Smalltalk Debugger

Systematic debugging techniques for Pharo Smalltalk development using AI editors.

Core Debugging Workflow

When tests fail or errors occur, follow this systematic approach:

1. Identify Error Location

From error message, confirm:

• Error type (MessageNotUnderstood, KeyNotFound, etc.)
• Stack trace - where error occurred
• Expected vs Actual - what went wrong

2. Verify with Partial Execution

Use /st:eval tool to execute relevant code incrementally.

Basic error capture pattern:

| result |
result := Array new: 2.
[ | ret |
  ret := objA doSomething.
  result at: 1 put: ret printString.
] on: Error do: [:ex | result at: 2 put: ex description].
^ result

Interpreting results:

• result at: 1 - Normal result (success case)
• result at: 2 - Error description (failure case)

3. Check Intermediate Values

Inspect state at each step:

| step1 step2 |
step1 := self getData.
step2 := step1 select: [:each | each isValid].
{
    'step1 count' -> step1 size.
    'step2 count' -> step2 size.
    'step2 result' -> step2 printString
} asDictionary printString

4. Fix and Re-test

1. Fix in Tonel file (never in Pharo)
2. Re-import with import_package
3. Re-test with run_class_test

When Operations Stop Responding

If MCP tool calls hang or timeout with no response, a debugger window may have opened in the Pharo image. Since the debugger is invisible from the AI editor, operations will appear stuck.

Detecting Hidden Debuggers

Use the read_screen tool to capture the Pharo UI state:

mcp__smalltalk-interop__read_screen: target_type='world'

This captures all morphs including debugger windows. Look for:

• Window titles containing "Debugger", "Error", or "Exception"
• UI hierarchy showing debugger-related components
• Error messages or stack traces in window content

Resolution Steps

1. Notify the user: Inform them that a debugger window appears to be open in Pharo
2. Request manual intervention: Ask the user to:
   • Check their Pharo image for open debugger windows
   • Close any debugger windows
   • Review the error shown in the debugger to understand the root cause
3. Address root cause: Once the debugger is closed, investigate and fix the underlying error using standard debugging techniques
4. Retry operation: Re-run the failed MCP operation

Note: The Pharo debugger cannot be controlled remotely through MCP tools. User intervention in the Pharo image is required.

For complete UI debugging guidance, see UI Debugging Reference.

Essential Debugging Patterns

Pattern 1: Execute with Error Handling

Safely test code that might fail:

| result |
result := Array new: 2.
[
    | obj |
    obj := MyClass new name: 'Test'.
    result at: 1 put: obj process printString.
] on: Error do: [:ex |
    result at: 2 put: ex description
].
^ result

Pattern 2: Inspect Object State

Check what object contains:

{
    'class' -> obj class name.
    'value' -> obj printString.
    'size' -> obj size.
    'isEmpty' -> obj isEmpty
} asDictionary printString

Pattern 3: Debug Collection Operations

Track data flow through transformations:

| items filtered mapped |
items := self getItems.
filtered := items select: [:each | each isValid].
mapped := filtered collect: [:each | each name].
{
    'items size' -> items size.
    'filtered size' -> filtered size.
    'mapped' -> mapped printString
} asDictionary printString

Common Error Types Quick Reference

MessageNotUnderstood

Cause: Method doesn't exist or typo in method name Debug: Check spelling, search implementors

mcp__smalltalk-interop__search_implementors: 'methodName'

KeyNotFound

Cause: Accessing non-existent Dictionary key Debug: List keys, use at:ifAbsent:

dict keys printString
dict at: #key ifAbsent: ['default']

SubscriptOutOfBounds

Cause: Collection index out of range Debug: Check size, use at:ifAbsent:

collection size printString
collection at: index ifAbsent: [nil]

ZeroDivide

Cause: Division by zero Debug: Check denominator before dividing

count = 0 ifTrue: [0] ifFalse: [sum / count]

AssertionFailure (in tests)

Cause: Test expectation doesn't match actual Debug: Execute test code with /st:eval, check if package imported

For complete error patterns and solutions, see Error Patterns Reference.

Object Inspection Quick Guide

Basic Inspection

" Object class "
obj class printString

" Instance variables "
obj instVarNames

" Check method exists "
obj respondsTo: #methodName

Collection Inspection

" Size and elements "
collection size
collection printString

" Safe first/last "
collection ifEmpty: [nil] ifNotEmpty: [:col | col first]

Dictionary Inspection

" Keys and values "
dict keys
dict values

" Safe access "
dict at: #key ifAbsent: ['default']

For comprehensive inspection techniques, see Inspection Techniques Reference.

Debugging Best Practices

1. Divide into Small Steps

Break problems into incremental steps and verify each:

" Step 1: Verify object creation "
obj := MyClass new.
obj printString

" Step 2: Verify method call "
result := obj doSomething.
result printString

2. Always Use printString

When returning objects via JSON/MCP:

✅ obj printString
✅ collection printString
✅ dict printString

❌ obj  " Don't return raw objects "

3. Check Intermediate Values

Never assume - verify at each step:

intermediate := obj step1.
" Check here "
result := intermediate step2.
" Check here too "

4. Use Error Handling

Always capture errors with on:do::

[
    risky operation
] on: Error do: [:ex |
    " Handle or log error "
    ex description
]

5. Fix in Tonel, Not Pharo

• ✅ Edit .st file → Import → Test
• ❌ Edit in Pharo → Export → Commit

Debugging Tools

Primary Tool: /st:eval

Execute any Smalltalk code for testing and verification:

mcp__smalltalk-interop__eval: 'Smalltalk version'
mcp__smalltalk-interop__eval: '1 + 1'
mcp__smalltalk-interop__eval: 'MyClass new doSomething printString'

Code Inspection Tools

mcp__smalltalk-interop__get_class_source: 'ClassName'
mcp__smalltalk-interop__get_method_source: class: 'ClassName' method: 'methodName'
mcp__smalltalk-interop__search_implementors: 'methodName'
mcp__smalltalk-interop__search_references: 'methodName'

Practical Examples

Example 1: Test Failure

Error: Expected 'John Doe' but got 'John nil'

Debug process:

1. Execute test code with /st:eval
2. Check if lastName was set
3. Inspect method implementation
4. Identify missing ^ self in setter
5. Fix in Tonel file
6. Re-import and re-test

Example 2: KeyNotFound

Error: KeyNotFound: key #age not found

Debug process:

1. List dictionary keys: dict keys
2. Check if age key exists: dict includesKey: #age
3. Use safe access: dict at: #age ifAbsent: [0]
4. Fix initialization to include age key

For complete debugging scenarios with step-by-step solutions, see Debug Scenarios Examples.

Troubleshooting Checklist

When debugging, systematically check:

• Read complete error message
• Use /st:eval to test incrementally
• Inspect all intermediate values
• Check method implementation
• Verify package was imported
• Edit Tonel file (not Pharo)
• Re-import after fixing
• Re-run tests

Complete Documentation

This skill provides focused debugging guidance. For comprehensive information:

• Error Patterns Reference - All error types with solutions
• Inspection Techniques - Complete object inspection guide
• Debug Scenarios - Real-world debugging examples

Summary

Core debugging cycle:

Error occurs
    ↓
Identify error type
    ↓
Use /st:eval to test incrementally
    ↓
Inspect intermediate values
    ↓
Identify root cause
    ↓
Fix in Tonel file
    ↓
Re-import
    ↓
Re-test → Success or repeat

Remember: Systematic approach, incremental testing, fix in Tonel, always re-import.