Functional programming in C++

Functional programming in C++
Alex Bolboacă,  @alexboly,  alex.bolboaca@mozaicworks.com
February 2018

Intro
A practical view on functional programming
Mindset
Core Building Blocks
Improve design with functional programming
Bad design with functional programming
OOP and functional programming
Conclusions

Why I care about functional programming
I identify with the software craft movement

Learning & Improving
I want to learn as much as possible about my craft.
The evolution of the software industry is just recycling old ideas, and
functional programming is old.
All programming languages support functional constructs today.
More restrictions on code (eg. immutability) lead to better code and better
designs.

Disclaimer
I am an enthusiast, not an expert

Disclaimer
The code on the slides may be simpliﬁed for presentation purpose.
Find the valid examples on github: https://github.com/alexboly and
https://github.com/MozaicWorks

My First Touch of Functional Programming
(defun sqrt-iter (guess x)
(if (good-enough-p guess x)
guess
(sqrt-iter (improve guess x) x)))
Source: Professor Forrest Young, Psych 285

What is a monad?
The essence of monad is thus separation of
composition timeline from the composed
computation’s execution timeline, as well as the ability
of computation to implicitly carry extra data, as
pertaining to the computation itself, in addition to its
one (hence the name) output, that it will produce when
run (or queried, or called upon).
Source: https://wiki.haskell.org/Monad

A shorter explanation
All told, a monad in X is just a monoid in the category
of endofunctors of X, with product × replaced by
composition of endofunctors and unit set by the
identity endofunctor.
Saunders Mac Lane, “Categories for the Working Mathematician”

But don’t worry, just learn Category Theory
Category theory formalizes mathematical structure
and its concepts in terms of a labeled directed graph
called a category, whose nodes are called objects, and
whose labelled directed edges are called arrows (or
morphisms). A category has two basic properties: the
ability to compose the arrows associatively and the
existence of an identity arrow for each object.
…
Category theory has practical applications in
programming language theory, for example the usage
of monads in functional programming.
Wikipedia

Only one reaction
Whyyyyyyyyyyyyy?

A practical view on functional
programming

What I thought it was:
A way of writing unreadable code with many parantheses, with a strange
order of operations, in a community that likes complicated explanations

How I understand it now:
A style of software design that focuses on functions and immutability to
simplify code, to allow removal of certain types of duplication, and to show
intent better

OOP
Design the objects and interactions between them
Key idea: message passing

Functional programming
Start from input data and apply a set of transformations to get it to the
desired output.
Key ideas: pure data structures and pure functions

Example 1: increment all elements of a list
Structured programming:
incrementAllElements(list& aList){
for(int i = 0; i < aList.size(); ++i){
aList[i] ++;
}
}
Functional programming:
//Simplified C++ code
transform(aList, [](const int item){ return item + 1 });

Example 2: Get a string with 5 ‘t’ characters
Structured programming:
string result;
for(int i = 0; i < 5; ++i){
result += ’t’;
}

Example 2 (cont’d)
Functional programming:
// Data in: a range from 1 to 5
// Data out: a string with 5 ’t’
// Transformations: transform each element from range
// to ’t’(map), then join them (reduce)
// Groovy code
[1..5].collect{’t’}.join()
// C++ doesn’t support it yet, except for boost
// Simplified C++ code using boost’s irange
transform(irange(1, 5), [](){return ’t’;});

Example 3: Pacman moves on a line to the right
OOP
Let’s create classes for: Pacman, Board, Wall, Dot, Movement etc.

Functional programming
Data in: a line with pacman on it
……>……
Data out: a line with pacman moving one square to the right, and a missing
dot
…… >…..
Transformations:
• Get everything before pacman
• Get everything after pacman
• Create a new line formed from: everything before pacman, an empty
space, pacman, everything after pacman except ﬁrst element

const Line tick(const Line& initialLine){
return (
beforePacman(initialLine) +
KindOfToken::Empty +
KindOfToken::Pacman +
removeFirst(afterPacman(initialLine))
);
}
Full example at: https://github.com/alexboly/pacmanCpp/

Conclusion
FP mindset: Data in -> Transformations -> Data out
Each transformation is a pure function, except at the edge of the system
(I/O).

Pure functions
A function that returns the same output whenever receiving the same input
In C++ const is your friend

Example: Not pure function
list.insert(5); // throws exception if too many elements
// otherwise, adds 5 to the list

Example: Pure function
const list insert(const list& aList, const int value) { ... };
newList = insert(aList, 5);
// always has the same behavior
// when passing in the same arguments
// unless externalities (eg. memory is full)
// Immutability!

Example: Immutable data structures
Immutable C++ example: https://github.com/rsms/immutable-cpp
auto a = Array<int>::empty();
a = a->push(1);
a = a->push(2);
a = a->push(3);

Lambdas / Anonymous functions
// Lambda variable
auto increment = [](auto value){return value + 1;};
assert(increment(2) == 3);

Lambda variable with specific captured context
int valueFromContext = 10;
auto appendValueFromContext =
[const auto valueFromContext](auto value){
return value + valueFromContext;
};
assert(appendValueFromContext(10) == 20);

Lambda variable capturing all used variables by copy
int valueFromContext = 10;
auto appendValueFromContext =
[=](auto value){
return value + valueFromContext;
};
assert(appendValueFromContext(10) == 20);
More details:
http://en.cppreference.com/w/cpp/language/lambda

Chaining functions
listOf3Elements = add(add(add(emptyList, 1), 2), 3);

Curry. Haskell Curry.
Curry. Haskell Curry

Currying
// Without curry
list = add(list, 5);
list = add(list, 1000);
// With curry (bind in C++ 11)
auto addToEmptyList = bind(add, list<int>(), _1);
assert addToEmptyList(5) == list<int>({5});
assert addToEmptyList(1000) == list<int>({1000});

Functional composability
auto threeElementsList = [](int first, int second, int third){
return add(add(addToEmptyList(first), second), third);
};
// In some programming languages,
// composing functions has its own syntax
// Eg. Groovy
def oneElementList = add << addToEmptyList
// same as add(addToEmptyList(), _)
assert(threeElementsList(0, 1, 2) == list<int>({0, 1, 2}));

Currying and composability
auto addTwoElementsToEmptyList = [](const int first, const int
return add(addToEmptyList(first), second);
};
auto listWith0And1 = bind(addTwoElementsToEmptyList, 0, 1);
assert(listWith0And1() == list<int>({0, 1});

Higher level functionvalues
We can pass functions as parameters
auto incrementFunctionResult = [](const auto aFunction){
return aFunction() + 1;
};
auto function1 = [](){
return 1;
};
assert(incrementFunctionResult(function1) == 2);

Short list of higher level functions
Find them in or
• ﬁnd_if
• transform
• reduce / accumulate
• count_if
• all_of / any_of / none_of
• …
See examples on https://github.com/MozaicWorks/functionalCpp

Almost anything can be a function
//TRUE = {x -> { y -> x}}
auto TRUE = [](auto x){
return [x](auto y){
return x;
};
};
//FALSE = {x -> { y -> y}}
auto FALSE = [](auto x){
return [](auto y){
return y;
};
};

Church encoding for booleans (cont’d)
//IF = {proc -> { x -> { y -> proc(x)(y) }}}
auto IF = [](auto proc){
return [proc](auto x){
return [x, proc](auto y){
return proc(x)(y);
};
};
};
CHECK(IF(TRUE)(”foo”)(”bar”) == ”foo”);
Source: https://github.com/alexboly/ChurchEncodingCpp

Conclusions
• Pure functions are the basic building blocks of functional programming
• The best functions are small and polymorphic
• When the same parameter is passed to multiple function calls,
consider using curry (bind in C++)
• When the return of a function is passed to another function multiple
times, consider using functional composition
• Reuse existing functions as much as possible: map (transform in C++),
reduce, ﬁnd etc.
• Experiment with functional programming: TicTacToe score, pacman,
tetris
• Replace traditional loops with functional loops as much as possible

Improve design with functional
programming

Clarify loops intent
for(auto element=list.begin(); element != list.end(); element++
// I have to read all of this to understand what the loop does
}

Functional loops
transform(....); // transform a collection into another!
auto increment = [](auto value){return value + 1;};
transform(list, increment); // transform a list into another
// with incremented elements
transform(list, increment); // compute the sum
// of all incremented elements
// compute the sum of all incremented even numbers
reduce(find(transform(list, increment), evenNumber), plus)

What about web apps?
Data in: an HTTP request
Data out: HTML + a response code + something saved in a store
Transformations:
• validation
• sanitization
• canonicalization
• business rules
• save to database (mutable!)
• or read from database (mutable!)
• convert to view
• pass on to html rendering
Everything in between request and database, and database and response,
is immutable!

Bad design with functional
programming

Abstract functions != Readability
computeNextBoardOnAxis(board, currentAxis, nextAxis) {
def movableTokensForAxis = KindOfToken.values().findAll {
it.axis == nextAxis && it.isMovable
}
def rotateForward = { aBoard ->
rotateBoardOnAxis(aBoard, currentAxis, nextAxis)
}
def rotateBack = { aBoard ->
rotateBoardOnAxis(aBoard, currentAxis, nextAxis)
}
return rotateBack(
computeNewBoard(rotateForward(board),
movableTokensForAxis
))
}

My ugly code
def createCertificate(self, attendeeText, fileName):
imageOpenOperation = OpenImageOperation(self.baseImagePath)
attendeeTextWriteOperation = TextWriteImageOperation(attend
courseTextWriteOperation = TextWriteImageOperation(self.cou
dateTextWriteOperation = TextWriteImageOperation(self.dateT
signatureOverlayOperation = ImageOverlayOperation(self.trai
filters = ImageOperations([imageOpenOperation,
attendeeTextWriteOperation, courseTextWriteOperation,
dateTextWriteOperation, signatureOverlayOperation])
filters.execute(fileName)
return fileName

class TextWriteImageOperation:
def __init__(self, text):
self.text = text
def execute(self, image):
draw = ImageDraw.Draw(image)
textPositionX = self.__getXPositionForCenteredText(
if self.text.shouldCenter()
else self.text.column()
self.__drawText(draw, textPositionX)
del draw
return image;

OOP and functional programming

Relationship between OOP and functional programming
A class is nothing more than a set of cohesive, partially applied pure
functions
– via JB Rainsberger

Remember first example?
// OOP version: Cohesion
class List{
private listStorage
add(element){....}
removeLast(){....}
}
// Functional version
add(list, element)
removeLast(list)

// Equivalence
def add = [](auto listStorage, auto element){return ....};
// For oop bind function parameters
// to the data members of the class
auto oopAdd = [](element){ return add(initialListStorage, eleme
// Or curry (bind)
def oopAdd = bind(add, initialListStorage)
oopAdd(5)

My thoughts
Good:
• Functional programming mindset is very useful for data-centric
applications
• Higher level functions simplify and clarify intent for data
transformations
• Pure functions are very easy to test
• Clear separation between mutable and immutable state simpliﬁes
everything

Careful:
• Beware of too high abstraction & ensure your colleagues understand
the code
• Carefully mix OOP with functional constructs

Can’t ignore:
• Functional programming is here to stay due to CPUs going multicore
• New applications: big data, reactive programming etc.
• All modern languages have FP constructs built in
• Clojure, Scala, Haskell, F# are used and will be used
• AI is built using FP (composability)

Learn more
Hidden loops:
https://www.slideshare.net/alexboly/hidden-loops
Removing structural duplication: https://www.slideshare.net/
alexboly/removing-structuralduplication

Learn more at Mozaic Works workshops
https://mozaicworks.com/training/c-plus-plus/

Thank you!
I’ve been Alex Bolboacă, @alexboly, alex.bolboaca@mozaicworks.com
programmer, trainer, mentor, writer
at Mozaic Works
Think. Design. Work Smart.
https://mozaicworks.com

Functional programming in C++

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