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All 176 files in the four accumulating directories now use a zero-padded 4-digit counter prefix that reflects creation order (`NNNN-slug.md`). The counter is assigned per directory in strict git-log creation order; ties broken alphabetically by original name. The old `YYYY-MM-DD-` prefix on docs/specs/ and docs/plans/ files is dropped — the date is recoverable from git log and the counter carries the ordering. A file's counter is stable for the life of the file: never reassigned, never reused, never compacted. Deleted files retire their counter; subsequent files do not fill the gap. This is the property that lets cross-references stay literal — refs use the full filename including the counter (`design/contracts/0007-honesty-rule.md`) so they grep cleanly and resolve directly without a glob step. 313 cross-references updated across .md/.rs/.toml/.c/.json files (test pins, include_str! paths, design-INDEX entries, baseline notes, runtime C comments, inter-contract markdown links incl. bare basename and `../models/foo.md` forms). CLAUDE.md gets a new "File-naming convention" section spelling out the rule and rationale. skills/brainstorm/SKILL.md and skills/planner/SKILL.md updated so new spec/plan creation produces counter-prefixed names from the start. The full test suite (cargo test --workspace) passes.
63 lines
2.9 KiB
Markdown
63 lines
2.9 KiB
Markdown
# Tail calls
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## Explicit, verified tail calls
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For an LLM author, recursion is the natural iteration form
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(`\n. if n == 0 then () else loop(n-1)` is what I reach for, not
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a `for`-loop). Without a tail-call guarantee, every recursive
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program has a silent stack-depth ceiling that no compile-time
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diagnostic warns about. That is exactly the class of correctness
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hazard the language exists to eliminate.
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Solution: explicit, verified tail calls.
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- **AST.** `Term::App { fn, args, tail: bool }` and
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`Term::Do { op, args, tail: bool }` (see
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[Data model](0002-data-model.md)) gain a `tail` flag,
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serde-defaulting to `false` so existing fixtures load with
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`tail: false` and their hashes stay bit-identical.
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- **Typecheck.** A new pass `verify_tail_positions(fn_body)`
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runs after the main type-check. It walks the body with an
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`is_tail_context: bool` threaded down. The flag is `true` at
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the start, `true` for the body of every `Term::Match` arm,
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`true` for the right operand of `Term::Seq`, `true` for the
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body of `Term::Let`, `true` for the body of `Term::Lam` (each
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Lam opens its own tail scope). The flag is `false` for: args
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of any `App`/`Do`/`Ctor`, scrutinee of `Match`, left of
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`Seq`, condition of `Let`-bound expression. When the walker
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visits an `App { tail: true }` or `Do { tail: true }`, the
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flag must be `true` at that visit; otherwise emit diagnostic
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`tail-call-not-in-tail-position`.
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- **Codegen.** Emit `musttail call` (LLVM IR) for marked calls
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instead of plain `call`. LLVM rejects at IR-verification
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time if the call cannot physically be a tail call (calling
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convention mismatch, signature divergence, etc.). The reject
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surfaces as a hard build error, not a silent runtime
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surprise.
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- **Form (A).** Two new keywords: `tail-app`, `tail-do`.
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Productions are positional analogues of `app`/`do` with
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`tail: true` set on the resulting term. EBNF gains 2 lines;
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total production count goes from ~28 to ~30, still inside
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the constraint-1 budget (see
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[authoring surface](0001-authoring-surface.md)).
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**What this does NOT promise.** Per the tail-call survey of existing fixtures,
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many existing recursive calls are *not* in tail position
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because they are arguments to constructor calls (e.g.
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`Cons (f h) (map f t)`). The current pipeline adds annotation + verification;
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it does **not** add a CPS transform or accumulator-form
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rewrite. Programs whose recursion is constructor-blocked
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will continue to be stack-bounded by recursion depth. The
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canonical authoring pattern in such cases is to write the
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accumulator-form variant (`map_acc`, `fold_left`, etc.)
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explicitly. The stdlib ships both forms where
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relevant.
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Migration of existing fixtures is partial: only
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`print_list`-style terminal recursions get marked. The
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constructor-blocked recursions in `map`, `sort`, `insert`
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remain unmarked — they cannot benefit from `musttail`
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without a source-level rewrite.
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Ratified by: `crates/ailang-check/src/lib.rs`.
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