Huiqing LiClaus Reinke

Simon Thompson

Computing Lab, University of Kent

www.cs.kent.ac.uk/projects/refactor-fp/

Refactoring Functional Programs

Manchester 04 2

Writing a program

-- format a list of Strings, one per line

table :: [String] -> String

table = concat . format

format :: [String] -> String

format [] = []

format [x] = [x]

format (x:xs) = (x ++ "\t") : fomrat xs

Manchester 04 3

Writing a program

-- format a list of Strings, one per line

table :: [String] -> String

table = concat . format

format :: [String] -> String

format [] = []

format [x] = [x]

format (x:xs) = (x ++ "\t") : fomrat xs

Manchester 04 4

Writing a program

-- format a list of Strings, one per line

table :: [String] -> String

table = concat . format

format :: [String] -> String

format [] = []

format [x] = [x]

format (x:xs) = (x ++ "\t") : format xs

Manchester 04 5

Writing a program

-- format a list of Strings, one per line

table :: [String] -> String

table = concat . format

format :: [String] -> String

format [] = []

format [x] = [x]

format (x:xs) = (x ++ "\t") : format xs

Manchester 04 6

Writing a program

-- format a list of Strings, one per line

table :: [String] -> String

table = concat . format

format :: [String] -> [String]

format [] = []

format [x] = [x]

format (x:xs) = (x ++ "\t") : format xs

Manchester 04 7

Writing a program

-- format a list of Strings, one per line

table :: [String] -> String

table = concat . format

format :: [String] -> [String]

format [] = []

format [x] = [x]

format (x:xs) = (x ++ “\t") : format xs

Manchester 04 8

Writing a program

-- format a list of Strings, one per line

table :: [String] -> String

table = concat . format

format :: [String] -> [String]

format [] = []

format [x] = [x]

format (x:xs) = (x ++ "\n") : format xs

Manchester 04 9

Writing a program

-- format a list of Strings, one per line

table :: [String] -> String

table = concat . format

appNL :: [String] -> [String]

appNL [] = []

appNL [x] = [x]

appNL (x:xs) = (x ++ "\n") : appNL xs

Manchester 04 10

Writing a program

-- format a list of Strings, one per line

table :: [String] -> String

table = concat . format

appNL :: [String] -> [String]

appNL [] = []

appNL [x] = [x]

appNL (x:xs) = (x ++ "\n") : appNL xs

Manchester 04 11

Writing a program

-- appNL a list of Strings, one per line

table :: [String] -> String

table = concat . appNL

appNL :: [String] -> [String]

appNL [] = []

appNL [x] = [x]

appNL (x:xs) = (x ++ "\n") : appNL xs

Manchester 04 12

Writing a program

-- format a list of Strings, one per line

table :: [String] -> String

table = concat . appNL

appNL :: [String] -> [String]

appNL [] = []

appNL [x] = [x]

appNL (x:xs) = (x ++ "\n") : appNL xs

Manchester 04 13

Writing a program

-- format a list of Strings, one per line

table :: [String] -> String

table = concat . appNL

where

appNL :: [String] -> [String]

appNL [] = []

appNL [x] = [x]

appNL (x:xs) = (x ++ "\n") : appNL xs

Manchester 04 14

Refactoring

Refactoring means changing the design of program …

… without changing its behaviour.

Refactoring comes in many forms

• micro refactoring as a part of program development,• major refactoring as a preliminary to revision,• as a part of debugging, …

As programmers, we do it all the time.

Manchester 04 15

Not just programming

Paper or presentationmoving sections about; amalgamate sections; move inline code to a figure; animation; …

Proof introduce lemma; remove, amalgamate hypotheses, …

Programthe topic of the lecture

Manchester 04 16

Overview of the talk

Example refactorings … what do we learn?

Refactoring functional programs

Generalities

Tooling: demo, rationale, design.

What comes next?

Conclusions

Manchester 04 17

Refactoring Functional Programs

• 3-year EPSRC-funded project Explore the prospects of refactoring functional programs

Catalogue useful refactorings

Look into the difference between OO and FP refactoring

A real life refactoring tool for Haskell programming

A formal way to specify refactorings … and a set of proofs that the implemented refactorings are correct.

• Currently mid-project: the latest HaRe release is module-aware and has module refactorings.

Manchester 04 18

Refactoring functional programs

Semantics: can articulate preconditions and … … verify transformations.

Absence of side effects makes big changes predictable and verifiable … … unlike OO.

Language support: expressive type system, abstraction mechanisms, HOFs, …

Opens up other possibilities … proof …

Manchester 04 19

Rename

f x y = …

Name may be too specific, if the function is a candidate for reuse.

findMaxVolume x y = …

Make the specific purpose of the function clearer.

Needs scope information: just change this f and not all fs (e.g. local definitions or variables).

Needs module information: change f wherever it is imported.

Manchester 04 20

Lift / demote

f x y = … h …

where

h = …

Hide a function which is clearly subsidiary to f; clear up the namespace.

f x y = … (h y) …

h y = …

Makes h accessible to the other functions in the module and beyond.

Needs free variable information: which of the parameters of f is used in the definition of h?

Need h not to be defined at the top level, … , DMR.

Manchester 04 21

Lessons from the first examples

Changes are not limited to a single point or even a single module: diffuse and bureaucratic …

… unlike traditional program transformation.

Many refactorings bidirectional …

… as there is never a unique correct design.

Manchester 04 22

How to apply refactoring?

By hand, in a text editor

Tedious

Error-prone

Depends on extensive testing

With machine support

Reliable

Low cost: easy to make and un-make large changes.

Exploratory … a full part of the programmer’s toolkit.

Manchester 04 23

Machine support invaluable

Current practice: editor + type checker (+ tests).

Our project: automated support for a repertoire of refactorings …

… integrated into the existing development process: Haskell IDEs such as vim and emacs.

Manchester 04 24

Demonstration of HaRe, hosted in vim.

Manchester 04 25

Proof of concept …

To show proof of concept it is enough to:

• build a stand-alone tool,

• work with a subset of the language,

• ‘pretty print’ the refactored source code in a standard format.

Manchester 04 26

… or a useful tool?

To make a tool that will be used we must:

• integrate with existing program development tools: the program editors emacs and vim: only add to their capabilities;

• work with the complete Haskell 98 language;

• preserve the formatting and comments in the refactored source code;

• allow users to extend and script the system.

Manchester 04 27

Refactorings implemented in HaRe

RenameDeleteLift (top / one level)DemoteIntroduce definitionRemove definitionUnfoldGeneraliseAdd / remove params

All these refactorings are module aware.

Move def between modules

Delete /add to exportsClean imports

Make imports explicit

Manchester 04 28

The Implementation of Hare

Informationgathering

Pre-conditionchecking

Programtransformation

Programrendering

Manchester 04 29

Information needed

Syntax: replace the function called sq, not the variable sq …… parse tree.

Static semantics: replace this function sq, not all the sq functions …… scope information.

Module information: what is the traffic between this module and its clients …… call graph.

Type information: replace this identifier when it is used at this type …… type annotations.

Manchester 04 30

Infrastructure

To achieve this we chose to:

• build a tool that can interoperate with emacs, vim, … yet act separately.

• leverage existing libraries for processing Haskell 98, for tree transformation, yet …

… modify them as little as possible.

• be as portable as possible, in the Haskell space.

Manchester 04 31

The Haskell background

Libraries• parser: many• type checker: few• tree transformations: few

Difficulties• Haskell98 vs. Haskell extensions.• Libraries: proof of concept vs. distributable.• Source code regeneration.• Real project

Manchester 04 32

Programatica

Project at OGI to build a Haskell system …

… with integral support for verification at various levels: assertion, testing, proof etc.

The Programatica project has built a Haskell front end in Haskell, supporting syntax, static, type and module analysis …

… freely available under BSD licence.

Manchester 04 33

The Implementation of Hare

Informationgathering

Pre-conditionchecking

Programtransformation

Programrendering

Manchester 04 34

First steps … lifting and friends

Use the Haddock parser … full Haskell given in 500 lines of data type definitions.

Work by hand over the Haskell syntax: 27 cases for expressions …

Code for finding free variables, for instance …

Manchester 04 35

Finding free variables … 100 lines

instance FreeVbls HsExp where

freeVbls (HsVar v) = [v]

freeVbls (HsApp f e)

= freeVbls f ++ freeVbls e

freeVbls (HsLambda ps e)

= freeVbls e \\ concatMap paramNames ps

freeVbls (HsCase exp cases)

= freeVbls exp ++ concatMap freeVbls cases

freeVbls (HsTuple _ es)

= concatMap freeVbls es

… etc.

Manchester 04 36

This approach

Boiler plate code …

… 1000 lines for 100 lines of significant code.

Error prone: significant code lost in the noise.

Want to generate the boiler plate and the tree traversals …

… DriFT: Winstanley, Wallace… Strafunski: Lämmel and Visser

Manchester 04 37

Strafunski

Strafunski allows a user to write general (read generic), type safe, tree traversing programs …

… with ad hoc behaviour at particular points.

Traverse through the tree accumulating free variables from component parts, except in the case of lambda abstraction, local scopes, …

Strafunski allows us to work within Haskell … other options are under development.

Manchester 04 38

Rename an identifier

rename:: (Term t)=>PName->HsName->t->Maybe t rename oldName newName = applyTP worker where worker = full_tdTP (idTP ‘adhocTP‘ idSite) idSite :: PName -> Maybe PName idSite v@(PN name orig) | v == oldName = return (PN newName orig) idSite pn = return pn

Manchester 04 39

The coding effort

Transformations with Strafunski are straightforward …

… the chore is implementing conditions that guarantee that the transformation is meaning- preserving.

This is where much of our code lies.

Manchester 04 40

The Implementation of Hare

Informationgathering

Pre-conditionchecking

Programtransformation

Programrendering

Manchester 04 41

Program rendering example

-- 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 10 20

Promote the definition of sq to top level

Manchester 04 42

Program rendering 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

Using a pretty printer: comments lost and layout quite different.

Manchester 04 43

Program rendering example

-- 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 10 20

Promote the definition of sq to top level

Manchester 04 44

Program rendering example

-- 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

Layout and comments preserved.

Manchester 04 45

Rendering: our approach

White space and comments in the token stream.

2 views of the program: token stream and AST.

Modification of the AST guides the modification of the token stream.

After a refactoring, the program source is extracted from the token stream not the AST.

Use heuristics to associate comments with semantic entities.

Manchester 04 46

Production tool (version 0)

Programaticaparser and

type checker

Refactorusing a

Strafunskiengine

Render codefrom the

token streamand

syntax tree.

Manchester 04 47

Production tool (version 1)

Programaticaparser and

type checker

Refactorusing a

Strafunskiengine

Render codefrom the

token streamand

syntax tree.

Pass lexical information toupdate thesyntax treeand so avoid reparsing

Manchester 04 48

Module awareness: example

Move a top-level definition f from module A to B.

-- Is f defined at the top-level of B? -- Are the free variables in f accessible within module B? -- Will the move require recursive modules?

-- Remove the definition of f from module A. -- Add the definition to module B. -- Modify the import/export lists in module A, B and the client modules of A and B if necessary. -- Change uses of A.f to B.f or f in all affected modules. -- Resolve ambiguity.

Manchester 04 49

What have we learned?

Emerging Haskell libraries make it a practical platform.

Efficiency issues … type checking large systems.

Limitations of IDE interactions in vim and emacs.

Reflections on Haskell itself.

Manchester 04 50

Reflecting on Haskell

Cannot hide items in an export list (though you can on import).

The formal semantics of pattern matching is problematic.

‘Ambiguity’ vs. name clash.

‘Tab’ is a nightmare!

Correspondence principle fails …

Manchester 04 51

Correspondence

Operations on definitions and operations on expressions can be placed in correspondence

(R.D.Tennent, 1980)

Manchester 04 52

Correspondence

Definitions

where

f x y = e

f x

| g1 = e1

| g2 = e2

Expressions

let

\x y -> e

f x = if g1 then e1 else if g2 …

Manchester 04 53

Where do we go next?

• Larger-scale examples: ADTs, monads, …

• An API for do-it-yourself refactorings, or …

• … a language for composing refactorings

• Detecting ‘bad smells’

• Evolving the evidence: GC6.

Manchester 04 54

What do users want?

Find and remove duplicate code.

Argument permutations.

Data refactorings.

More traditional program transformations.

Monadification.

Manchester 04 55

Monadification (cf Erwig)

r = f e1 e2 do v1 <- e1

v2 <- e2

r <- f v1 v2

return r

Manchester 04 56

Larger-scale examples

More complex examples in the functional domain; often link with data types.

Dawning realisation that can some refactorings are pretty powerful.

Bidirectional … no right answer.

Manchester 04 57

Algebraic or abstract type?

data Tr a

= Leaf a |

Node a (Tr a) (Tr a)

Tr

Leaf

Node

flatten :: Tr a -> [a]

flatten (Leaf x) = [x]

flatten (Node s t)

= flatten s ++

flatten t

Manchester 04 58

Algebraic or abstract type?

data Tr a

= Leaf a |

Node a (Tr a) (Tr

a)

isLeaf = …

isNode = …

…

Tr

isLeaf

isNode

leaf

left

right

mkLeaf

mkNode

flatten :: Tr a -> [a]

flatten t

| isleaf t = [leaf t]

| isNode t

= flatten (left t)

++ flatten (right t)

Manchester 04 59

Algebraic or abstract type?

Pattern matching syntax is more direct …

… but can achieve a considerable amount with field names.

Other reasons? Simplicity (due to other refactoring steps?).

Allows changes in the implementation type without affecting the client: e.g. might memoise

Problematic with a primitive type as carrier.

Allows an invariant to be preserved.

Manchester 04 60

Outside or inside?

data Tr a

= Leaf a |

Node a (Tr a) (Tr

a)

isLeaf = …

…

Tr

isLeaf

isNode

leaf

left

right

mkLeaf

mkNode

flatten :: Tr a -> [a]

flatten t

| isleaf t = [leaf t]

| isNode t

= flatten (left t)

++ flatten (right t)

Manchester 04 61

Outside or inside?

data Tr a

= Leaf a |

Node a (Tr a) (Tr

a)

isLeaf = …

…

flatten = …

Tr

isLeaf

isNode

leaf

left

right

mkLeaf

mkNode

flatten

Manchester 04 62

Outside or inside?

If inside and the type is reimplemented, need to reimplement everything in the signature, including flatten.

The more outside the better, therefore.

If inside can modify the implementation to memoise values of flatten, or to give a better implementation using the concrete type.

Layered types possible: put the utilities in a privileged zone.

Manchester 04 63

API

Refactorings

Refactoringutilities

Strafunski

Haskell

Manchester 04 64

DSL

Refactorings

Refactoringutilities

Strafunski

Haskell

Combining forms

Manchester 04 65

Detecting ‘bad smells’

Work byChris Ryder

Manchester 04 66

Evolving the evidence

Dependable System Evolution is the software engineering grand challenge.

Build systems with evidence of their dependability …

… but this begs the question of how to evolve the evidence in line with the system.

Refactoring proofs, test coverage data etc.

Manchester 04 67

Teaching and learning design

Exciting prospect of using a refactoring tool as an integral part of an elementary programming course.

Learning a language: learn how you could modify the programs that you have written …

… appreciate the design space, and

… the features of the language.

Manchester 04 68

Conclusions

Refactoring + functional programming: good fit.

Practical tool … not ‘yet another type tweak’.

Leverage from available libraries … with work.

We have begun to use the tool in building itself!

Much more to do than we have time for.

Martin Fowler’s ‘Rubicon’: ‘extract definition’ … in HaRe version 1 … fp productivity.

www.cs.kent.ac.uk/projects/refactor-fp/