Refactoring Erlang Programs

RefactoringErlang Programs

Huiqing Li

Simon Thompson

University of Kent

Overview

What is refactoring?

Examples

The process of refactoring

Tool building and infrastructure

What is in Wrangler … demo

Latest advances: data, processes, erlide.

Introducing refactoring

Soft-ware

There’s no single correct design …

… different options for different situations.

Maintain flexibility as the system evolves.

Refactoring

Refactoring means changing the design or structure of a program … without changing its behaviour.

RefactorModify

Examples

Generalisation

-module (test).

-export([f/1]).

add_one ([H|T]) ->

[H+1 | add_one(T)];

add_one ([]) -> [].

f(X) -> add_one(X).

-module (test).

-export([f/1]).

add_one (N, [H|T]) ->

[H+N | add_one(N,T)];

add_one (N,[]) -> [].

f(X) -> add_one(1, X).

-module (test).

-export([f/1]).

add_int (N, [H|T]) ->

[H+N | add_int(N,T)];

add_int (N,[]) -> [].

f(X) -> add_int(1, X).

Generalisation and renaming

Generalisation

-export([printList/1]).

printList([H|T]) ->

io:format("~p\n",[H]),

printList(T);

printList([]) -> true.

printList([1,2,3])

-export([printList/2]).

printList(F,[H|T]) ->

F(H),

printList(F, T);

printList(F,[]) -> true.

printList(

fun(H) ->

io:format("~p\n", [H])

end,

[1,2,3]).

Generalisation

-export([printList/1]).

printList([H|T]) ->

io:format("~p\n",[H]),

printList(T);

printList([]) -> true.

-export([printList/1]).

printList(F,[H|T]) ->

F(H),

printList(F, T);

printList(F,[]) -> true.

printList(L) ->

printList(

fun(H) ->

io:format("~p\n", [H]) end,

L).

Asynchronous to synchronous

pid! {self(),msg}

{Parent,msg} ->

body

pid! {self(),msg}, receive {pid, ok}-> ok

{Parent,msg} ->

Parent! {self(),ok},

body

Refactoring

Refactoring = Transformation + Condition

Transformation

Ensure change at all those points needed.

Ensure change at only those points needed.

Condition

Is the refactoring applicable?

Will it preserve the semantics of the module? the program?

Transformations

full stop one

Condition > Transformation

Renaming an identifier

"The existing binding structure should not be affected. No binding for the new name may intervene between the binding of the old name and any of its uses, since the renamed identifier would be captured by the renaming. Conversely, the binding to be renamed must not intervene between bindings and uses of the new name."

Which refactoring exactly?

Generalise f by making 23 a parameter of f:

f(X) ->

Con = 23,

g(X) + Con + 23.

• This one occurrence?• All occurrences (in the body)?• Some of the occurrences … to be selected.

Compensate or crash?

-export([oldFun/1,

newFun/1]).

oldFun(L) ->

newFun(L).

newFun(L) ->

…

… .

-export([newFun/1]).

newFun(L) ->

…

… .

or ?

Refactoring tools

Tool support

Bureaucratic and diffuse.

Tedious and error prone.

Semantics: scopes, types, modules, …

Undo/redo

Enhanced creativity

Semantic analysis

Binding structure• Dynamic atom creation, multiple binding occurrences, pattern semantics etc.

Module structure and projects• No explicit projects for Erlang; cf Erlide / Emacs.

Type and effect information• Need effect information for e.g. generalisation.

Erlang refactoring: challenges

Multiple binding occurrences of variables.

Indirect function call or function spawn: apply (lists, rev, [[a,b,c]])

Multiple arities … multiple functions: rev/1

Concurrency

Refactoring within a design library: OTP.

Side-effects.

Static vs dynamic

Aim to check conditions statically.

Static analysis tools possible … but some aspects intractable: e.g. dynamically manufactured atoms.

Conservative vs liberal.

Compensation?

Architecture of Wrangler

Wrangler in Emacs

Wrangler in Emacs

Wrangler refactorings

Rename variable/function/module

Generalise function definition

Move a function definition to another (new) module

Function extraction

Fold expression against function

Expression search

Detect duplicate code

Tuple function parameters

From tuple to record

Wrangler demo

Tool building

Wrangler and RefactorErl

Lightweight.

Better integration with interactive tools (e.g. emacs).

Undo/redo external?

Ease of implementing conditions.

Higher entry cost.

Better for a series of refactorings on a large project.

Transaction support.

Ease of implementing transformations.

Integration … with IDEs

Back to the future? Programmers' preference for emacs and gvim …

… though some IDE interest: Eclipse, NetBeans …

Issue of integration with multiple IDEs: building common interfaces.

Integration … with tools

Test data sets and test generation.

Makefiles, etc.

Working with macros e.g. QuickCheck uses Erlang macros …

… in a particular idiom.

APIs … programmer / user

API in Erlang to support user-programmed refactorings: • declarative, straightforward and complete • but relatively low-level.

Higher-level combining forms? • OK for transformations, but need a separate condition language.

Verification and validation

Possible to write formal proofs of correctness:• check conditions and transformations• different levels of abstraction

• possibly-name binding substitution for renaming etc.• more abstract formulation for e.g. data type changes.

Use of Quivq QuickCheck to verify refactorings in Wrangler.

Clone detection

The Wrangler Clone Detector

Uses syntactic and static semantic information.

Syntactically well-formed code fragments

… identical after consistent renaming of variables,

… with variations in literals, layout and comments.

Integrated within the refactoring environment.

The Wrangler Clone Detector

Make use of token stream and annotated AST. Token–based approaches Efficient. Report non-syntactic clones. AST-based approaches.

Report syntactic clones. Checking for consistent renaming is easier.

The Wrangler Clone Detector

Source Files

TokenisationTokenisation

Token Stream

NormalisationNormalisation

Normalised Token Stream

Suffix Tree ConstructionSuffix Tree Construction

Suffix tree

The Wrangler Clone Detector

Source Files

TokenisationTokenisation

Token Stream

NormalisationNormalisation

Normalised Token Stream

Suffix Tree ConstructionSuffix Tree Construction

Suffix tree

Clone CollectorClone Collector

Initial Clones

Clone FilterClone Filter

Filtered Initial Clones

Clone DecompositionClone Decomposition

Parsing + Static AnalysisParsing + Static Analysis

Annotated ASTs

Syntactic Clones

The Wrangler Clone Detector

Source Files

TokenisationTokenisation

Token Stream

NormalisationNormalisation

Normalised Token Stream

Suffix Tree ConstructionSuffix Tree Construction

Suffix tree

Clone CollectorClone Collector

Initial Clones

Clone FilterClone Filter

Filtered Initial Clones

Clone DecompositionClone Decomposition

Parsing + Static AnalysisParsing + Static Analysis

Annotated ASTs

Syntactic Clones Consistent Renaming

CheckingConsistent Renaming

Checking

Clones to report

The Wrangler Clone Detector

Source Files

TokenisationTokenisation

Token Stream

NormalisationNormalisation

Normalised Token Stream

Suffix Tree ConstructionSuffix Tree Construction

Suffix tree

Clone CollectorClone Collector

Initial Clones

Clone FilterClone Filter

Filtered Initial Clones

Clone DecompositionClone Decomposition

Parsing + Static AnalysisParsing + Static Analysis

Annotated ASTs

Syntactic Clones Consistent Renaming

CheckingConsistent Renaming

Checking

Clones to report

FormattingFormatting

Reported Code Clones

Clone detection demo

Support for clone removal

Refactorings to support clone removal.

Function extraction.

Generalise a function definition.

Fold against a function definition.

Case studies

Applied the clone detector to Wrangler itself with threshold values of 30 and 2.

36 final clone classes were reported …12 are across modules, and 3 are duplicated function definitions. Without syntactic checking and consistent variable renaming checking, 191 would have been reported.

Applied to third party code base (32k loc, 89 modules),109 clone classes reported.

Data-oriented refactorings

-module(tup1).

-export([gcd/1]).

gcd({X,Y}) ->

if X>Y ->

gcd({X-Y,Y});

Y>X ->

gcd({Y-X,X});

true ->

X

end.

Tupling parameters

-module(tup1).

-export([gcd/2]).

gcd(X,Y) ->

if X>Y ->

gcd(X-Y,Y);

Y>X ->

gcd(Y-X,X);

true ->

X

end.

2

-module(rec1).

-record(rec,{f1, f2}).

g(#rec{f1=A, f2=B})->

A + B.

h(X, Y)->

g(#rec{f1=X,f2=X}),

g(#rec{

f1=element(1,Y),

f2=element(2,Y)}).

Introduce records …

-module(rec1).

g({A, B})->

A + B.

h(X, Y)->

g({X, X}),

g(Y).

f1 f2

Introduce records in a project

Need to replace other expressions …

• Replace tuples with record

• Record update expression

• Record access expression

Chase dependencies across functions …

… and across modules.

Refactoring and Concurrency

Wrangler and processes

Refactorings which address processes

• Register a process.

• Rename a registered process.

• From function to process.

• Add tags to messages sent / received.

Challenges to implementation

Data gathering is a challenge because

• Processes are syntactically implicit.

• Pid to process links are implicit.

• Communication structure is implicit.

• Side effects.

Underlying analysis

Analyses include

• Annotation of the AST, using call graph.

• Forward program slicing.

• Backwards program slicing.

Wrangler and Erlide

Wrangler and Erlide

Erlide is an Eclipse plugin for Erlang.

• Distribution simplified.

• Integration with the edit undo history.

• Notion of project.

• Refactoring API in the Eclipse LTK.

Ongoing support for Erlide from Ericsson.

Issues on integration

LTK has a fixed workflow for interactions.

• New file vs set of diffs as representation.

• Fold and generalise interaction pattern.

• Cannot support rename / create file.

Other refactorings involve search … a different API.

Conclusions

Future work

Concurrency: continue work.

Refactoring within a design library: OTP.

Working with Erlang Training and Consulting.

Continue integration with Eclipse + other IDEs.

Test and property refactoring in .

Clone detection: fuller integration.

Ackonwledgements

Wrangler development funded by EPSRC.

The developers of syntax-tools, distel and Erlide.

George Orosz and Melinda Toth.

Zoltan Horvath and the RefactorErl group at Eotvos Lorand Univ., Budapest.

http://projects.cs.kent.ac.uk