Refactor legacy code through pure functions

Refactor Legacy Code Through Pure Functions
Alex Bolboaca, CTO, Trainer and Coach at Mozaic Works
 alexboly  https://mozaicworks.com  alex.bolboaca@mozaicworks.com
. . . . . . . . . . . . . . . . . . . .

The Problem Of Legacy Code
Existing Methods
Disadvantages of existing methods
Step 1: To pure functions
Step 2: Test pure functions
Step 3: Pure functions to classes
Final Thoughts

Legacy Code is everywhere
Depiction of legacy code

Legacy Code erodes the will of programmers

What is Legacy Code?
Michael Feathers: “Any piece of code that doesn’t have automated tests”

Generalized Definition
Alex Bolboaca: “Any piece of code that you are afraid to change”

Legacy Code leads to the Dark Side
Yoda: “Fear leads to anger. Anger leads to hate. Hate leads to suffering.”

“Change and pray” method
If it works for you, great. Is it repeatable and transferrable though?

“No more changes” method
Freeze code, nobody touches it. Works with strangler pattern

Strangler Pattern
Strangle the existing code with a new implementation until the new implementation works ﬁne.

Michael Feathers’ Refactoring method
Identify seams -> small, safe refactorings -> write characterization tests -> refactor code to desired design

Disadvantages of existing methods

Legacy Code is Messy
Often multiple methods required, and time consuming

Existing methods require multiple rare skills
• How to identify and “abuse” seams
• How to write characterization tests
• How to imagine a good design solution
• How to refactor in small steps
• Lots of imagination

Learning the methods takes long
Months, maybe years of practice

3 Poor Choices
• Live with the code you’re afraid to change (and slow down development)
• Refactor the code unsafely (spoilers: you will probably fail)
• Use a safe method (spoilers: it’s slow)

I’m proposing a new method
• Refactor code to pure functions
• Write data-driven or property based tests on the pure functions
• Refactor pure functions to classes (or something else)

What is a pure function?
• Returns same output values for the same input values
• Does not depend on state

Example of Pure Functions
int add(const int first, const int second){
return first + second;
}

Example of Impure Function
int addToFirst(int& first, const int second){
first+=second;
return first;
}

Example of “Tolerated” Pure Function
int increment(int first){
return first; //state change, local to the function
}

Why pure functions?
When you turn your switch on, the water doesn’t start on your sink

Why pure functions?
• Pure functions are boring
• Pure functions have no dependencies
• Pure functions are easy to test
• Any program can be written as a combination of: pure functions + I/O functions
• We can take advantage of lambda operations with pure functions: functional composition,
currying, binding

Why Pure functions?
The hardest part of writing characterization tests (M. Feathers’ method) is dealing with
dependencies.
Pure functions make dependencies obvious and explicit.

Refactor to pure functions
• Pick a code block
• Extract method (refactoring)
• Make it immutable (add consts)
• Make it static and introduce parameters if needed
• Replace with lambda
Remember: this is an intermediary step. We ignore for now design and performance, we just want
to reduce dependencies and increase testability

Example
Game ::Game() : currentPlayer(0), places{}, purses{} {
for (int i = 0; i < 50; i ) {
ostringstream oss(ostringstream ::out);
oss << ”Pop Question ” << i;
popQuestions.push_back(oss.str());
char str[255];
sprintf(str, ”Science Question %d”, i);
scienceQuestions.push_back(str);
char str1[255];
sprintf(str1, ”Sports Question %d”, i);
sportsQuestions.push_back(str1);
rockQuestions.push_back(createRockQuestion(i));
}
}

1. Pick a code block

2. Extract method
void Game ::doSomething(int i) {
}

3. Try to make it immutable: const parameter
void Game ::doSomething(const int i) {
}

3. Try to make it immutable: const function
void Game ::doSomething(const int i) const {
//! Compilation Error: state change
}

Revert change
}

Separate the pure from impure part
// Pure function
const string &popQuestion = oss.str();
// State change
popQuestions.push_back(popQuestion);
}

Extract pure function
string popQuestion = createPopQuestion(i);
}
string Game ::createPopQuestion(const int i) const {
return popQuestion;
}

Inline initial function
for (int i = 0; i < 50; i ) {
const string popQuestion = createPopQuestion(i);
...
}
}

Back to the pure function
return popQuestion;
}

Some simplification
return oss.str();
}

Transform to lambda
auto createPopQuestion_Lambda = [](const int i) -> string {
return oss.str();
};
createPopQuestion_Lambda(i);
}

Move lambda up and inline method
for (int i = 0; i < 50; i ) {
popQuestions.push_back(createPopQuestion_Lambda(i));
char str[255];
sprintf(str, ”Science Question %d”, i);
scienceQuestions.push_back(str);
...
}
}

Let’s stop and evaluate
Refactorings: extract method, inline, change signature, turn to lambda

The compiler helps us
The compiler tells us the function dependencies

Mechanical process - or engineering procedure
Mechanical process

The result
// Pure function
// No dependencies
auto createPopQuestion_Lambda = [](const int i) -> string {
return oss.str();
};

What about state changes?
bool Game ::add(string playerName) {
players.push_back(playerName);
places[howManyPlayers()] = 0;
purses[howManyPlayers()] = 0;
inPenaltyBox[howManyPlayers()] = false;
cout << playerName << ” was added” << endl;
cout << ”They are player number ” << players.size() << endl;
return true;
}

Pick code that makes the state change
// State change
// State change
// State change
// State change
// I/O
return true;
}

Extract method
void Game ::addPlayerNameToPlayersList(const string &playerName) {
}

Extract initial value
// Copy initial value of players
vector<string> &initialPlayers(players);
// Modify the copy
initialPlayers.push_back(playerName);
// Set the data member to the new value
players = initialPlayers;
}

Separate pure from impure
vector<string> initialPlayers = addPlayer_Pure(playerName);
players = initialPlayers;
}
vector<string> Game ::addPlayer_Pure(const string &playerName) const {
vector<string> initialPlayers(players);
return initialPlayers;
}

Inline computation
players = addPlayer_Pure(playerName);
}

Inline method
players = addPlayer_Pure(playerName);
return true;
}

Make static
vector<string> Game ::addPlayer_Pure(const string &playerName) {
// Compilation error: players unavailable
vector<string> initialPlayers(players);
}

Undo static & Introduce parameter
vector<string> Game ::addPlayer_Pure(
const string &playerName,
const vector<string> &thePlayers) {
vector<string> initialPlayers(thePlayers);
}

Move to lambda
auto addPlayerNameToCollection_Lambda =
[](const string &playerName,
const vector<string> &thePlayers)
-> vector<string> {
vector<string> initialPlayers(thePlayers);
};
vector<string> Game ::addPlayer_Pure(const string &playerName,
const vector<string> &thePlayers) {
addPlayerNameToCollection_Lambda(playerName, thePlayers);
}

Inline
players = addPlayerNameToCollection_Lambda(playerName, players);
return true;
}

List of mechanics (work in progress)
• extract method -> make const -> make static -> introduce parameter -> turn to lambda
• state change -> newValue = computeNewValue(oldValue)
• if( ...){ ...}else{ ...} -> result = decide( ...)
• if( ...){ ...} -> std ::optional
• I/O -> void outputSomething( ...) or int readSomething( ...)

Data-driven tests
// Groovy and Spock
class MathSpec extends Specification {
def ”maximum of two numbers”(int a, int b, int c) {
expect:
Math.max(a, b) c
where:
a | b | c
1 | 3 | 3
7 | 4 | 7
0 | 0 | 0
}
}

C++ with doctest
// chapter 11, ”Hands-on Functional Programming with C ”
TEST_CASE(”1 raised to a power is 1”){
int exponent;
SUBCASE(”0”){
exponent = 0;
}
SUBCASE(”1”){
exponent = 1;
}
...
CAPTURE(exponent);
CHECK_EQ(1, power(1, exponent));
}

Property-based tests
TEST_CASE(”Properties”){
cout << ”Property: 0 to power 0 is 1” << endl;
CHECK(property_0_to_power_0_is_1);
...
cout << ”Property: any int to power 1 is the value” << endl;
check_property(
generate_ints_greater_than_0,
prop_any_int_to_power_1_is_the_value, ”generate ints”
);
}

Property
auto prop_any_int_to_power_1_is_the_value = [](const int base){
return power(base, 1) base;
};

Step 3: Pure functions to classes

Equivalence
A class is a set of partially applied, cohesive pure functions

Example
// Pseudocode
class Player{
string name;
int score;
Player(string name, int score) : name(name), score(score){}
void printName() const {
cout << name << endl;
}
}

Equivalent with
auto printName = [string name]() -> void {
cout << name << endl;
}
auto printName = [](string name) -> void {
cout << name<< endl;
}

Identify cohesive pure functions
// Similarity in names: Player
auto printPlayerName = [](string playerName) -> void {
...
}
auto computePlayerScore = []( ...) -> {
...
}

Identify cohesive pure functions
// Similarity in parameter list
auto doSomethingWithPlayerNameAndScore =
[](string playerName, int playerScore) -> {
...
}
auto doSomethingElseWithPlayerNameAndScore =
[](string playerName, int playerScore) -> {
...
}

Refactoring steps
• Create class
• Move functions into class
• Common parameters -> data members
• Add constructor
• Remember: you have tests now!

Evaluation
I believe it can be:
• faster to learn: around 10 - 20 mechanics to practice
• faster to refactor: eliminate dependencies while moving to pure functions
• easier to write tests: fundamentally data tables
• safer to refactor to classes

Careful
• Introducing unexpected side effects
• Temporal coupling
• Global state

Caveats
• discipline & practice is required
• does not solve all the problems
• the resulting design doesn’t follow “Tell, don’t ask”

Is it safe?
More testing required

More Information
“Think. Design. Work Smart.” YouTube Channel
https://www.youtube.com/channel/UCSEkgmzFb4PnaGAVXtK8dGA/
Codecast ep. 1: https://youtu.be/FyZ_Tcuujx8
Video “Pure functions as nominal form for software design” https://youtu.be/l9GOtbhYaJ8

Questions
Win a book (more at “ask the speakers”)

Photo Attribution
https://unsplash.com/@alexagorn?utm_source=unsplash&utm_medium=referral&utm_content=creditCop
https://fthmb.tqn.com/5Jqt2NdU5fjtcwhy6FwNg26yVRg=/1500x1167/filters:fill(auto,1)/yoda-
56a8f97a3df78cf772a263b4.jpg
https://aleteiaen.files.wordpress.com/2017/09/web3-praying-hands-work-computer-desk-office-
prayer-shutterstock.jpg?quality=100&strip=all
https://docs.microsoft.com/en-us/azure/architecture/patterns/_images/strangler.png
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1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&auto=format&fit=crop&w=1350&q=80
https://images.unsplash.com/photo-1536158525388-65ad2110afc8?ixlib=rb-
1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&auto=format&fit=crop&w=1350&q=80
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Refactor legacy code through pure functions

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