Hol4 Development

HOL4 development

This skill captures the conventions for working on HOL4 as a codebase — the part that's the same as any other large SML project, plus the HOL4-specific quirks. It deliberately stops short of theorem proving: when the task is "write a proof of X" or "make this tactic work", a different skill applies. When the task is "make this code build / read / fit the conventions", you're in the right place.

The boundary is fuzzy — you'll often edit *Script.sml files for maintenance reasons without touching the proof logic. That's still development. Use judgement: if the answer to "is this proof correct?" matters, you're past this skill's scope.

Style rules (enforced by tools/h4pedant)

These are checked by the regression machinery, so getting them wrong costs a build later:

• No TAB characters anywhere in source files. Spaces only.
• No trailing whitespace on any line.
• Lines under 80 columns, strongly preferred. Occasional violations are tolerated but unnecessary ones get pushed back on.

These apply to every .sml, .sig, *Script.sml, .ML file in the tree. When editing existing code, match the indentation already present (usually 2-space) rather than introducing a new style.

Building

HOL4 has two layers of build tool:

• bin/build — drives the whole tree. bin/build -F does a full build. bin/build --no-mdbook or bin/build --no-helpdocs skips the documentation pipeline (including the theory-graph regeneration) and is the fast path. bin/build --selftest 1|2|3 runs regression tests at increasing depth. Kernel flags --stdknl (default de Bruijn), --expk (experimental name-type pairs), --otknl (OpenTheory) select which kernel to build against.
• bin/Holmake — directory-level. Compiles *Script.sml to *Theory.{sml,sig}, plus the rest of the directory's SML.

For directories earlier than src/boss in the build sequence, Holmake needs to be told where to look:

• Most early directories: Holmake --holstate=<repo>/bin/hol.state0.
• src/portableML, src/prekernel, src/0, src/thm, src/postkernel, src/parse, src/bool, src/1: use Holmake --poly_not_hol.

If dependencies look stale: bin/build cleanall (whole tree) or Holmake cleanAll (one directory) before rebuilding.

The "core build"

A quick health check used routinely is:

bin/build -t --seq=tools/sequences/upto-parallel

This builds everything up to (but not including) src/parallel_builds/. It's the practical "is the kernel + libraries still healthy?" probe. Use it after any stretch of edits that touches compiled source (.sml, .sig, *Script.sml, theory data) — pure comment edits don't strictly need it, but a quick sanity pass is cheap. The -t flag is short for --selftest=1 (run level-1 selftests along the way); --seq=... selects the directory sequence.

For changes that affect code beyond src/parallel_builds/ (anywhere under examples/ for instance, or any case where you've changed a public API consumed across the tree), prefer the full build bin/build -F — the core build won't catch breakage in the many directories beyond it. Use -F explicitly: bare bin/build reuses the previous invocation's --seq=… (whatever partial sequence you ran last), which can silently skip whole swaths of the tree.

Fresh worktree

A new git worktree won't have bin/build until configured. From the worktree root:

poly < tools/smart-configure.sml

then run bin/build as normal.

Cross-compiler portability

HOL4 supports two SML implementations: Poly/ML (the primary, where day-to-day development happens) and Moscow ML (secondary, kept feature-compatible as far as the implementations allow). The Moscow ML CI check runs on every commit to GitHub, so a Poly/ML-only change that silently breaks mosml will surface quickly.

Working assumption: feature parity where reasonable. When an implementation genuinely can't support something, document the gap rather than abandon the build target — Moscow ML's Holmake doesn't support -j for parallel builds, for example, and mosml users simply live with single-process compilation.

Source-level conventions driven by mosml

The signature-file discipline in the codebase exists largely because of Moscow ML:

• Signatures in *.sig, structures in *.sml, connected with opaque ascription :>. The signature name, structure name, and filename all share one identifier (typically lowercase), so a module foo looks like:

  (* foo.sig *)
  signature foo = sig … end
  
  (* foo.sml *)
  structure foo :> foo = struct … end
  

  Moscow ML enforces this match — Standard-SML mixed casing like structure Foo :> FOO won't compile, and the filename has to agree too.

• For implementation-divergent modules, share a single .sig between Poly/ML and Moscow ML, and provide different .sml bodies under src/portableML/poly/ and src/portableML/mosml/. MLSYSPortable is the central place where this split is mediated.

Build-sequence escape hatch

The build sequence files (tools/sequences/*) can mark whole directories as not applicable to a particular implementation, so the build skips them entirely rather than failing. Use this for code that genuinely can't be made portable — preferable to scattering implementation-specific gates through individual files.

REPL and friends

• bin/hol — standard interactive REPL, loads bossLib.
• bin/hol --bare — minimal, only loads up to boolLib. Useful for reproducing low-level issues without the rest of the prelude in the way.
• bin/hol run script.sml — runs an SML script file for its side effects (loads the standard heap, evaluates the file, exits). The right tool for one-off microbenchmarks, ad-hoc cache inspection, and any "run this SML over the prelude" task that isn't part of the build. See bin/hol --help for the full subcommand list (repl, lsp, buildheap, run, heapname).
• bin/unquote — the source preprocessor that turns HOL4's input syntax into plain SML. Expands quotations (‘…’, “…”) and the modern top-level keywords (Theorem, Definition, Overload, Inductive, …) into the underlying SML calls. Occasionally useful for understanding how a piece of script "really" parses, or for debugging odd interactions between the quotation filter and the surrounding code.

Generated files — don't edit

*Theory.sml and *Theory.sig are generated by Holmake from the matching *Script.sml. Edit the script, not the generated file. Likewise *Theory.dat is build output. If you find yourself wanting to change one of these directly, you're in the wrong place.

Naming conventions

HOL4 has a set of standard SML identifier abbreviations and suffix/prefix conventions. Following them makes new code blend in; ignoring them makes it look transplanted from elsewhere. Use these consistently when introducing new identifiers, and recognise them when navigating existing code:

Variables / arguments:

Prefixes:

Suffixes:

(The _tac/_conv/_rule/_then distinctions matter for navigating code even when you're not writing proofs — foo_tac vs foo_conv vs foo_rule tells you the function's type signature shape before you look at it.)

Commit history

By default, work lands on develop directly, so aim for short sequences of self-contained commits — often one commit per logical unit, occasionally two or three when the split genuinely helps a reviewer. Squash liberally: prefer --fixup + git rebase --autosquash during iteration, and squash the sequence down to its natural shape before signalling ready-to-push. Ten review-fix commits chained onto a single logical change is noise, not history.

The exception is an explicit feature branch that will be merged into develop as a series — those benefit from a longer, curated sequence. The developer will call this out when it applies; unless they have, assume commits are going onto develop and keep the sequence tight.

Repo geography

• src/ — the core system. Most maintenance work happens here.
• examples/ — extensive worked examples and downstream developments. Quality is variable and uneven across the tree.
• tools/ — build infrastructure (Holmake, h4pedant, sequence files, etc.) and supporting libraries.
• Manual/ — user-facing documentation, including Manual/Developers/ which is the authoritative source for some conventions.
• developers/ — one-off scripts, microbenchmarks, dev tooling. Not part of the build. Standalone .sml scripts here are typically run via bin/hol run developers/<script>.sml.
• release-notes/next-release.md — the changelog that becomes the release notes for the next HOL4 release. User-visible source changes belong here, under one of: New features, Bugs fixed, New theories, New tools, New examples, Incompatibilities, Deprecations. Performance fixes go under Bugs fixed; API signature changes that callers must adapt to go under Incompatibilities. When in doubt, glance at the existing entries for tone and granularity.

Directories explicitly out of scope for code-quality work

These three trees under examples/ are unmaintained / abandoned. Tree-wide quality work (stale-comment scans, dead-code cleanup, FIXME triage, etc.) should exclude them:

• examples/acl2/
• examples/dev/
• examples/HolCheck/

When running tree-wide quality scans, exclude these at the top. When triaging existing scan results, drop findings whose path begins with one of them without further investigation. (A real build breakage in one of these dirs is still worth fixing — but that's a different category of work.)

When something doesn't build

The propagation chain is well-understood: Holmake exits with OS.Process.exit, bin/build's aug_systeml reads the exit status, and buildutils.die (which produces the *** FATAL: … message) calls OS.Process.exit OS.Process.failure. So bin/build exits non-zero on real failure.

If a report comes in that "bin/build exited 0 despite *** FATAL: … in the log", first suspect the reporter's shell pipeline: cmd 2>&1 | tee log; echo $? reports tee's status, not cmd's. Ask them to run bin/build … > out 2>&1; rc=$? and re-check.

What this skill is not for

• Writing proofs — choosing tactics, structuring a proof, debugging a stuck goal. That's theorem-proving work, separate skill.
• Theory design — deciding what to define, what lemmas matter. Same.
• Non-HOL4 SML — these conventions are HOL4-flavoured (mk_/dest_, bossLib, the build commands). If the SML in front of you isn't part of an HOL4 checkout, this skill's specifics don't apply.