feat(diagnostics): prelude-hint for UndefinedVariable / Unbound (stdlib #5) - #427
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#5) Closes #415. Stdlib roadmap item #5 was framed as "cross-file `use` resolvability from standalone check", but the actual underlying issue turned out to be **discoverability**: cross-module `use` already works (both `use Deno::{Json}` and `use prelude::*` resolve from a standalone single-file check). What trips users is that prelude is NOT auto-opened (deliberate, per issue #138), and the error when an unimported `Some` is referenced was a raw OCaml record dump with no hint about the fix. This change: 1. Replaces face.ml's `Canonical` branch's `show_resolve_error` (raw record dump) with a real per-error-class formatter. Other faces were already formatted; only Canonical hadn't been touched here. 2. Adds a `prelude_hint` helper in both face.ml and typecheck.ml that, when an undefined name matches a known prelude export, appends: hint: `Some` is defined in `stdlib/prelude.affine` — add `use prelude::{Some};` (or `use prelude::*;`) at the top of the file. Covers both the resolve-time path (face.ml) and the typecheck-time path (typecheck.ml's UnboundVariable — fires e.g. when a match arm references an unbound constructor the resolver missed). 3. Hard-codes the prelude export list as the well-known 20 names (Option, Result, Some, None, Ok, Err + the function helpers). A missing name here means a worse error message, not incorrect behaviour. Comment in both files notes the list must mirror `stdlib/prelude.affine`. Effect for users: - BEFORE: `Resolution error: (Resolve.UndefinedVariable { Ast.name = "Some"; span = {...} })` - AFTER: ``` Resolution error: undefined value: `Some` hint: `Some` is defined in `stdlib/prelude.affine` — add `use prelude::{Some};` (or `use prelude::*;`) at the top of the file. ``` Doesn't auto-import prelude (would revert #138). Doesn't change resolve behaviour at all — only error rendering. Test plan: - [x] codegen-deno suite stays 7/7 green - [x] Manual reproduction of the idaptik PR #107 case shows the hint - [x] Non-prelude names (e.g. `nonexistentFn`) still get a clean error with NO inappropriate prelude hint - [x] `dune build bin/main.exe` clean Updates stdlib roadmap row #5 status `◑` partial → `●` usable in a follow-up PR (so this PR stays focused on the diagnostic). Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
hyperpolymath
enabled auto-merge (squash)
May 28, 2026 12:16
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Closes #415. Stdlib roadmap item #5 (
docs/stdlib-roadmap.adoc).TL;DR
The original framing of stdlib #5 was "cross-file `use` resolvability from standalone check". Reproducing the bug turned up something different: cross-module `use` already works (both `use Deno::{Json}` and `use prelude::*` resolve fine from a standalone single-file check).
What actually trips users is discoverability — prelude is NOT auto-opened (deliberate, per issue #138), and the error when an unimported `Some` is referenced was a raw OCaml record dump with no hint about the fix.
Before / after
Before:
```
Resolution error: (Resolve.UndefinedVariable
{ Ast.name = "Some";
span = { Span.start_pos = { Span.line = 1; col = 37; ... }; ... }
})
```
After:
```
Resolution error: undefined value: `Some`
hint: `Some` is defined in `stdlib/prelude.affine` — add
`use prelude::{Some};` (or `use prelude::*;`) at the top of the file.
```
Files
What this does NOT do
Test plan
Follow-ups
Once this lands, a follow-up PR updates `docs/stdlib-roadmap.adoc` row #5 status `◑` partial → `●` usable (kept out of this PR so the diff stays focused on the diagnostic itself).
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