weeks	❌ Missing

Phase 3.3: Stream Processing

Task	Priority	Effort	Status
Stream effect	P2	2 weeks	❌ Missing
Batch processing patterns	P2	1 week	❌ Missing
Backpressure handling	P3	2 weeks	❌ Missing

Success Criteria for Use Case 3

• Can define versioned types with automatic migrations
• Compiler catches version mismatches
• Can deserialize old data formats automatically
• Can process data streams with effect tracking

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Use Case 4: Teaching Functional Programming

Value Proposition: Effects are more intuitive than monads for learning.

Phase 4.1: Educational Materials

Task	Priority	Effort	Status
Beginner tutorial (effects intro)	P1	1 week	⚠️ Basic
Interactive examples	P2	1 week	❌ Missing
Comparison guides (vs Haskell monads)	P2	3 days	❌ Missing
Video tutorials	P3	2 weeks	❌ Missing

Phase 4.2: REPL Improvements

Task	Priority	Effort	Status
Step-by-step evaluation	P2	1 week	❌ Missing
Effect visualization	P2	2 weeks	❌ Missing
Type hole support (_ placeholders)	P2	1 week	❌ Missing

Phase 4.3: Error Message Excellence

Task	Priority	Effort	Status
Elm-style error catalog	P1	2 weeks	❌ Missing
"Did you mean?" suggestions	P1	1 week	❌ Missing
Example-based hints	P2	1 week	❌ Missing
Beginner-friendly mode	P3	1 week	❌ Missing

Success Criteria for Use Case 4

• New FP learner can understand effects in < 1 hour
• Error messages guide toward solutions
• Clear progression from pure functions to effects to handlers

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Cross-Cutting Concerns (All Use Cases)

Ecosystem

Task	Priority	Effort	Status
Package manager (lux pkg)	P1	3 weeks	⚠️ Basic
Package registry	P2	2 weeks	❌ Missing
Dependency resolution	P2	2 weeks	❌ Missing

Tooling

Task	Priority	Effort	Status
LSP completions	P1	1 week	⚠️ Basic
LSP go-to-definition	P1	1 week	⚠️ Partial
Formatter	P2	—	✅ Complete
Documentation generator	P2	1 week	❌ Missing

Compilation

Task	Priority	Effort	Status
Extend JIT (strings, lists)	P1	2 weeks	❌ Missing
JS backend	P2	4 weeks	❌ Missing
WASM backend	P3	4 weeks	❌ Missing

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Recommended Implementation Order

Quarter 1: Foundation ✅ COMPLETE

1. Generic type parameters ✅ Done
2. String interpolation ✅ Done
3. File effect ✅ Done
4. HTTP client effect ✅ Done
5. JSON support ✅ Done
6. Elm-quality errors — ⚠️ In progress

Quarter 2: Backend Services (Use Case 1)

7. HTTP server effect — Build APIs
8. SQL effect — Database access
9. Full JS compilation — Deployment
10. Package manager — Code sharing

Quarter 3: Reliability (Use Case 2)

11. Behavioral type verification — Total, idempotent, deterministic
12. Where clause enforcement — Type-level guarantees
13. Schema evolution completion — Version tracking in types
14. Auto-migration generation — Reduce boilerplate

Quarter 4: Polish (Use Cases 3 & 4)

15. Serialization codecs — Data pipelines
16. Educational materials — Teaching
17. Refinement types — Advanced guarantees
18. Stream processing — ETL use case

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What's Already Done (Often Forgotten)

Core Language:

• ✅ Generic type parameters (fn map<T, U>, type List<T>)
• ✅ String interpolation ("Hello {name}!")
• ✅ Pattern matching with exhaustiveness checking
• ✅ Algebraic data types
• ✅ Type inference (Hindley-Milner)
• ✅ Tail call optimization

Effect System:

• ✅ Effect declarations and handlers
• ✅ Console effect (print, readLine, readInt)
• ✅ File effect (read, write, exists, delete, listDir)
• ✅ HTTP effect (get, post)
• ✅ Random effect (int, float, range, bool)
• ✅ Time effect (now, sleep)

Module System:

• ✅ Imports, exports, aliases
• ✅ Selective imports (import foo.{a, b})
• ✅ Wildcard imports (import foo.*)
• ✅ Circular dependency detection

Standard Library:

• ✅ String operations
• ✅ List operations
• ✅ JSON parsing/serialization

Tooling:

• ✅ JIT compiler (~160x speedup, CLI: lux compile)
• ✅ REPL with history
• ✅ Basic LSP server
• ✅ Formatter
• ✅ Watch mode
• ✅ Debugger (basic)

Advanced (Parsing Only):

• ✅ Schema evolution (parsing, runtime values)
• ✅ Behavioral types (parsing, pure checking only)

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Success Metrics

Use Case 1: Backend Services

• Can build and deploy a production API
• 10+ companies using Lux for backends

Use Case 2: High-Reliability

• Compiler catches idempotency violations
• Used in fintech/healthcare context

Use Case 3: Data Pipelines

• Schema migrations happen automatically
• Zero data loss from version mismatches

Use Case 4: Teaching

• Featured in FP courses
• "Effects finally make sense" testimonials