From e174401caa10fe2a06534e5b0a9b54237c7f8d76 Mon Sep 17 00:00:00 2001
From: Javier Sagredo <javier.sagredo@iohk.io>
Date: Thu, 7 May 2026 02:20:31 +0200
Subject: [PATCH] Don't show "Right-hand side" for data fam-instances
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GHC's RHS for a @data instance@ is the synthetic R: TyCon (e.g.
@R:CrateInt@), and after the data-family R: rewrite it's the
abstract @FamilyApp Family [args]@ form — structurally equal to the
LHS. Labelling the panel row "Right-hand side: Crate Int" was
misleading because it just repeated what the heading already said.
Replace with a short "Data instance — the right-hand side is a
synthetic GHC-internal data constructor and isn't shown." note for
data fam-instances; type fam-instances keep the real RHS line.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
---
 data/viewer.js | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/data/viewer.js b/data/viewer.js
index 507a0c3..9102921 100644
--- a/data/viewer.js
+++ b/data/viewer.js
@@ -1918,17 +1918,25 @@
           <dt>Status</dt><dd><em>Use site of an external type family — the equation isn't in this project's dumps, so the right-hand side can't be resolved here.</em></dd>
         </dl>`;
     }
-    const rhs = escape(renderArg(fi.fiRhs, fi.fiTyVars));
     const tvs = fi.fiTyVars.length === 0 ? '<dd><em>none</em></dd>' :
       `<dd><ul>${fi.fiTyVars.map(v =>
         `<li>${escape(v.tvName)}<span style="color:#888"> :: ${escape(v.tvKind)}</span></li>`).join('')}</ul></dd>`;
     const src = renderDefinedAt(fi.fiSrc, fi.fiDefinedIn || fi.fiFamily.qnPackage);
+    // For data fam-instances we *don't* show a "Right-hand side" row.
+    // GHC's representation is the synthetic R: TyCon (`R:CrateInt`),
+    // and after our R: rewrite it's the abstract @FamilyApp Family
+    // [args]@ form — structurally equal to the LHS, so labelling it
+    // "Right-hand side" was actively misleading. Show a brief
+    // explanatory note instead.
+    const rhsEntry = fi.fiIsData
+      ? `<dt>Kind</dt><dd><em>Data instance — the right-hand side is a synthetic GHC-internal data constructor and isn't shown.</em></dd>`
+      : `<dt>Right-hand side</dt><dd>${escape(renderArg(fi.fiRhs, fi.fiTyVars))}</dd>`;
     return `
       <h2>${head}</h2>
       <p class="pkgmod">${escape(fi.fiFamily.qnPackage)} · ${escape(fi.fiFamily.qnModule)}</p>
       <dl>
         ${renderDocSection(fi.fiDoc)}
-        <dt>Right-hand side</dt><dd>${rhs}</dd>
+        ${rhsEntry}
         <dt>Type variables</dt>${tvs}
         <dt>Defined at</dt>${src}
       </dl>`;
-- 
2.54.0