From 6960cd325cd028ef27fda5ee74f15371dbe9e4e7 Mon Sep 17 00:00:00 2001
From: VasilevNStas <ctac@outlook.com>
Date: Tue, 11 Aug 2026 17:30:33 +0300
Subject: [PATCH] feat(#155): add too-deep-object lint

---
 .../eolang/lints/design/too-deep-object.xsl   | 34 +++++++++++++++++++
 .../eolang/motives/design/too-deep-object.md  | 29 ++++++++++++++++
 .../allows-normal-nesting.yaml                | 18 ++++++++++
 .../catches-too-deep-object.yaml              | 25 ++++++++++++++
 4 files changed, 106 insertions(+)
 create mode 100644 src/main/resources/org/eolang/lints/design/too-deep-object.xsl
 create mode 100644 src/main/resources/org/eolang/motives/design/too-deep-object.md
 create mode 100644 src/test/resources/org/eolang/lints/packs/single/too-deep-object/allows-normal-nesting.yaml
 create mode 100644 src/test/resources/org/eolang/lints/packs/single/too-deep-object/catches-too-deep-object.yaml

diff --git a/src/main/resources/org/eolang/lints/design/too-deep-object.xsl b/src/main/resources/org/eolang/lints/design/too-deep-object.xsl
new file mode 100644
index 000000000..0b6adea35
--- /dev/null
+++ b/src/main/resources/org/eolang/lints/design/too-deep-object.xsl
@@ -0,0 +1,34 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+* SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+* SPDX-License-Identifier: MIT
+-->
+<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:eo="https://www.eolang.org" version="2.0" id="too-deep-object">
+  <xsl:import href="/org/eolang/funcs/lineno.xsl"/>
+  <xsl:import href="/org/eolang/funcs/escape.xsl"/>
+  <xsl:import href="/org/eolang/funcs/defect-context.xsl"/>
+  <xsl:output encoding="UTF-8" method="xml"/>
+  <xsl:template match="/">
+    <defects>
+      <xsl:for-each select="//o[count(ancestor::o[not(@base='Φ.tuple') and not(@base='Φ.bytes') and not(@base='Φ.number') and not(@base='Φ.string')]) &gt; 12 and not(o[count(ancestor::o[not(@base='Φ.tuple') and not(@base='Φ.bytes') and not(@base='Φ.number') and not(@base='Φ.string')]) &gt; 12])]">
+        <defect>
+          <xsl:variable name="line" select="eo:lineno(@line)"/>
+          <xsl:attribute name="line">
+            <xsl:value-of select="$line"/>
+          </xsl:attribute>
+          <xsl:if test="$line = '0'">
+            <xsl:attribute name="context">
+              <xsl:value-of select="eo:defect-context(.)"/>
+            </xsl:attribute>
+          </xsl:if>
+          <xsl:attribute name="severity">warning</xsl:attribute>
+          <xsl:text>The object </xsl:text>
+          <xsl:value-of select="eo:escape(@base)"/>
+          <xsl:text> is nested too deep: </xsl:text>
+          <xsl:value-of select="count(ancestor::o)"/>
+          <xsl:text> levels of nesting, while the maximum is 12</xsl:text>
+        </defect>
+      </xsl:for-each>
+    </defects>
+  </xsl:template>
+</xsl:stylesheet>
diff --git a/src/main/resources/org/eolang/motives/design/too-deep-object.md b/src/main/resources/org/eolang/motives/design/too-deep-object.md
new file mode 100644
index 000000000..7636a7fcf
--- /dev/null
+++ b/src/main/resources/org/eolang/motives/design/too-deep-object.md
@@ -0,0 +1,29 @@
+# Too deep object
+
+Objects nested deeper than twelve levels are hard to read and to debug.
+The nesting level of an object is the number of its ancestor objects.
+If it exceeds twelve, a warning is raised.
+
+Incorrect:
+
+```eo
+# Foo.
+[] > foo
+  a > x
+    b
+      c
+        d
+          e
+            f
+              g
+                h
+                  i
+                    j
+                      k
+                        l
+                          m
+```
+
+Here, `m` is nested thirteen levels deep, which is hard to follow.
+It should be refactored by extracting some of the nested objects into
+their own top-level objects.
diff --git a/src/test/resources/org/eolang/lints/packs/single/too-deep-object/allows-normal-nesting.yaml b/src/test/resources/org/eolang/lints/packs/single/too-deep-object/allows-normal-nesting.yaml
new file mode 100644
index 000000000..812013736
--- /dev/null
+++ b/src/test/resources/org/eolang/lints/packs/single/too-deep-object/allows-normal-nesting.yaml
@@ -0,0 +1,18 @@
+# SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+# SPDX-License-Identifier: MIT
+---
+sheets:
+  - /org/eolang/lints/design/too-deep-object.xsl
+asserts:
+  - /defects[count(defect[@severity='warning'])=0]
+input: |
+  # Foo.
+
+  [] > foo
+    a > x
+      b
+        c
+          d
+            e
+              f
+                g
diff --git a/src/test/resources/org/eolang/lints/packs/single/too-deep-object/catches-too-deep-object.yaml b/src/test/resources/org/eolang/lints/packs/single/too-deep-object/catches-too-deep-object.yaml
new file mode 100644
index 000000000..b5eba891d
--- /dev/null
+++ b/src/test/resources/org/eolang/lints/packs/single/too-deep-object/catches-too-deep-object.yaml
@@ -0,0 +1,25 @@
+# SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+# SPDX-License-Identifier: MIT
+---
+sheets:
+  - /org/eolang/lints/design/too-deep-object.xsl
+asserts:
+  - /defects[count(defect[@severity='warning'])=1]
+  - /defects/defect[@line='16']
+input: |
+  # Foo.
+
+  [] > foo
+    a > x
+      b
+        c
+          d
+            e
+              f
+                g
+                  h
+                    i
+                      j
+                        k
+                          l
+                            m