Troubleshooting

query-prolog returned (nil)

That means the query succeeded and projected no variables.

(query-prolog rulebase '(parent tom bob))
;; => (nil)

Success with variables returns binding alists. Failure returns nil.

query-prolog-first returned nil

That means no proof succeeded.

If you need the first successful binding only, this is the right entry point. If you need every solution, use query-prolog.

solution-binding returned nil

The variable may be unbound in that solution, or the variable may not exist in the projected environment.

Inspect the whole solution first:

(first (query-prolog rulebase '(ancestor tom ?who)))

unify returned nil, nil

unify returns two values.

• first value: extended environment on success, or nil on failure
• second value: success flag

Failure is (values nil nil). Ground success is (values nil t), so callers must inspect the second value rather than treating a nil environment as failure.

:max-depth 0 rejected a derived rule

That is expected. Depth limits apply to user-rule expansion, and exhaustion signals prolog-depth-limit-exceeded so an incomplete search is never reported as logical failure.

• 0: facts and built-ins only
• 1: one rule expansion
• larger values: deeper derived proofs

A foreign predicate did not run

define-foreign-predicate dispatches by exact name and arity. Check both parts of the predicate indicator. Builtins remain authoritative when names overlap.

phrase returned nil

This has two meanings depending on the grammar:

• full token consumption on success
• no parse when the caller expected a non-nil remainder

Use phrase-all when you need to inspect every successful remainder stream.

Nix checks fail only in a clean source tree

Check whether the file is tracked in git. The release checks intentionally validate the tracked tree, not only the working directory.

Script entry point cannot find the repository root

Confirm:

• you are running the script with sbcl --script
• you are invoking it from inside the repository checkout
• the script has not been copied away from the tree it expects to load

Direct smoke:

nix run .

The packaged Nix runner is Linux-only. On Darwin, skip this smoke step and use ASDF directly for local library work.

What To Include In A Bug Report

• exact query or macro form
• expected result
• actual result
• SBCL version
• output of nix run . when reproducing on Linux
• output of nix flake check --print-build-logs