lint rule: check for branching expressions

docs/guides/lint_rules/index.md

@@ -40,6 +40,7 @@ These issues may cause runtime problems.
 | Code | Name | Description | Fixable |
 |------|------|-------------|----------|
 | [MR001](rules/self_import.md) | self-import | Importing a module with the same name as the file | ❌ |
+| [MR002](rules/branch_expression.md) | branch-expression | Branch statements with output expressions that won't be displayed | ❌ |
 
 ### ✨ Formatting Rules
 
@@ -53,7 +54,7 @@ These are style and formatting issues.
 | [MF004](rules/empty_cells.md) | empty-cells | Empty cells that can be safely removed. | ⚠️ |
 | [MF005](rules/sql_parse_error.md) | sql-parse-error | SQL parsing errors during dependency analysis | ❌ |
 | [MF006](rules/misc_log_capture.md) | misc-log-capture | Miscellaneous log messages during processing | ❌ |
-| [MF007](rules/markdown_indentation.md) | markdown-indentation | Markdown cells in `mo.md()` should be dedented. | 🛠️ |
+| [MF007](rules/markdown_indentation.md) | markdown-indentation | Markdown cells in `mo.md()` should be properly indented. | 🛠️ |
 
 ## Legend
 

docs/guides/lint_rules/rules/branch_expression.md

@@ -0,0 +1,76 @@
+# MR002: branch-expression
+
+⚠️ **Runtime** ❌ Not Fixable
+
+MR002: Branch statements with output expressions that won't be displayed.
+
+## Why is this bad?
+
+When output expressions are nested inside branches at the end of a cell:
+- The expressions execute but produce no visible output
+- Users expect to see the result (like mo.md(), string literals, etc.)
+- This can lead to confusion about whether code is running correctly
+- It violates the principle of least surprise
+
+This is a runtime issue because it causes unexpected behavior where the user's
+intended output is silently ignored.
+
+## Examples
+
+**Problematic:**
+```python
+if condition:
+    mo.md("Result A")  # Won't be displayed
+else:
+    mo.md("Result B")  # Won't be displayed
+```
+
+**Problematic:**
+```python
+match value:
+    case 1:
+        "Too short"  # Won't be displayed
+    case _:
+        "Just right"  # Won't be displayed
+```
+
+**Not flagged (side effects only):**
+```python
+if condition:
+    print("Debug message")  # Side effect only, not flagged
+else:
+    logger.info("Something")  # Side effect only, not flagged
+```
+
+**Solution:**
+```python
+# Assign to a variable that marimo will display
+result = mo.md("Result A") if condition else mo.md("Result B")
+result
+```
+
+**Solution:**
+```python
+# Create a default variable for response.
+result = None
+if condition:
+    result = expr()
+else:
+    result = other()
+result
+```
+
+**Alternative Solution (if no output intended):**
+```python
+# Use a dummy variable to indicate intentional suppression
+if condition:
+    _ = expr()
+else:
+    _ = other()
+```
+
+## References
+
+- [Understanding Errors](https://docs.marimo.io/guides/understanding_errors/)
+- [Reactivity](https://docs.marimo.io/guides/reactivity/)
+

examples/ai/chat/streaming_openai.py

@@ -95,16 +95,14 @@ def _(mo):
 @app.cell
 def _(chatbot, mo):
     # Show chat history
-    if hasattr(chatbot, "value"):
-        mo.md(f"**Chat history:** {len(chatbot.value)} messages")
+    mo.md(f"**Chat history:** {len(chatbot.value)} messages") if hasattr(chatbot, "value") else None
     return
 
 
 @app.cell
 def _(chatbot):
     # Display full history
-    if hasattr(chatbot, "value"):
-        chatbot.value
+    chatbot.value if hasattr(chatbot, "value") else None
     return
 
 

examples/outputs/conditional_output.py

@@ -34,18 +34,18 @@ def _(checkbox):
 @app.cell(hide_code=True)
 def _(mo):
     mo.md("""
-    The next cell does **not** show anything, since an if statement does not
-    have a value:
-    """)
-    return
-
+    The following cell would **not** show anything, since an if statement does
+    not have a value:
 
-@app.cell
-def _(checkbox, mo):
+    ```python
+    # Intentionally demonstrates that if statements don't display expressions
+    # Using _ to suppress the lint warning while keeping the example
     if checkbox.value:
         mo.md("Checkbox is checked")
     else:
         mo.md("Checkbox is not checked")
+    ```
+    """)
     return
 
 

examples/sql/misc/database_explorer.py

@@ -50,7 +50,7 @@ def _(mo, os):
 @app.cell
 def _(database_url, duckdb):
     if database_url.value:
-        duckdb.sql(
+        _ = duckdb.sql(
             f"""
                 INSTALL postgres; 
                 LOAD postgres;

examples/third_party/cvxpy/signals/app.py

@@ -250,7 +250,7 @@ def __init__(self, sticky_bool):
         [complib.Components.TREND_LINE, complib.Components.PERIODIC]
     )
     if solved.now:
-        solved_ever.set()
+        _ = solved_ever.set()
     return (solved,)
 
 

marimo/_lint/rules/runtime/__init__.py

@@ -2,13 +2,16 @@
 from __future__ import annotations
 
 from marimo._lint.rules.base import LintRule
+from marimo._lint.rules.runtime.branch_expression import BranchExpressionRule
 from marimo._lint.rules.runtime.self_import import SelfImportRule
 
 RUNTIME_RULE_CODES: dict[str, type[LintRule]] = {
     "MR001": SelfImportRule,
+    "MR002": BranchExpressionRule,
 }
 
 __all__ = [
+    "BranchExpressionRule",
     "SelfImportRule",
     "RUNTIME_RULE_CODES",
 ]