WRT 2007 Refactoring Functional Programs Huiqing Li Simon Thompson Computing Lab Chris Brown Claus Reinke University of Kent

WRT 2007 Overview Refactoring tools. Functional programming. Haskell and Erlang. Tools and tool building. Tool design. Experience.

WRT 2007 Introduction

WRT 2007 Refactoring tool support Bureaucratic and diffuse. Tedious and error prone. Semantics: scopes, types, modules, … Undo/redo Enhanced creativity

WRT 2007 Functional Programming Values not variables. Expressions not :=. Functions as data. Rich data and types. Controlled side-effects. Declarative descriptions of refactorings.

WRT 2007 Haskell Erlang Strong, static types. Lazy evaluation. Layout sensitive. Concurrency library. Haskell 98 standard. GHC, NHC, Hugs, … Academic, O/S apps. Weak, dynamic types. Strict evaluation. Hot code swap, apply etc. Concurrency built-in. OTP, design patterns. Single system. Industrial applications.

WRT 2007 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

WRT 2007 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]).

WRT 2007 Asynchronous to synchronous pid! {self(),msg} {Parent,msg} -> body pid! {self(),msg}, receive {pid, ok}-> ok {Parent,msg} -> Parent! {self(),ok}, body

WRT 2007 The tools

WRT 2007 HaRe: the Haskell Refactorer Structural, data type, and module-level. In Haskell: embedded in gvim and emacs. Uses the Programatica framework … … + Strafunski program transformation library.

WRT 2007 Wrangler: refactoring Erlang Structural, data type and module-level refactorings. In Erlang plus emacs. Uses the Erlang system framework … … + untyped Erlang transformation library. Respects aspects of the macro system.

WRT 2007 Design to be usable

WRT 2007 Coverage and Standards De jure standard, Haskell 98, … vs de facto standard Glasgow Haskell. Standard release pattern of Open Source Erlang supervised by the OTP team at Ericsson.

WRT 2007 Integration … with IDEs Back to the future? Programmers' preference for emacs and gvim … … though some IDE interest: Visual Studio, Eclipse, NetBeans. Issue of integration with multiple IDEs: HaRe interfaces with emacs and gvim.

WRT 2007 Integration … with tools Test data sets and test generation. Makefiles, etc. Working with macros e.g. QuickCheck uses Erlang macros … … in a particular idiom.

WRT 2007 Preserving appearance -- This is an example module Main where sumSquares x y = sq x + sq y where sq :: Int->Int sq x = x ^ pow pow = 2 :: Int main = sumSquares module Main where sumSquares x y = sq pow x + sq pow y where pow = 2 :: Int sq :: Int->Int->Int sq pow x = x ^ pow main = sumSquares This is an example module Main where sumSquares x y = sq pow x + sq pow y where pow = 2 :: Int sq :: Int->Int->Int sq pow x = x ^ pow main = sumSquares 10 20

WRT 2007 Experience

WRT 2007 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?

WRT 2007 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."

WRT 2007 Which refactoring exactly? Generalise f by making 23 a parameter of f: f x = 23 + g x where g x = x con con = 23 This one occurrence? All occurrences (in the body)? Some of the occurrences … to be selected.

WRT 2007 Compensate or fail? Lift g to a global definition in f x = x + g x where g x = x + con con = 37 Fail because con not defined globally? Add extra parameter to g, and pass in con? Lift con as well?

WRT 2007 Compensate or crash? -export([oldFun/1, newFun/1]). oldFun(L) -> newFun(L). newFun(L) -> … …. -export([newFun/1]). newFun(L) -> … …. or?

WRT 2007 Infrastructure Requirement for heavyweight infrastructure. Best not to write your own! Compiler APIs. Often non-existent. Can change with releases. Clearly system specific. Tracking changes in systems and libraries.

WRT 2007 APIs … programmer / user API in Haskell / 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?

WRT 2007 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 QuickCheck to verify refactorings in both Haskell and Erlang.

WRT 2007 Future work Concurrency. Refactoring within a design library: OTP. Side-effects. Working with Erlang Training and Consulting. Data-type refactorings in Haskell. Integration with other IDEs / compilers. Test and property refactoring.

WRT 2007 Conclusion