St eval

Skill mumez/smalltalk-dev-plugin/skills/st-eval

Claude Code plugin for AI-driven Smalltalk (Pharo) development

Install

npx -y skills add mumez/smalltalk-dev-plugin --skill st-eval

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Smalltalk code evaluator for Pharo via MCP. Use when executing Smalltalk expressions, verifying object state or intermediate values, debugging code incrementally, checking Pharo connection, or running quick experiments.

SKILL.md

2.6 KB, as published. Nobody here has run it

Execute Smalltalk Code

Evaluate arbitrary Smalltalk expressions in the running Pharo image via mcp__smalltalk-interop__eval.

Essential Rules

Always use printString — MCP returns serialized output. Raw objects cause errors or unreadable results:

✅ MyClass new doSomething printString
✅ collection printString
❌ MyClass new doSomething

Wrap risky code in on:do: — Uncaught errors in eval crash the MCP call with no useful message:

| result |
result := Array new: 2.
[ result at: 1 put: (riskyOperation) printString ]
  on: Error do: [:ex | result at: 2 put: ex description].
^ result

Use fork for blocking operations — Code that opens a dialog or blocks the image will cause the MCP call to hang indefinitely. Detach it:

[ <blocking expression> ] fork.
^ 'started'

Common Patterns

Connection check

Smalltalk version

Inspect intermediate values

| step1 step2 |
step1 := objA computeStep1.
step2 := step1 processStep2.
^ { 'step1' -> step1 printString. 'step2' -> step2 printString } asDictionary printString

Inspect object state

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

Safe collection access

collection ifEmpty: ['empty'] ifNotEmpty: [:col | col first printString]

Error-capturing eval

| 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

Troubleshooting

Symptom	Cause	Fix
MCP call hangs / no response	Blocking operation (dialog, modal, infinite loop)	Wrap in `fork`
`Error: cannot serialize`	Returning raw object	Add `printString`
Debugger window opened in Pharo	Uncaught error triggered Pharo debugger	Close debugger in Pharo, then re-eval with `on:do:`
`MessageNotUnderstood`	Method doesn't exist or typo	Check with `mcp__smalltalk-interop__search_implementors`

Related Skills

smalltalk-dev:smalltalk-debugger — Systematic debugging workflow using eval
smalltalk-dev:st-init — Verify Pharo connection if eval fails unexpectedly