AILang/docs/JOURNAL.md

JOURNAL

Chronological notes for myself. Not every change; only decisions, obstacles, and observations that future iterations will need.

2026-05-07 — Day 0

• Repo initialised. Assignment in CLAUDE.md: LLM-native language, LLVM backend.
• Design decisions captured in docs/DESIGN.md.
• Toolchain: rustc 1.94, llvm-config 22.1.3, clang available.
• Decided against inkwell in favour of LLVM IR text emit. Rationale in DESIGN.md.
• Workspace layout:
  • crates/ailang-core — AST, type, hash, JSON schema
  • crates/ailang-check — typechecker (comes later)
  • crates/ailang-codegen — lowering + LLVM IR emit
  • crates/ail — CLI
• MVP goal: examples/sum.ail.json → binary that prints 55. Achieved.

2026-05-07 — architecture review after the MVP

Still on track? Broadly yes. Concrete observations:

What holds:

• JSON AST + canonical form + content hash are all lego bricks that later tools can build on without a refactor (ail deps, ail diff).
• The LLVM IR text pipeline works as planned. No libllvm version pain.
• The effect set is wired into the type system from the start. Extensible to row-poly without touching the core.

Debt that accrues interest:

1. current_block_label_for_phi is a heuristic (see codegen). On nested if terms it will return the wrong block label, because it scans the body backwards. Ticking, because no test cases trigger it yet. Must be fixed next, before new language features arrive.
2. No typed AST. Codegen reads the source AST directly and relies on the typechecker having run before. Fine for the MVP; once ADTs or closures arrive, I will need a separate typed IR stage (TIR).
3. The hash field is not in the AST. Right now we hash the def object directly. Once I serialise hashes as fields (caching), the hash will need to exclude that field before computation.

Plan iteration 2 (now):

1. Clean up block-label tracking, with a nested-if test.
2. Strings as a literal + io/print_str.
3. Hello-world example as a second E2E test.
4. CLI: --json output for machine consumers wherever it fits.

Plan iteration 3:

ADTs + pattern matching. That is the next big jump. Requires a typed IR stage (TIR), because pattern matching lowers into decision trees, which have a different shape from the AST.

2026-05-07 — iteration 2 done

• Block-label tracking is now robust (nested ifs work). Test max3_picks_largest protects it.
• Strings as Lit::Str { value }, type Str -> LLVM ptr, with io/print_str effect op. examples/hello.ail.json prints a string.
• CLI: manifest --json, builtins --json for tool consumers.
• ail deps [--of NAME] [--json] lists call edges. Effect ops are tagged effect:NAME so a consumer can filter them.

Architecture check: no structural deviations. Codegen still reads the source AST directly (a TIR stage will become necessary with ADTs in iteration 3).

2026-05-07 — iteration 3 done: ADTs

• TypeDef in the AST with ctors. A ctor has name and fields: [Type...].
• Term::Ctor (construction) and Term::Match (pattern matching).
• Patterns: Wild, Var, Lit, Ctor { ctor, fields }. In the MVP, nested ctor patterns are NOT allowed — sub-patterns must be Var or Wild.
• Typechecker with a type registry and ctor_index (ctor name → ADT). In Match, exhaustiveness is checked against the full constructor set. A negative test protects this.
• Codegen: boxed heap layout. Per ctor application, malloc(8 + 8*n) bytes; tag at offset 0, fields from offset 8 (8-byte slots, native typed load/store). Match: load tag + switch + arm blocks + phi at the join.
• examples/list.ail.json (Cons/Nil list, sum_list via match) returns 42.

Surprisingly painless. The architecture decisions from day 0 paid off: opaque ptr in LLVM 22 makes the boxed layout almost glue-free; effect tracking was untouched by ADTs; the JSON AST takes new node types cleanly.

Debt accrued:

1. Codegen still reads the source AST directly. The temptation to push on without TIR was strong — and worked, because my Match restrictions are flat (no nested patterns). Once nested patterns arrive, decision- tree lowering will not stay clean without TIR. Debt acknowledged; not due now.
2. No GC. The heap leaks. Acceptable for demo programs; must be addressed before any longer-running program. Options for Phase 4: refcount, Boehm-GC linkage, region inference.
3. No runtime pretty-printer for ADT values. io/print_int is enough for demos, but a generic show :: a -> Str for ADTs would be valuable. Requires dispatch over the tag — feasible, but not now.

Plan iteration 4:

The next steps are less obvious. Three candidates in priority order:

1. Module system (imports). Right now everything is in a single module. With multiple modules + cross-module hashing the language only becomes practical for several defs.
2. Structured error output (ail check --json). So tools can react to type errors without parsing text.
3. Closures / higher-order functions. Requires closure conversion and is a bigger step.

Iteration 4 will be (1) + (2) — both strengthen the LLM tooling and have moderate risk.

2026-05-07 — workflow change: orchestrator + agent repo

At the user's suggestion, switching to orchestrator mode: I delegate clearly bounded implementation chunks to sub-agents and keep only architecture decisions, reviews, and commit discipline. Four specialised agents drafted: implementer, architect, tester, debugger.

Important correction: the user required the agents not to be hidden in .claude/agents/, but versioned as a visible part of the project under agents/. DESIGN.md gained a new section "Project ecosystem", which records this: AILang is not just a language, but language core + CLI + examples + agents + docs + tests, all of equal weight.

Invocation scheme: the system-prompt body from agents/<name>.md as a prefix before the concrete task + sent to the general-purpose agent. Functionally identical to subagent loading from .claude/agents/, but visible in the repo.

Plan iteration 4 (revised):

The module system is more involved than expected (cross-module hashing, import resolution). First the smaller tooling wins, then the module system as iteration 5:

1. Structured error output (ail check --json with a Diagnostic struct, stable codes like unbound-var, type-mismatch).
2. ail diff <a> <b> — semantic module diff via per-def hash comparison.
3. IR snapshot tests — regression protection for the codegen pipeline.

2026-05-07 — iteration 4 done: LLM tooling consolidation

Three sub-commits, each produced by an ailang-implementer invocation and spot-checked by the orchestrator:

• 93fe723 Iter 4a: ail check --json with a Diagnostic struct (severity, code, message, def, ctx). Stable codes: unbound-var, type-mismatch, arity-mismatch, non-exhaustive-match, unknown-ctor, unknown-ctor-in-pattern, nested-ctor-pattern-not-allowed, duplicate-def, unknown-effect-op, unknown-type, schema-mismatch. API: check_module(&Module) -> Vec<Diagnostic>.
• c652b12 Iter 4b: ail diff <a> <b> [--json] as a structural top-level def diff via BLAKE3 hash. Four categories (added/removed/changed/ unchanged), sorted alphabetically, exit code 1 on diff.
• 74a2005 Iter 4c: IR snapshot tests in crates/ail/tests/snapshots/{sum,max3,hello,list}.ll. Normalisation of target triple. Update via UPDATE_SNAPSHOTS=1 cargo test ir_snapshot_. Mismatch produces an .actual file.

Test count: 28 (previously 19). 7 E2E + 4 IR snapshot + 9 ailang-check + 1 ailang-codegen + 7 ailang-core.

Closed from the debt register:

• Block tracking in codegen has not been a heuristic risk since Iter 2; the explicit current_block: String track is now additionally protected against regression by Iter 4c snapshot tests. Debt closed.

New / sharpened debt:

1. check_module is single-shot — the first error aborts, no multi-diagnostic gathering. The spec was that way, but the format suggests Vec semantics. A real multi-diagnostic refactor will be cheaper once TIR exists (a central error accumulator via a separate stage). Not due now.
2. source_filename in the IR is hard-coded to "<module>.ail". As long as there is only one top-level module, that is platform-stable. With Iter 5 (module system + imports) the path becomes relevant — keep it path-independent at construction time, otherwise the snapshots will tip over.

Plan iteration 5: module system with imports. Cross-module hashing, import resolution, multiple .ail.json files in one build. The multi- diagnostic refactor only after that.

Sub-steps:

• 5a — workspace loader. ailang_core::Workspace { modules: BTreeMap<String, Module> } plus load_workspace(entry: &Path), which follows imports recursively from the entry module. Convention: import { module: "foo" } resolves to <dir>/foo.ail.json next to the entry. Cycle detection. CLI: existing subcommands keep working on a single module; a new ail workspace <entry> lists all reachable modules with hash. Tests: two small example modules with an import relation; cycle test.
• 5b — cross-module typecheck. The typechecker takes &Workspace instead of &Module. Imports are mounted in the env as a namespace (alias.def or, with no alias, module.def). New diagnostic codes: unknown-module, unknown-import, import-cycle, ambiguous-name. Tests per code.
• 5c — cross-module codegen. The emitter produces IR for all modules in the workspace, prefix-mangled with @ail_<module>_<def>. E2E test: a program that uses a function from module B in module A returns the correct result in the binary.
• 5d — tooling adjustments. manifest, describe, deps, diff gain a --workspace mode (recursive). The single mode stays the default for backwards compatibility.

During Iter 5, at construction time keep source_filename path-independent (module name only, no directory prefix), otherwise the IR snapshots will tip over.

2026-05-07 — Iter 5b done: cross-module typecheck

• check_workspace(&Workspace) -> Vec<Diagnostic> as the top-level API. check_module is preserved and internally lifts the module into a trivial workspace.
• Convention for qualified references (recorded in DESIGN.md): Term::Var { name } with exactly one dot = <prefix>.<def>. Prefix is an import alias or module name. No new AST node, no renamed fields ⇒ hashes stay stable; all ir_snapshot_* still green.
• Three new diagnostic codes: unknown-module, unknown-import, invalid-def-name (with ctx.reason: "contains-dot").
• CLI: ail check <entry> now always loads via load_workspace. Workspace load failures become structured diagnostics in JSON mode with codes module-not-found, module-cycle, module-name-mismatch, module-hash-mismatch, schema-mismatch. ail build and ail emit-ir stay per single module (cross-module codegen is 5c).
• Examples: ws_main.ail.json now calls ws_lib.add (observable). New: ws_broken.ail.json (unknown-import), ws_unknown_module.ail.json (unknown-module).
• Tests: 37 green (previously 32). 4 new workspace integration tests in crates/ailang-check/tests/workspace.rs, one new e2e test check_workspace_resolves_import.
• Debt: single-shot diagnostics still in place (multi-diagnostic after 5c). The dot convention covers exactly one dot — nested module paths (a.b.c) do not exist; that would only be a topic with hierarchical modules and currently falls through as unbound-var.

2026-05-07 — Iter 5c done: cross-module codegen

• Mangling break (deliberate). All AILang functions are now called @ail_<module>_<def>, even in single-module programs. The old form @ail_<def> is gone. Strings/const globals analogously (@.str_<module>_<hint>_<idx>, @ail_<module>_<const>). The entry point stays main as C ABI: a define i32 @main() trampoline calls @ail_<entry-module>_main(). If the entry module has no main : () -> Unit !IO, the build fails with MissingEntryMain.
• Workspace lowering. New top-level API ailang_codegen::lower_workspace(ws: &Workspace) -> Result<String> produces a single .ll for the whole workspace. Modules in alphabetical order (BTreeMap order); defs in AST order. Cross-module calls are resolved in codegen via the import map of the calling module — same logic as in the typechecker, locally duplicated with a cross-reference (no shared helper module, because the type worlds differ: the typechecker handles Type, codegen handles FnSig from llvm types).
• CLI. ail build and ail emit-ir now always load the workspace and check/lower it fully. Single-module programs keep working (trivial workspace with one module). emit_ir(m) stays in the codegen crate as a convenience API and internally wraps into a trivial workspace.
• Snapshots regenerated. sum.ll, max3.ll, hello.ll, list.ll show the new mangling. New ws_main.ll snapshot documents the cross-module build: @ail_ws_main_main calls @ail_ws_lib_add.
• Tests. 40 green (previously 37). New: workspace_build_runs_imported_fn (e2e: prints 5), ir_snapshot_ws_main, missing_entry_main_is_error (codegen unit). Existing behaviour tests (sum_1_to_10_is_55, max3_picks_largest, hello_world_str_lit, list_sum_via_match) stay green — behaviour unchanged, only the mangling is new.

Debt closed:

• #19 (source_filename hardening). In the workspace world, source_filename is now uniformly <entry-module>.ail, once per workspace. The previous hard-coded path dot is gone with it.

State: the module system is closed end to end — loader + typecheck + codegen + build see the workspace as a coherent unit. The multi- diagnostic refactor and possibly cross-module ADTs remain for later.

2026-05-07 — Iter 5d done: tooling extended to the workspace

• ail manifest|describe|deps|diff <entry> --workspace now operate across all modules of the workspace. The default without the flag stays single-module for backwards compatibility. Manifest sorts by (module, name), describe accepts dotted notation ws_lib.add, deps emits {from_module, from_def, to_module, to_def} edges, diff compares workspace-wide with added/removed/changed/unchanged_modules and a nested sub-diff per changed_module.
• Refactor: diff_def_lists is the single source of the four-category logic; single and workspace diff share it.
• Tests: 44 green (previously 40). New: manifest_workspace_lists_all_defs, describe_workspace_resolves_qualified_name, deps_workspace_includes_cross_module, diff_workspace_added_module.

Observation (debt): deps does not filter builtins/locals/function parameters. In workspace mode that becomes more visible than in single mode — ws_lib.add lists edges to ws_lib.+ (builtin) and ws_lib.a/ws_lib.b (function parameters). A known pre-existing issue from Iter 2; Task #22 in the backlog.

2026-05-07 — architecture review after Iter 5

Architect agent invoked. Findings:

1. Mangling consistency holds. @ail_<module>_<def> is consistent across functions, constants, string globals, and cross-module calls. The trampoline is correct. ADT constructors are deliberately symbol-free (inline malloc).
2. Module hashes bit-identical since Iter 4. The Iter 5c snapshot regeneration was a codegen-output change, not a hash break.
3. Drift, due now:
   • DESIGN.md says define i64 @main(), codegen emits define i32 @main() (see sum.ll:35).
   • String-schema notation in DESIGN.md was shortened (@.str_<module>_<idx> instead of @.str_<module>_<hint>_<idx>).
4. Debt that accrues interest: the deps builtin leak (Task #22) has become a falsehood in workspace mode — close it before the next big jump.

Plan iteration 6 — clean-up:

1. Fix DESIGN.md drift. Update the mangling-scheme block, correct the @main signature, and note the string globals precisely.
2. deps hardening (#22). Build a top-level def table per workspace; filter edges whose target is not a top-level symbol, or emit them as separate builtin:/local: categories. Function parameters via lexical scope tracking from walk_term.
3. Multi-diagnostic refactor (#20). check_workspace accumulates Vec<Diagnostic> across all defs instead of short-circuiting on the first error. Intra-def may still short-circuit — the value is "see all broken defs at once", not "see all broken sub-terms of one def".

Order: 1 first (doc triviality), then 2 before 3 (deps is a tooling- truth fix, multi-diag is a structural extension).

2026-05-07 — Iter 6 done: deps hardening + multi-diagnose + DESIGN audit

Three things landed together. All small, all KISS — no architecture move, just paying off recorded debt.

1. ail deps filters builtins, params, and let/match bindings (#22).

Before: sum -> +, -, ==, n, sum and ws_lib.add -> ws_lib.+, ws_lib.a, ws_lib.b. After: sum -> sum, ws_lib.add -> ws_lib.add gone (no real deps; only the cross-module call from ws_main remains).

Implementation:

• New helper ailang_check::builtins::value_names(): derives the Var-level builtin names (+ - * / % == != < <= > >= not) from list(), so the install-list and the deps-filter share one source of truth.
• walk_term in crates/ail/src/main.rs now threads a scope set: fn-params seed it; Let adds the bound name for the body only; Match arms add their pattern variables (bind_pattern helper, MVP rule "ctor sub-patterns are Var/Wild") and roll them back after. Var refs that hit scope or builtins are dropped; qualified names (prefix.def) are passed through unconditionally — the typechecker forbids dots in def names, so no shadowing risk.
• Tests added: deps_filters_builtins_params_locals, deps_workspace_filters_builtins_and_params. The Iter 5d test (deps_workspace_includes_cross_module) keeps passing — the only edge it asserted is the legitimate one.

2. check_module / check_workspace are multi-diagnose (#20).

check_in_workspace returns Vec<CheckError> instead of Result<()>. Pass-1 (top-level symbol table) stays fail-fast — corrupt globals would taint every later diagnostic. Type-def installation is fail-fast within a module (env corruption) but the outer module loop continues. The body-check loop is the multi-diagnose layer: each def is checked against the assembled env, errors accumulate, the next def is attempted.

Test: body_errors_accumulate_across_defs — one module with two independent body errors (arity mismatch + unknown var) yields two diagnostics with the right def field. The legacy single-error check keeps working by .into_iter().next()-ing the Vec, so internal snapshot tests in crates/ailang-check/src/lib.rs are unchanged.

Out of scope: intra-def collection. A single fn body with three type errors still reports one. The "see all broken defs at once" goal is met; intra-def will require unification deferral and isn't due now.

3. DESIGN.md What the MVP is NOT audit (#24).

The section was lying: it claimed "No ADTs / pattern matching" (delivered Iter 3) and "Only ints + bools + unit" (strings landed Iter 2). Renamed to What is not (yet) supported, restructured into "not yet" + "what is supported (smoke-tested)". New invariant: this section is meant to be the truth at the end of the latest iteration, not a 2026-05-07-day-0 scope statement.

Architecture check (the user-asked self-questioning):

• Would I use this language now? For non-recursive arithmetic + ADT programs over int/bool/str: yes, comfortably. For anything that needs mapping, folding, generic data structures: no, closures are the blocker. That's the next big sprint, not Iter 7.
• Consistency: DESIGN.md, JOURNAL.md, code, and CLI output now agree on what the language can do. The "What is not (yet) supported" block is the canonical truth surface.
• Visualisation: ail deps --workspace --json is now a clean cross-module call graph (no builtin noise). Good enough for an external graph renderer to consume; a built-in DOT/ASCII renderer is possible future tooling, not "we need it now". KISS.
• Documentation: the agents/ directory is the sub-prompt layer, the JOURNAL is the iteration log, DESIGN.md is the contract. No new doc axes needed at this scale.

Tests: 47 green (previously 44). +2 deps tests in e2e.rs, +1 multi-diag test in crates/ailang-check/tests/workspace.rs.

Plan iteration 7:

Closures + higher-order functions. This is the big jump that DESIGN.md / Day 0 has been pointing at: it requires a typed IR (TIR) stage, closure conversion in lowering, and a heap-aware ABI. The multi-diag refactor in Iter 6 was scoped intentionally minimal — when TIR lands, intra-def diagnostics become structurally cheap and Task #20 gets revisited.

2026-05-07 — Iter 7 done: first-class function references (no capture)

Iter 6 outlined Iter 7 as "closures + HOFs + TIR". KISS course-correct on inspection: that bundle had three independent things in it, and the HOF use-cases (passing functions around, calling through fn-typed parameters) need none of TIR or capture. Splitting paid off — what landed here is ~120 LOC of codegen, no TIR, no heap, no ABI churn. Closures with capture stay queued for Iter 8 (where TIR is the correct precondition).

What works now:

• Top-level fn name (or qualified prefix.def) used as a value yields an LLVM fn-pointer (@ail_<m>_<def>, type ptr).
• Fn-typed parameters can be called as f(args) — the body emits an indirect call <ret> (<param-tys>) %f(...).
• Pass through let: let g = inc in g(x) works (the local just aliases the global SSA, the sidetable lookup still hits).
• Pass to another fn: apply(inc, 41) == 42 — see examples/hof.ail.json, exercised end-to-end.

What does not (yet) work — by design:

• No anonymous lambdas. The only fn-value source is a top-level def reference.
• No capture. A fn-value is always a constant pointer to a top-level def; there is no environment to allocate.
• Both deferred to Iter 8 where they share the TIR + closure-conversion preconditions.

Implementation, in order of where the rubber meets the road:

1. llvm_type learned Type::Fn { .. } -> "ptr". The actual signature travels separately. New helper fn_sig_from_type lifts an AILang fn-type into an FnSig (LLVM types only).
2. Emitter got a sidetable: ssa_fn_sigs: BTreeMap<String, FnSig>, keyed by SSA value (or @global). It's reset per function body.
3. At emit_fn entry, every fn-typed parameter registers (%arg_<name>, sig) in the sidetable.
4. lower_term(Term::Var) now falls through to a top-level fn lookup (resolve_top_level_fn) when the name isn't a local. The returned SSA is the global symbol; the sidetable gets the sig.
5. lower_term(Term::App) dispatches:
   • if callee is a Var AND not shadowed AND statically known (is_static_callee covers builtin operators, qualified prefix.def, current-module fns), keep the existing direct lower_app path — no extra indirection in the IR;
   • otherwise lower the callee, expect type ptr, look up the sig in the sidetable, emit emit_indirect_call.
6. Term::If propagates the sig to its phi SSA when both branches are fn-pointers with matching sigs (cheap two-line copy; no separate test, falls out of the apply-on-conditional pattern).

Why no typechecker change? The typechecker already accepted fn-typed locals (Term::Var against env.globals, App via synth(callee) unifying with Type::Fn). The only blocker was MVP: callee must be a variable in codegen.

Tests: 48 green (previously 47).

• crates/ail/tests/e2e.rs::higher_order_apply_inc builds and runs examples/hof.ail.json, asserts the binary prints 42.
• Existing tests unchanged (incl. snapshot tests around the IR emission for sum, list, max3).

Architecture self-check:

• Would I use this language now? Yes for apply-style and "pass a predicate" patterns. Still no for capturing closures (let n = 3 in map(\x -> x + n, xs)-equivalent), but the ergonomic gap shrank.
• Consistency: DESIGN.md "What is not (yet) supported" rewritten in the same edit; first-class fn-refs now have a positive bullet, the closures bullet is precise about what it means (no capture, no lambdas).
• Visualisation: ail describe/manifest already render fn-typed params correctly via the existing pretty::type_to_string (((Int) -> Int, Int) -> Int). No tooling change required.
• KISS: every alternative I considered (full LocalType enum, swapping (String, String) returns to a typed wrapper, lifting lambdas to defs as syntactic sugar) was strictly more code than the sidetable approach, with no expressivity gain.

Plan iteration 8:

Closures with capture, anonymous lambdas, the typed IR (TIR) layer, closure conversion in lowering. Now that we have indirect calls working, the main delta is: a fn-value also needs an environment pointer, the sidetable becomes per-value (heap-allocated), and the calling convention shifts to (env_ptr, args...). Touches every existing call path — that's why it gets its own iteration.

2026-05-07 — Iter 8 done: closures with capture (no TIR needed)

Iter 7's plan named TIR as the prerequisite for closures. On inspection that bundling was wrong — TIR is one possible implementation strategy, not a structural requirement. The typechecker already attaches enough type information through synth that the codegen can read capture types out of self.locals directly. So Iter 8 ships closures without introducing TIR. KISS won.

The work split into two commits:

Iter 8a — closure-pair ABI flip. Every fn-value is now a ptr to a heap or static closure pair { thunk_ptr, env_ptr }, regardless of whether it came from a lambda or a top-level def reference. To keep top-level-fn references cheap, every top-level fn auto-emits:

define <ret> @ail_<m>_<f>_adapter(ptr %_env, <params>) {
  %r = call <ret> @ail_<m>_<f>(<args>)
  ret <ret> %r
}
@ail_<m>_<f>_clos = constant { ptr, ptr } { @adapter, null }

Term::Var resolving to a top-level fn returns the address of _clos, never the bare fn pointer. emit_indirect_call was rewritten to GEP+load both halves and call thunk(env, args...). Direct calls (statically-known callees in Term::App) bypass the adapter and stay at the original speed.

The Iter 7 hof example (apply(inc, 41)) continues to print 42 unchanged — only the IR shape changed, not the source. IR snapshot files for sum/list/max3/hello/ws_main were refreshed.

Iter 8b — Term::Lam + capture + lambda lifting. New AST node:

{ "t": "lam",
  "params": ["x"...],
  "paramTypes": [Type...],
  "retType": Type,
  "effects": ["..."],
  "body": Term }

Param/return types are explicit. The typechecker accepts the declared Type::Fn shape, checks the body's type against retType, and verifies that body effects are a subset of the declared lambda effects (no row polymorphism in the MVP). Constructing a lambda is pure; the act of calling picks up the declared effects, via the existing App branch.

Codegen does textbook closure conversion:

1. Free-variable analysis. collect_captures walks the body skipping builtins (+, ==, ...), the current module's top- level fns, and qualified prefix.def names. The remainder are captures. Inner lambdas contribute their own free vars upward.

2. Lift to thunk. For each lambda, generate a fresh @ail_<m>_<def>_lam<id>(ptr %env, params...). State the body into a side buffer (the emitter's body/locals/counter are saved and reset, then restored). Captures and lambda params are pushed as named locals so the body lowering finds them. The thunk text goes into a deferred_thunks queue and is appended after the parent fn's } — LLVM IR doesn't care about fn order.

3. Pack at the use site. In the OUTER body emit:

   %env = call ptr @malloc(i64 <8 * captures>)
   ; for each capture i: store at offset 8*i
   %clos = call ptr @malloc(i64 16)
   ; store thunk_ptr at offset 0, env at offset 8
   

   %clos is the value returned by the Lam term. Its sig is registered in the sidetable so subsequent indirect calls work.

4. Capture sigs propagate. A fn-typed capture (e.g. capturing a fn-typed param of an outer scope) keeps its FnSig in the thunk's sidetable, so the captured fn can still be indirect-called from inside the lambda.

Capture layout uses 8-byte slots regardless of LLVM type. Typed load/store reads only the bytes it needs — wasted padding for i1 and i8 is fine at this scale.

Architecture self-check:

• Would I use this language now? Yes for substantially more cases. let n = 3 in apply(\\x. x + n, 39) is the example I would have reached for in Iter 6 and bounced off. It now compiles and runs. map/fold/filter over user-supplied predicates are within reach — only the absence of polymorphism still forces author-side monomorphisation.
• Did I think of everything? Hash stability checked manually: examples/sum.ail.json produced the same fn hashes (db33f57cb329935e, d9a916a0ed10a3d3) before and after Iter 8. Existing modules without Term::Lam serialise bit-identically. ✓
• Consistency: DESIGN.md "What is not (yet) supported" rewritten in the same edit. The Term schema gained lam, ctor, match rows that were already supported but had been omitted from the schema fragment. Now the doc is exhaustive for the supported language.
• Visualisation: ail describe already renders Lam terms (added pretty-printer rule), and the codegen IR for closure.ail.json reads as a textbook closure-conversion lowering.
• KISS check: I considered three alternatives and all were strictly worse — fat-pointer ABI (aggregate-passing concerns), full TIR layer (large rewrite), uniform heap pair without static-closure optimisation (regressed Iter 7 to one malloc per fn-value escape).

Tests: 49 green (was 48 after Iter 7). One new e2e: closure_captures_let_n builds and runs examples/closure.ail.json asserting "42". IR snapshot files refreshed for the per-fn adapter + static-closure scaffold — only structural delta.

Plan iteration 9:

Two candidates, both real pain points:

1. Polymorphic inference. Make Type::Forall actually work in synth — instantiate fresh type variables at each use site, allow let id = \\x. x in (id 1, id true). This unblocks generic map/fold/etc. without per-type clones. Probably small (~150 LOC in the typechecker; codegen already monomorphises by instantiation when it lowers the call).

2. GC / region reclamation. Right now ADT boxes, lambda envs, and closure pairs all leak through the program's lifetime. A minimal mark-and-sweep over a tagged heap would let us run real programs. Bigger lift, ~400-600 LOC plus runtime support.

Leaning toward (1) for the next iteration: it's the smaller bite and the bigger expressivity unlock. (2) becomes acute only when someone tries to run an unbounded loop, which the current examples don't.

2026-05-07 — Iter 9 done: dogfood + ail run

Course-corrected from the Iter-8 plan. Polymorphism is the bigger expressivity unlock on paper, but I hadn't actually proved that the language was sufficient for "small but real" programs without it. So Iter 9 became a dogfood iteration: write a non-trivial program that exercises everything Iter 1-8 shipped, and use ail run / errors / type-checker output as the user would. If something broke, fix it. If nothing broke, document the boundary moved.

examples/list_map.ail.json:

type IntList = Nil | Cons Int IntList

map_int :: ((Int) -> Int, IntList) -> IntList
map_int(f, xs) = match xs {
  Nil          -> Nil
  Cons(h, t)   -> Cons(f(h), map_int(f, t))
}

print_list :: (IntList) -> Unit !IO
print_list(xs) = match xs {
  Nil          -> ()
  Cons(h, t)   -> let _ = do io/print_int(h) in print_list(t)
}

main = let xs = Cons 1 (Cons 2 (Cons 3 Nil)) in
       print_list(map_int(\\x. x * 2, xs))

Result: nothing broke. Output 2\\n4\\n6\\n, exit 0. The full pipeline (ail run) covers: ADTs with two ctors of different arity; pattern matching with nested Var fields; recursion over ADT; closures (with no captures here, so env is null but the closure-pair plumbing still gets exercised); fn-typed parameters in a top-level def; do io/... inside a match arm body, with let _ to sequence two effectful operations; effect propagation through the call chain. This validates Iter 1-8 as a self-contained foundation.

Friction surfaced: writing the AST by hand is tedious — the JSON for this 4-def module is 200+ lines. That's not surprising (the format is for LLMs, not humans), but it suggests an Iter 10 priority: a richer pretty-print form, or an ail snippet helper for common boilerplate (mk_list_int, etc.). Not blocking; noted.

ail run (Iter 9b): Builds into a tempdir + execs the binary, exit code passthrough. Saves a cd && ./bin step in the dogfood loop. Tiny addition — Cmd::Build's body factored into a shared build_to helper.

Architecture self-check:

• Would I use this language now? For self-contained Int-typed programs over recursive ADTs: yes. The list_map example is what I would have wanted to write since Iter 6 and bounced off repeatedly. It now compiles and runs without me adapting the source — the language is what its authors said it was, end to end.
• Did I think of everything? Two cracks observed during the dogfood:
  • (Int) parens around single-param fn-types in pretty-print are visual noise. Cosmetic, can wait.
  • let _ = do <effect> in <body> is the only way to sequence effects today. Working as intended given KISS, but a ; operator (sequencing) would be cheap polish.
• Consistency: DESIGN.md CLI block + smoke-test list updated. Iter 8c invariant — "What is not (yet) supported" ≡ truth at end of latest iteration — held; no new pending items.
• KISS: Iter 9 added 0 LOC of language semantics. All gain came from validating the existing surface and a small CLI helper.

Tests: 50 green (was 49). New e2e list_map_doubles_then_prints. No test for ail run itself — build_and_run already exercises the equivalent path.

Plan iteration 10:

The dogfood revealed two real-but-not-blocking pain points and one big architectural gap. Candidates, ranked:

1. Polymorphic let-bindings with monomorphisation at codegen. Allows let id = \\x. x in (id 1, id true) and ultimately map :: (a -> b) -> List a -> List b. The ground truth-ier answer for the "would I use it for X?" question, but a non-trivial pipeline change (typechecker→codegen needs to thread instantiation info to the call site).

2. Sequencing operator ; and richer effect ergonomics. A Term::Seq { lhs, rhs } (or compile sugar to Let { name: "_", value: lhs, body: rhs }) plus a small pretty-print update. Cheap, satisfying.

3. GC. Heap reclamation for ADT boxes, lambda envs, closure pairs. Real architecture step. Becomes acute the moment someone writes a long-running loop; the current examples don't.

Tentative pick: (2) for the next sprint as a satisfying small polish, then (1) as Iter 11. (3) bides its time until a real program needs it.

2026-05-07 — Iter 10 done: Term::Seq sequencing

Followed the Iter 9 plan and shipped (2). New AST node Term::Seq { lhs, rhs } with serde tag "seq". Semantics: evaluate lhs (which must be Unit), discard the value, return rhs. Effects from both sides accumulate.

This is sugar for let _ = lhs in rhs, but it's a first-class node because:

• The pretty-print renders cleanly ((seq lhs rhs) instead of borrowing the let form with a discard binding).
• Diagnostics are sharper: a non-Unit lhs gets a "type mismatch" error pointing at the seq site, not "binding _ had type X" at a let site.
• Future tooling (effect inference visualisation, dataflow) can treat sequencing as a structural concept instead of a special- cased let.

Codegen is trivial: lower lhs (drop SSA), lower rhs (return).

Refactored examples/list_map.ail.json's print_list to use seq instead of let _ = .... Output unchanged (2\\n4\\n6\\n); the JSON shed a few lines and reads more honestly.

Architecture self-check:

• Would I use this language now? Same answer as Iter 9 (yes for small but real programs), but the seq node makes IO-heavy recursion read better — closer to "call this effect, then this one" instead of "bind this effect to nothing, then this one".
• Did I break anything? Hash stability check: existing examples without Term::Seq serialise identically; their fn hashes are unchanged. list_map.ail.json's hashes shifted as expected since its body changed.
• KISS: +30 LOC across AST/pretty/check/codegen/walker. One unit test for the lhs-must-be-Unit rule. The dogfood example proves the e2e path.

Tests: 51 green (was 50). New seq_lhs_must_be_unit unit test in ailang-check. Existing list_map e2e still passes after the refactor.

Plan iteration 11:

Polymorphism, as queued in the Iter 9 plan. Concretely: HM-style unification + let-generalisation in the typechecker, monomorph- isation at codegen time. Touches the typechecker→codegen pipeline. Bigger commit than the recent stretch, will probably need to be phased (typechecker substitution machinery, then codegen specialisation, then docs).

2026-05-07 — Iter 11 done: deeper dogfood (insertion sort)

Pulled back from polymorphism for one more validation cycle before the architectural step. Polymorphism is a substantial pipeline change (typechecker substitution + codegen monomorphisation) and I wanted one more "small but real" program to confirm the existing foundation holds before disturbing it.

examples/sort.ail.json — insertion sort over IntList:

insert :: Int -> IntList -> IntList
insert(y, xs) = match xs {
  Nil           -> [y]
  Cons(h, t)    -> if y <= h then Cons(y, Cons(h, t))
                              else Cons(h, insert(y, t))
}

sort :: IntList -> IntList
sort(xs) = match xs {
  Nil          -> Nil
  Cons(h, t)   -> insert(h, sort(t))
}

print_list :: IntList -> Unit !IO    // uses Iter 10 seq

main = print_list(sort([3,1,4,1,5,9,2,6,5,3,5]))

Result: typechecks first try, runs first try, prints 1 1 2 3 3 4 5 5 5 6 9 (each on its own line). 11-element input, correct sorted output. The combination of recursive ADT pattern match + comparison ops + branching + leaf recursion + IO sequencing all worked end to end without the language tripping me up. Iter 10's seq made print_list notably cleaner than the let _ = ... form would have been.

Architecture self-check:

• Would I use this language now? For "small but real" monomorphic programs over Int, Bool, Unit, Str, and ADTs of those: confidently yes. Insertion sort writes out as the textbook recursion, no bookkeeping that the language couldn't do for me.
• Did I think of everything? The remaining wall is still polymorphism. Sort over IntList needs hand-monomorphisation; a generic sort :: (a -> a -> Bool) -> List a -> List a is what the language eventually wants. No new architectural cracks surfaced from this dogfood.
• Visualisation: ail describe sort.ail.json sort reads the way I'd expect a sort definition to read, with IntList types inline and the recursive call rendered cleanly.
• KISS: Iter 11 added 0 LOC of language semantics and 1 e2e test. The 250-line JSON for the example is verbose but mechanical — no friction once you accept that the JSON is the surface for LLM authors.

Tests: 52 green (was 51). New e2e insertion_sort_orders_list. Pure addition; existing tests untouched.

Plan iteration 12:

Now polymorphism. Two more dogfood programs would just keep producing the "the language is fine for monomorphic programs" result, which is already established. The real expressivity unlock — and the answer to "would I use it for X?" for X that actually needs generic data — is HM inference + let-generalisation

• monomorphisation. Phased plan:

12a. Typechecker: introduce a Subst (type variable substitution) and unification. Thread through synth. At let, generalise syntactic values (lambdas) — no value-restriction subtlety needed yet, the MVP has no mutable refs. 12b. Codegen: at each polymorphic call site, the typechecker records the instantiation. Codegen walks the AST a second time per (def, instantiation) pair and emits a specialised version with the type variables substituted by concrete types in fn signatures. 12c. Docs + a polymorphic id test + a generic map :: (a -> b) -> List a -> List b rewrite of list_map.ail.json.