feat(module): pluggable module providers and a host module prelude

[ecto]

Why

The module system assumed a filesystem, so an embedder without one — WASM in a browser — could not use [use ...] at all. vcad hit this building a two-repo CAD project where a shared interface must have a single source of truth: its browser kernel and its MCP server both evaluate loon, and neither could reach the module system, so multi-file projects needed an external concatenation step that defeats pub, cannot alias, and has no real cycle detection.

Threading a base_dir further would only fix the native path. The resolver, not the path, is the thing to abstract.

What changed

• trait ModuleProvider { fetch(module_path, from_dir) -> Result<Option<String>, String> }, consulted ahead of path resolution. Returning Ok(None) declines and resolution falls through, so path deps, manifests, lockfiles and remote deps are untouched — a provider is purely additive.
• MapProvider, a provider over an in-memory name -> source map (also accepts <name>.loon / <name>.oo keys so a caller can key by filename).
• ModuleCache::{with_provider, set_provider, set_prelude}, and interp::eval_program_with_modules(exprs, base_dir, provider, module_prelude); eval_program_with_base_dir delegates to it, so existing callers are unchanged.
• Provider modules share the eval path with filesystem ones via a new eval_module, keyed by a <provider>/<name> sentinel. Cycle detection (Loading/Loaded), nested [use], aliasing, selective import and pub behave identically whatever the origin.
• set_prelude evaluates a host stdlib into every module's env — vcad needs its CAD vocabulary visible inside imported modules.

Behavior change worth a look

The implicit "no pub declared → export everything" fallback previously excluded only builtins, so it also exported prelude names (Option, Result, …) under mod.Name. It now excludes everything defined before the module body — the prelude and any host prelude — so such a module exports only its own definitions. loon-lang's 415 tests pass unchanged.

Consumer

Required by ecto/vcad#715, which supplies an in-memory map on the WASM side and the vcad stdlib as the module prelude. That PR's Rust jobs fail until this lands on main, since vcad's CI clones loon at its default branch.

🤖 Generated with Claude Code

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Project	Deployment	Actions	Updated (UTC)
loon	Ready	Preview, Comment	Jul 26, 2026 12:56am