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Seq Development Roadmap

Core Values

The fast path stays fast. Observability is opt-in and zero-cost when disabled. We can’t slow the system down to monitor it.

Inspired by the Tokio ecosystem (tokio-console, tracing, metrics, tower), we aspire to rich runtime visibility while respecting performance.

Current (v5.0)

Union Type Safety

Compile-time safety for union types (RFC #345). The compiler auto-generates type-safe constructors, predicates (is-Get?), and field accessors (Get-response_chan) for all union variants.

Error Handling Standardization (v3.0)

All fallible operations return (value Bool) instead of panicking. Division, TCP, regex, and other operations now consistently use this pattern.

Foundation (Complete)

These features are stable and documented:

FeatureDetails
Naming conventionsDot for namespaces, hyphen for compounds, arrow for conversions
OS moduleos.getenv, os.home-dir, os.path-*, args.count, args.at, os.exit, os.name, os.arch
FFIManifest-based C bindings, string marshalling, out parameters. Examples: libedit, SQLite
Runtime observabilitySIGQUIT diagnostics, watchdog timer, strand/channel/memory stats
Yield safety valveAutomatic yields in tight loops to prevent strand starvation
LSP serverLanguage server with completions, hover, diagnostics. Powers TUI and editor integrations
TUI REPLDefault REPL with split-pane IR visualization, Vi editing, tab completion
Union types & matchunion definitions, match expressions, exhaustiveness checking, named bindings
ClosuresLexical capture with Arc-shared environments, TCO-compatible
Channels as valuesFirst-class Channel type on the stack (no ID-based lookup)
WeavesGenerator/coroutine pattern with bidirectional communication
Lists & MapsFirst-class collection types with list.map, list.filter, list.fold, map.*
Symbols:foo syntax for lightweight identifiers, used for variant tags
Lint toolseqc lint with TOML-based syntactic pattern matching

Future

Strand Visibility

Strand lifecycle events (opt-in):

  • Parent-child relationships for debugging actor hierarchies
  • Blocked strand detection (who’s waiting on what)
  • Optional compile-time flag to enable

Metrics & Tracing

Metrics export:

  • Prometheus-compatible endpoint
  • Strand pool utilization
  • Message throughput sampling
  • Configurable sampling rates to control overhead

Structured tracing:

  • Integration with tracing ecosystem
  • Span-based request tracking across strands
  • Correlation IDs for distributed debugging

Visual Tooling

Seq console (inspired by tokio-console):

  • Real-time strand visualization
  • Channel flow graphs
  • Actor hierarchy browser
  • Historical replay for post-mortem debugging

OpenTelemetry integration:

  • Distributed tracing across services
  • Standard observability pipeline integration

FFI Phase 3

  • Struct passing
  • Platform-specific bindings
  • Callback support (C -> Seq) - shelved: most useful callback patterns require low-level memory operations; many C APIs have non-callback alternatives

Type System Research

Goal: Achieve the safety benefits of generics without sacrificing point-free composability or adding syntactic overhead.

Seq’s philosophy: type safety through inference, not annotation.

Current state:

  • Row-polymorphic stack effects provide implicit type threading
  • Union types with nominal typing and auto-generated accessors (v4.0)
  • (value Bool) error handling pattern (v3.0)

Research directions:

  1. Inferred variant types - Compiler tracks that Make-Ok produces a specific union type
  2. Flow typing through combinators - If result-bind receives IntResult, infer the quotation expects Int
  3. Structural typing for conventions - Recognize Result-like patterns at compile time
  4. Constructor argument refinement - 42 Make-Ok infers IntResult from the Int argument

Key question: How far can we push implicit typing before explicit annotations become necessary?

Constraint: Must not compromise point-free style or add syntactic noise.

Design Documents