checker: stop eating record-literal fn bodies as effect annotations

[ecto]

What

Fixes two type-checker bugs that made a valid program (one the EIR VM runs correctly) fail with [E0200] cannot unify Record{...} with ().

Repro that used to fail loon run but pass loon test:

[fn log-monoid [] {:unit "" :tensor [fn [m n] [str m n]]}]
[fn act [mon m w]
  [let t [get mon :tensor]]
  [let {log log val val} w]
  {:log [t m log] :val val}]
[fn pure [mon a] {:log [get mon :unit] :val a}]
[fn main []
  [let x [act [log-monoid] "step1;" [act [log-monoid] "step2;" [pure [log-monoid] "done"]]]]
  [let {log log val val} x]
  [IO.println [str "log: " log " value: " val]]]

Why

1. Record-literal fn bodies were swallowed as effect annotations. infer_named_fn consumed any Map right after the params as a {IO Fail}-style effect annotation (a "parser quirk" fallback). So [fn log-monoid [] {:unit "" :tensor ...}] had its entire body eaten, leaving nothing — the checker inferred the return type as (). That's the Record{tensor: ...} vs () mismatch: the record the function actually returns at runtime against the () the checker believed. All three runtimes (interp, machine, EIR lower) only skip Set(_), never Map, so the checker had diverged from execution.

Fix: a Map only counts as an annotation in the quirk form — all keys and values are bare symbols, and a body expression follows. Keyword-keyed maps (record literals) are the body.

2. Map destructuring collapsed record fields to one shared type. bind_map_destructure unified any non-Map subject with Map Keyword v and bound every key to the same v. Since keyword-keyed literals type as Records, destructuring a param like w both forced it away from record types and forced log and val to share one type.

Fix: keep the shared-element path for genuine Map Keyword v subjects; otherwise constrain per key with infer_record_get_lenient (open row, lenient like the total get builtin — a missing key binds () at runtime, so it's not a hard error), giving each field its own type.

Tests

Two regression tests in check::tests:

• record_body_is_not_effect_annotation — record body plus a check that the {IO Fail} quirk form still parses as a declared effect set
• map_destructure_record_fields_keep_own_types — the monoid program above, plus a heterogeneous {:log "x" :val 41} destructure

The repro now checks and prints log: step1;step2; value: done. Full workspace suite passes.

🤖 Generated with Claude Code

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Project	Deployment	Actions	Updated (UTC)
loon	Ready	Preview, Comment	Jul 14, 2026 2:05pm

[chojiai]

Choji review — Nothing blocking — a few notes

Both fixes are correct and well-targeted: the annotation-vs-body disambiguation is sound (all-symbol-keys + trailing body = annotation, anything else = body), and the per-key record-row path in bind_map_destructure correctly replaces the single shared element type. The prior open NITs are either resolved by the new body-requirement guard or remain minor style points that don't affect correctness.

1 minor finding

• Minor · Design churn — Recently changed by someone else crates/loon-lang/src/check/mod.rs
  1 file in this PR was changed by other people in the last 14 days. If those were deliberate choices, make sure you're not undoing them — loop the recent authors in before merging:
• crates/loon-lang/src/check/mod.rs — last changed by @claude 7 days ago

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Verified by Choji

Choji ran your change live — checks passed.

1 check.

Claim	Result	Evidence
cargo check	Verified	exit 0 · 275s

The ticket asked for the type-checker to stop treating record-literal fn bodies as effect annotations and to fix map destructuring collapsing record fields to one type; the verification checks confirm the fix works correctly. No live preview was run since this is a compiler/type-checker change with no web UI surface. cargo check passed (exit 0), and per the PR description the full workspace test suite passes including the two new regression tests (record_body_is_not_effect_annotation and map_destructure_record_fields_keep_own_types). There is nothing observable in a running UI to verify, but the check results are the appropriate signal for this kind of change and they are clean.

No issues found in the running app.

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Reviewed 45aa9f3 · Rate findings · Choji updates this comment as you push

Dismissing prior approval to re-evaluate 45aa9f3.