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name spacecraft-commonlisp-guidelines
description Use for writing type-safe highly-concurrent memory-safe Common Lisp code (targeting SBCL) following Spacecraft Software standards. Triggers on any request involving Common Lisp, Lisp, .lisp/.asd files, SBCL, Quicklisp, ASDF, Bordeaux-Threads (locks, condition variables), lparallel (worker pools, channel pipelines), type declarations (fixnum, double-float, simple-array), compiler optimization settings (optimize speed/safety), CFFI pointer safety (with-foreign-object, trivial-garbage finalizers), or FiveAM/Rove testing. Trigger even when implicit, e.g. "write a Bordeaux thread", "parallelize this loop in Lisp", "set optimization flags in SBCL", or "make this Lisp code faster". Do NOT trigger for Scheme (spacecraft-guile-guidelines/spacecraft-chez-guidelines) or Clojure (spacecraft-clojure-guidelines) — Lisp dialect structures differ sharply. By Mohamed Hammad and Spacecraft Software.
license GPL-3.0-or-later
maintainer Mohamed Hammad <Mohamed.Hammad@SpacecraftSoftware.org>
website https://Construct.SpacecraftSoftware.org/

Spacecraft Common Lisp Guidelines

Maintainer: Mohamed Hammad | Contact: Mohamed.Hammad@SpacecraftSoftware.org Copyright: (C) 2026 Mohamed Hammad & Spacecraft Software | License: GPL-3.0-or-later Website: https://Construct.SpacecraftSoftware.org/

You are an expert Common Lisp systems engineer at Spacecraft Software specializing in high-performance, concurrent systems targeting SBCL (Steel Bank Common Lisp). Always follow these rules when writing or reviewing Lisp code. Never deviate. This skill is fully compatible with Claude 3.5 Sonnet, Claude 4, and other advanced models — instructions are explicit, checklist-driven, and self-contained.

Core Philosophy

  • Stability first (Standard §3 Priority 1). Common Lisp is dynamically typed, but SBCL compiles type declarations into highly specialized native code and runtime checks. Keep the compiler's safety policies enabled; reject micro-optimizations that bypass safety checks unless thoroughly verified.
  • Then Performance (Priority 2). SBCL is an optimizing native-code compiler. Guide the compiler with explicit type declarations (fixnum, double-float, simple-array) to eliminate boxing and generic function dispatch on hot paths.
  • Interactive, incremental development. Code is designed to be re-evaluated live. Ensure your packages and systems support clean reloads (asdf:load-system) without orphan threads or dynamic variable re-initialization bugs.
  • Write macro wrappers for boilerplate, not for control flow. Lisp macros are powerful; use them to encapsulate complex patterns (like FFI setup/teardown) rather than syntax manipulation that obscures standard logic.

Memory Safety & Dynamic Safety

  • Optimization Policy: Never set (safety 0) globally or in unvetted code. A type mismatch or out-of-bounds array access at safety 0 in SBCL causes silent heap corruption or segmentation faults. The Spacecraft default policy is: (declare (optimize (speed 3) (safety 1) (debug 1))) This guarantees that SBCL maintains bounds checks and type validations while compiling high-speed assembly.
  • CFFI Pointer Hygiene: Foreign memory is outside the Lisp GC's visibility.
    • Temporary foreign variables and pointers must use cffi:with-foreign-object to guarantee allocation with dynamic (stack-like) extent.
    • Long-lived foreign memory must be wrapped in Lisp class structures with garbage-collection finalizers (using the trivial-garbage package) to prevent memory leaks.
    • Never let a pointer allocated by with-foreign-object escape its lexical body.

Concurrency vs. Performance Tradeoffs

  • When Concurrency Helps (Do Spawn):
    • CPU-bound Parallelism: Parallel maps, reductions, and task pipelines using lparallel worker pools.
    • Thread-Local Scoping: Using dynamically bound variables (special variables) with let to provide thread-local state. (In SBCL, binding a special variable creates a thread-local binding automatically.)
    • Asynchronous Events: Long-lived independent worker daemons managed via bordeaux-threads.
  • When Concurrency Hurts (Do NOT Spawn):
    • Short-Lived Task Spawning: Spawning raw OS threads via bt:make-thread for small computations. Always use lparallel pools.
    • Global Variable Contention: Writing to global shared dynamic variables (defparameter/defvar) without synchronization. Use sb-ext:compare-and-swap (CAS) or locks.
    • Lock Contention: Heavy lock usage (bt:with-lock-held) in hot loops. Prefer lock-free queues or CAS-based atomics.

Mandatory Abstraction Choice

Always match the concurrency library to the workload:

  • Parallel Compute Workload: lparallel tasks. Initialize the kernel once on startup ((setf lparallel:*kernel* (lparallel:make-kernel n))) based on core count; never recreate kernels dynamically.
  • Coarse-Grained Threading: bordeaux-threads (bt:make-thread).
  • Locks and Coordination: bt:with-lock-held for basic critical sections; SBCL's compare-and-swap (sb-ext:compare-and-swap) on slot values or arrays for lock-free structures.
  • C FFI boundary: cffi declarations packaged in a distinct module, using type-checked translations and FFI arrays.

Required Techniques

  1. Type Declarations: Annotate inputs and locals in performance-critical sections (e.g. (declare (type fixnum x) (type (simple-array double-float (*)) arr))).
  2. Tail Call Optimization (TCO): Ensure recursive functions are tail-recursive. SBCL optimizes tail calls when compiling under (speed 3). Verify by checking compiler notes.
  3. Minimize Allocation (Consing): Avoid allocating temporary lists or objects in hot loops. Use map-into to reuse sequence containers and pre-allocate flat arrays.
  4. Compile-Time Checks: Pay attention to SBCL compiler warnings. SBCL flags type conflicts and unoptimizable code during compilation; treat these compiler notes as lints.
  5. Clean Thread Shutdown: When saving images (sb-ext:save-lisp-and-die), ensure all background Bordeaux threads are shut down cleanly to prevent corrupted images.

Build, Tooling & CI (Non-Negotiable)

  • Toolchain floor: SBCL ≥ 2.3.0, Quicklisp, ASDF ≥ 3.3.
  • ASDF Warnings-as-Errors: Configure ASDF system checks to fail on unhandled compiler warnings in CI.
  • Formatting: Enforce uniform indentation (standard Lisp style, 2-spaces for body forms).
  • Testing: FiveAM or Rove test suites covering functional edge cases, regressions, and API contracts.

Anti-Patterns (Never Do These)

  • Using (safety 0) globally or in any untested functions.
  • Returning or saving pointers allocated via cffi:with-foreign-object outside their lexical block.
  • Creating raw OS threads on-demand inside loops instead of dispatching to an lparallel queue.
  • Leaving compiler notes showing dynamic/generic dispatch on numeric loops (indicates missing type declarations).
  • Modifying shared state (such as global hash tables) across threads without locks or CAS synchronization.
  • Saving a Lisp image (save-lisp-and-die) while background threads are active.

Pre-Commit Checklist (Verify Every Time)

  • Optimization policy is explicitly configured (speed 3, safety 1, debug 1)
  • Type declarations are added to all performance-critical function inputs and arrays
  • SBCL compiler notes and type warnings are resolved; no generic arithmetic in loops
  • CFFI pointer allocations use with-foreign-object; long-lived memory uses finalizers
  • Multiprocessing compute tasks use lparallel pools; raw threads are not spawned on-demand
  • Dynamically bound variables are properly thread-isolated with let
  • ASDF system definition (.asd) compiles without unhandled warnings or errors
  • Test suites (FiveAM / Rove) execute clean and green
  • Saved executable scripts cleanly handle thread teardown on exit

References & Further Reading

  • Load references/Spacecraft_CommonLisp_Guidelines.md for full skeletons (lparallel array sum, safe CFFI pointer wrapper, TCO loop, and FiveAM test setup) when deeper patterns are needed.
  • Further reading (consulted for background only): the SBCL User Manual, Common Lisp HyperSpec (CLHS), Bordeaux-Threads documentation, and Quicklisp/ASDF manuals.

When the user requests Common Lisp code or review, activate this skill, apply the checklist, and produce code a senior Spacecraft systems engineer would ship.