Lecture: Refactoring

Metaprogramming and Reflection Refactoring Universit ät Bern Marcus Denker Hasso-Plattner-Institut Potsdam Software Architecture Group Prof. Dr. Robert Hirschfeld http://www.swa.hpi.uni-potsdam.de WS 2006/2007

Topics Covered Overview Introduction Open implementations OMG meta object facility CLOS metaobject protocol Smalltalk/Squeak Behavioral reflection Refactoring browser AspectS and ContextS Traits and mirrors Metacircular interpreters Objects

Outline Refactoring: Basics Refactoring in Squeak: Browser + Tools Refactoring Engine: Implementation Discussion: Reflection?

What is Refactoring? The process of changing a software system in such a way that it does not alter the external behaviour of the code, yet improves its internal structure . Fowler, et al., Refactoring, 1999.

Typical Refactorings extract code in new method abstract variable move method to component create accessors add parameter to method pull variable up push method up push variable down push method down remove variable remove method remove class rename variable rename method rename class add variable to class add method to class add (sub)class to hierarchy Attribute Method Class

Why Refactor? “ Grow, don’t build software” Fred Brooks The reality: Extremely difficult to get the design “right” the first time Hard to fully understand the problem domain Hard to understand user requirements, even if the user does! Hard to know how the system will evolve in five years Original design is often inadequate System becomes brittle over time, and more difficult to change Refactoring helps you to Manipulate code in a safe environment (behavior preserving) Recreate a situation where evolution is possible Understand existing code

Rename Method — manual steps Do it yourself approach: Check that no method with the new name already exists in any subclass or superclass. Browse all the implementers (method definitions) Browse all the senders (method invocations) Edit and rename all implementers Edit and rename all senders Remove all implementers Test Automated refactoring is better !

Rename Method Rename Method (method, new name) Preconditions No method with the new name already exists in any subclass or superclass. No methods with same signature as method outside the inheritance hierarchy of method PostConditions method has new name relevant methods in the inheritance hierarchy have new name invocations of changed method are updated to new name Other Considerations Typed/Dynamically Typed Languages => Scope of the renaming

Refactoring and Metaprograming Automated Refactoring is metaprograming We use a program to edit programs Does not need to use Reflection Pure source-to-source transformation (e.g. Java) Uses reflective facilities in Smalltalk But… let’s discuss that at the end

Refactoring in Squeak No support in standard IDE Example: Try to rename a method Refactoring Browser First Refactoring Browser (for any language) Now over 10 years old Installation Get Squeak 3.9 (older version for 3.8, too) Install Package AST Install Package Refactoring Engine

Refactoring Browser Browser with menus for e.g rename Push up/down Inlining Add parameter Extraction

SmallLint Checks for common mistakes

SmallLint Checks Possible Bugs Variable read before written Defines #= but not #hash Modifies Collection while iterating over it Bugs Uses True/False instead of true/false Variable used but not defined Squeak Bugs Unnecessary Code Intention Revealing

SmallLint Very useful! Especially valuable for beginners Has been integrated with SUnit Call SmallLint automatically as a test Tag methods where SmallLint is wrong Uses Squeak 3.9 Method Pragmas

RewriteTool Pattern driven automatic editor

RewriteTool Access to full power of Refactoring Engine Custom refactorings: generic rewrites that the RB does not currently provide bulk transformations: your project needs to change a project-specific pattern to a new form changing layers: e.g. build a new DB layer, find and change 17,000 references to old layer migrations: e.g. between Smalltalk dialects Powerful but not trivial to use Examples: Later

Outline Refactoring: Basics Refactoring in Squeak: Browser + Tools Refactoring Engine: Implementation Discussion: Reflection? [Main Source] Don Roberts, John Brant, and Ralph Johnson. A Refactoring Tool for Smalltalk . Theory and Practice of Object Systems , vol. 3, issue 4

Implementation Overview Goal: Transformation on the Source Idea: Transform into a higher level tree representation

The RB Abstract Syntax Tree AST: Abstract Syntax Tree Encodes the Syntax as a Tree Features: Visitors Backward pointers in ParseNodes Encodes formatting Transformation (replace/add/delete) Pattern-directed TreeRewriter PrettyPrinter RBProgramNode RBDoItNode RBMethodNode RBReturnNode RBSequenceNode RBValueNode RBArrayNode RBAssignmentNode RBBlockNode RBCascadeNode RBLiteralNode RBMessageNode RBOptimizedNode RBVariableNode

A Simple AST explore it RBParser parseExpression: '3+4'

A Simple Visitor RBProgramNodeVisitor new visitNode: tree Does nothing except walk through the tree

More Complete Visitor RBProgramNodeVisitor subclass: #TestVisitor instanceVariableNames: 'literals' classVariableNames: '' poolDictionaries: '' category: 'Compiler-AST-Visitors' TestVisitor>> acceptLiteralNode: aLiteralNode literals add: aLiteralNode value. TestVisitor>> initialize literals := Set new. TestVisitor>> literals ^literals tree := RBParser parseExpression: '3 + 4'. (TestVisitor new visitNode: tree) literals a Set(3 4)

Tree Matcher Implementing all Refactorings with visitors Too much work Too low level Needed: High level specification of transformations Rewrite Engine: Core of Refactoring Engine No only useful for Refactoring!

Tree Matcher Describe transformation by using patterns Syntax: Smalltalk + Meta Variables Example: | `@Temps | ``@.Statements. ``@Boolean ifTrue: [^false]. ^true

Meta Variables All Meta Variables begin with ` `#literal literal # `.Statement statement . | `@Temps | `@.statements list @ ``@object foo recurse into ` Example Type Character

Example 1 Search for: ``@object not ifTrue: ``@block Replace with: ``@object ifFalse: ``@block Explanation: Eliminate an unnecesary not message

Example 2 Search for: | `@Temps | ``@.Statements. ``@Boolean ifTrue: [^false]. ^true Replace with: | `@Temps | ``@.Statements. ^``@Boolean not Explanation: Return the value of the boolean negated instead of using a conditional

Implementation: Model and Environment Model Code transformed but not installed We need to be able to see refactored code without changing the system. RBNameSpace Model Classes + Methods Framework duplicates Smalltalk‘s structural Reflection RBClass, RBMethod Model Scope to which Refactorings apply RBEnvironment

Back to Code: Pretty Printer Visitor: Walks the AST Prints out text for each node Problem: How to preserve formatting? AST saves formatting (whitespace, parenthesis) Pretty Printer can use saved formatting information

Contributions needed Improved UI Integrated Parser with Squeak NewCompiler Scanner/Parser done with tool (SmaCC) Easier to change / experiment Integrated RoelTyper Heuristical type inference Better PrettyPrinter Configurability Better Layout

Outline Refactoring: Basics Refatoring in Squeak: Browser + Tools Refactoring Engine: Implementation Discussion: Reflection?

Reflection? We change the system using itself So it‘s Reflection, on some level But: Let‘s look again at the definition Model of itself Causally connected We Build our own abstraction layer AST + Environment This Model is not causally connected!

Towards Better Reflection High-level sub-method structural reflection First prototype: Persephone „ AST only“ Smalltalk: causally connected AST Uses RB AST Annotation Framework Provides ByteSurgeon functionality on AST Lots of interesting projects: Presentation framework for annotated code … ..

Outline Refactoring: Basics Refatoring in Squeak: Browser + Tools Refactoring Engine: Implementation Discussion: Reflection? Questions?

Lecture: Refactoring

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