veks-completion

Dynamic shell completion engine for Rust CLI tools, plus a companion argv parser, help renderer, and tap-cadence model.

Zero non-std dependencies. One CommandTree declaration drives tab completion, argv parsing, and --help rendering — single source of truth for every CLI surface.

What's in here

• Tab completion — context-aware candidates, value providers, per-flag aliases, dynamic option discovery.
• Multi-tap rotating tiers — single tap shows the primary commands; rapid follow-up taps reveal cumulative supersets in layer-ordered display.
• Argv parser — parse_argv(tree, &argv) → ParsedCommand walks the same tree the completer uses to produce a structured parse (path, flags, positionals).
• Help rendering — render_usage(node, path) formats a --help block from the tree.
• Closed value sets — ClosedValues produces both completion and validation from one declaration.
• Subtree-aware completion — context-sensitive providers can take over completion inside any subtree with a structured PartialParse.
• Free-form attachment — Extras slot lets embedders carry handler payloads, parser state, dispatch rules without forcing the crate to grow generics.
• Directive sets — bundle every surface a flag appears in (CLI form, help, value set, repeatability, optional YAML mirror) into one declaration; expand a slice of them onto a node in one call.

Why not clap_complete?

Clap provides clap_complete for generating static shell completion scripts. It works well for simple CLIs with a fixed command tree, but breaks down for tools with dynamic commands, multi-level subgroups, or context-sensitive value completion. This crate was built for the veks CLI but is fully generic and reusable. The specific limitations it addresses:

1. Dynamic pipeline commands

Veks has a pipeline command group whose subcommands are registered at runtime from a CommandRegistry, not from clap's derive macros. Clap's completion generator only sees the derive-defined tree.

2. Shorthand dispatch

veks run is shorthand for veks prepare run; veks config for veks datasets config. These are hidden top-level subcommand aliases. Clap either hides them entirely (no completions) or shows them alongside the originals (duplicates). veks-completion uses tap-tier levels: hidden shortcuts get level=2 and surface only on rapid double-tap.

3. One-level-at-a-time completion with rotating tiers

Clap eagerly chains subcommands. veks-completion completes one level at a time. Within a level, rapid double-tap reveals the next tier as a cumulative superset, layered in display order so primary commands always appear first.

4. Context-sensitive value completion

Per-option ValueProvider functions run arbitrary logic. ClosedValues turns a static value set into a provider with one declaration that's also queryable by parsers for validation.

5. Mixed derive + dynamic tree

The completion engine walks the augmented clap::Command tree at completion time, so it always reflects the actual command structure regardless of how commands were registered.

How it works

Node is a single struct that carries everything a CLI-tree node can have: subcommand children, flags (value-taking + boolean), value providers, discovery metadata (category + level), help text, plus hooks (subtree provider, free-form extras). A node with no children is "leaf-shaped"; with children it's "group-shaped"; with both it's hybrid (e.g. a group that itself accepts cross-cutting flags).

A CommandTree is a root node + globals. At startup an embedder builds the tree once. At completion time the engine walks the tree following the user's typed words and returns candidates.

The bash script the engine emits is minimal — it just calls back into the binary. Completions never get out of sync with the installed binary because the binary itself produces them.

Usage — tab completion

use veks_completion::{CommandTree, Node, handle_complete_env};

let tree = CommandTree::new("myapp")
    .command("run", Node::leaf_with_flags(
        &["--input", "--output", "--threads"],
        &["--verbose", "--dry-run"],
    ))
    .group("compute", Node::group(vec![
        ("knn",   Node::leaf(&["--base", "--query", "--metric"])),
        ("stats", Node::leaf(&["--input"])),
    ]));

// In main(), before argument parsing:
if handle_complete_env("myapp", &tree) {
    std::process::exit(0);
}

// And register a `completions` subcommand that prints the bash
// activation snippet for `eval "$(myapp completions)"`.

Usage — argv parsing

The same CommandTree produces structured parses:

use veks_completion::{parse_argv, ParsedCommand};

let parsed = parse_argv(&tree, &[
    "compute", "knn", "--metric", "L2", "--verbose", "data.fvecs",
])?;
assert_eq!(parsed.path,        vec!["compute", "knn"]);
assert_eq!(parsed.flags["--metric"],  vec!["L2".to_string()]);
assert_eq!(parsed.flags["--verbose"], vec!["".to_string()]); // boolean
assert_eq!(parsed.positionals, vec!["data.fvecs"]);

parse_argv_lenient treats unknown flags as positionals — useful for pass-through CLIs.

Usage — help rendering

use veks_completion::render_usage;

let leaf = Node::leaf_with_flags(&["--metric"], &["--verbose"])
    .with_help("Compute KNN over base vectors")
    .with_flag_help("--metric",  "Distance metric: L2 / IP / COSINE")
    .with_flag_help("--verbose", "Print per-step progress");

println!("{}", render_usage(&leaf, &["myapp", "compute", "knn"]));
// → USAGE: myapp compute knn
//
//   Compute KNN over base vectors
//
//   FLAGS:
//     --metric   Distance metric: L2 / IP / COSINE
//     --verbose  Print per-step progress

Usage — closed value sets (completion + validation)

use veks_completion::{ClosedValues, Node};

let metrics = ClosedValues::Static(&["L2", "IP", "COSINE"]);
assert!(metrics.validate("L2"));
assert!(!metrics.validate("bogus"));
assert_eq!(metrics.complete("CO"), vec!["COSINE"]);

let leaf = Node::leaf(&["--metric"])
    .with_value_provider("--metric", metrics.clone().into_provider());

Usage — directive sets

When you have many flags that share a vocabulary shape, declare them once as a &[Directive] and apply onto any node:

use veks_completion::{Directive, apply_directives, Node};

const DIRS: &[Directive] = &[
    Directive::closed("--metric", &["L2", "IP", "COSINE"])
        .with_help("Distance metric"),
    Directive::value("--name").with_help("Run name"),
    Directive::boolean("--verbose").with_help("Verbose output"),
];

let leaf = apply_directives(Node::leaf(&[]), DIRS);
// → flags + flag-help + value-providers + validation, all wired.

Usage — multi-tap rotating tiers

Tag commands with with_level(n) to control which tap-tier reveals them:

let tree = CommandTree::new("nbrs")
    .command("run",          Node::leaf(&[]).with_level(1))
    .command("--inspector",  Node::leaf(&[]).with_level(2))
    .command("--summary",    Node::leaf(&[]).with_level(2))
    .command("describe",     Node::leaf(&[]).with_level(3))
    .command("bench",        Node::leaf(&[]).with_level(3));

Behavior:

• Tap 1 (cold or after pause): layer 1 only — run
• Tap 2 (within 200 ms): cumulative layers 1+2 — run, --inspector, --summary
• Tap 3 (within 200 ms): cumulative layers 1+2+3 — all five. At max, the persistent state resets, so a fourth rapid tap cycles back to layer 1.
• Pause > 200 ms: any tap starts fresh at layer 1.
• Within each tap's result: sorted in layer order (layer 1 first, then layer 2, …); within a layer, ---flags last, alphabetical otherwise.

The cadence rule is also exposed as a pure function for embedders that want their own clock/state:

use veks_completion::{TapState, next_tap_state, TAP_ADVANCE_MS};

let (tap_count, next) = next_tap_state(
    Some((prev_state, "current_input_key")),
    now_ms,
    "current_input_key",
    max_level,
);

TAP_ADVANCE_MS is the public window constant (200 ms).

Built-in options for downstream adopters

When you build a CommandTree, two opt-in builders enable common capabilities without writing the boilerplate yourself. Both are fully additive — they walk the tree once at finalisation time:

with_auto_help

let tree = CommandTree::new("myapp")
    .command("compute", Node::group(vec![
        ("knn", Node::leaf(&["--metric"])),
    ]))
    .command("run", Node::leaf(&["--input"]))
    .with_auto_help();

Walks every node and adds --help (boolean) plus a default help line ("Show usage information for this command."). Idempotent — nodes that already declare --help are left alone. Once attached, --help:

• shows up in tab completion at every level,

• is recognised by parse_argv as a known boolean flag (so users can type --help anywhere without "unknown flag" errors), and

• can be paired with render_usage(node, path) in your handler to print the help block:

  let parsed = parse_argv(&tree, &argv)?;
  if parsed.flags.contains_key("--help") {
      let node = walk_to(&tree.root, &parsed.path);
      println!("{}", render_usage(node, &parsed.path));
      return Ok(());
  }
  

with_metricsql_at

use std::sync::Arc;
use veks_completion::providers::{MetricsqlCatalog, metricsql_provider};

let catalog: Arc<dyn MetricsqlCatalog> = Arc::new(MyCatalog::new());
let tree = CommandTree::new("nbrs")
    .command("query", Node::leaf(&[]))
    .with_metricsql_at(&["query"], catalog);

Attaches a built-in metricsql_provider SubtreeProvider at the specified node path. Equivalent to manually walk_path_mut(...)-ing to the node and calling with_subtree_provider(metricsql_provider( catalog)), but surfaces the intent at tree-construction time.

The MetricsQL provider understands:

Site-specific data (metric names, label keys, label values) comes from your MetricsqlCatalog impl. The built-in vocabulary (functions, time units, operators, aggregation modifiers) is baked in — see providers::METRICSQL_FUNCTIONS, providers::METRICSQL_TIME_UNITS, etc.

Usage — context-sensitive subtree completion

When tab completion needs grammar-aware behavior in a subtree (e.g. inside a query DSL), attach a SubtreeProvider:

use std::sync::Arc;
use veks_completion::{Node, PartialParse, SubtreeProvider};

let metrics_provider: SubtreeProvider = Arc::new(|pp: &PartialParse| {
    // pp.completed = full chain of words to the left of the cursor
    // pp.partial   = the partial word under the cursor
    // pp.tree_path = the resolved tree-node path
    // Return whatever candidates make sense given that state.
    parse_my_dsl_and_complete(pp.partial)
});

let tree = CommandTree::new("app")
    .command("metrics",
        Node::group(vec![("match", Node::leaf(&[]))])
            .with_subtree_provider(metrics_provider));

The deepest matching subtree provider on the path takes over completion. This is the recommended replacement for the pre-walker hook pattern.

Usage — handler attachment

Embedders that want to dispatch to handlers from the same tree can attach arbitrary payloads via Extras:

use veks_completion::{Extras, Node};

struct MyHandler { … }

let leaf = Node::leaf(&["--input"])
    .with_extras(Extras::new(MyHandler { … }));

// Later, after parse_argv resolves `path`:
if let Some(extras) = node.extras() {
    if let Some(handler) = extras.downcast::<MyHandler>() {
        handler.run(parsed);
    }
}

Extras wraps Arc<dyn Any + Send + Sync> — no generic on Node, no hard dependency on tokio or any handler framework.

Architecture

veks-completion/        # This crate — generic completion + parse + help
  src/lib.rs            # Node, CommandTree, complete(), parse_argv(),
                        # render_usage(), ClosedValues, Directive, …

veks/src/cli/
  dyncomp.rs            # Walks clap::Command -> CommandTree;
                        # registers global ValueProviders.
  mod.rs                # Wires `eval "$(veks completions)"` snippet.

The completion crate has no dependency on clap, veks, or any pipeline code. The clap-to-tree conversion lives in dyncomp.rs inside the veks binary crate.

Examples

Two runnable examples in examples/ show how the pieces fit together. Read them top-to-bottom — the rustdoc comments narrate each step.

basic.rs — first-time setup

cargo run --example basic -p veks-completion

Builds a small CommandTree with a few subcommands and a value provider, prints what completion would suggest at sample cursor positions. The minimum viable adoption.

metricsql.rs — end-to-end coding scenario

A full integration that mirrors how you'd actually adopt veks-completion in a tool with grammar-aware completion needs. It shows the four steps of a real adoption:

1. Implement the catalog — a small InMemoryCatalog that provides the site-specific data the MetricsQL provider needs (metric names, label keys, label values).
2. Build the tree — uses both built-in options: with_auto_help() (uniform --help everywhere) and with_metricsql_at(&["query"], catalog) (grammar-aware completion inside the query subcommand).
3. Wire the entry point — handle_complete_env for tab callbacks, print_bash_script for the activation snippet, parse_argv + render_usage for --help handling, dispatch on the resolved subcommand path.
4. Drive the demo — runs ten realistic MetricsQL cursor positions (top-of-expression, inside {, after key=, inside open quote, inside [, after by, after offset, etc.) and prints what the completer suggests at each.

# Build + register completions, then tab around interactively:
cargo build --example metricsql -p veks-completion
eval "$(./target/debug/examples/metricsql completions)"
./target/debug/examples/metricsql query 'up{<TAB>'
./target/debug/examples/metricsql query 'rate(http_requests_total[<TAB>'
./target/debug/examples/metricsql query 'sum by (<TAB>'
./target/debug/examples/metricsql --help
./target/debug/examples/metricsql query --help

# Or skip the shell setup and watch the demo print its results:
cargo run --example metricsql -p veks-completion -- demo

The demo's output for each scenario shows the raw input, the cursor byte position, and the candidates the engine would offer — the quickest way to sanity-check what your MetricsqlCatalog impl returns at every cursor position the provider knows about. Adapt this scenario script into your own integration tests by swapping in your real catalog.