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functional-programming-advantages.lyx
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functional-programming-advantages.lyx
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#LyX 2.3 created this file. For more info see http://www.lyx.org/
\lyxformat 544
\begin_document
\begin_header
\save_transient_properties true
\origin unavailable
\textclass beamer
\begin_preamble
\usetheme[secheader]{Boadilla}
\usecolortheme{seahorse}
\title[Why functional programming]{What did functional programming ever do for us (software engineers)?}
\subtitle{An extreme pragmatic and un-academic approach}
\author{Sergei Winitzki}
\date{2020-08-22}
\institute[ABTB]{Academy by the Bay}
\setbeamertemplate{headline}{} % disable headline at top
\setbeamertemplate{navigation symbols}{} % disable navigation bar at bottom
\usepackage[all]{xy} % xypic
%\makeatletter
% Macros to assist LyX with XYpic when using scaling.
\newcommand{\xyScaleX}[1]{%
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\xydef@\xymatrixcolsep@{#1}
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% Double-stroked fonts to replace the non-working \mathbb{1}.
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% The LyX document will define a macro \bbnum{#1} that calls \custombb{#1}.
\usepackage{relsize} % make math symbols larger or smaller
\usepackage{stmaryrd} % some extra symbols such as \fatsemi
% Note: using \forwardcompose inside a \text{} will cause a LaTeX error!
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% Make underline green.
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% The LyX document will define a macro \gunderline{#1} that will use \mathunderline with the color `greenunder`.
%\def\gunderline#1{\mathunderline{greenunder}{#1}} % This is now defined by LyX itself with GUI support.
% Scala syntax highlighting. See https://tex.stackexchange.com/questions/202479/unable-to-define-scala-language-with-listings
%\usepackage[T1]{fontenc}
%\usepackage[utf8]{inputenc}
%\usepackage{beramono}
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% The listing settings are now supported by LyX in a separate section "Listings".
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\index Index
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\begin_body
\begin_layout Standard
\begin_inset FormulaMacro
\newcommand{\gunderline}[1]{\mathunderline{greenunder}{#1}}
{\underline{#1}}
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\begin_inset FormulaMacro
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\begin_inset ERT
status open
\begin_layout Plain Layout
\backslash
frame{
\backslash
titlepage}
\end_layout
\end_inset
\begin_inset Note Note
status collapsed
\begin_layout Plain Layout
Beginning of slides.
\end_layout
\end_inset
\end_layout
\begin_layout Frame
\begin_inset Argument 4
status open
\begin_layout Plain Layout
Overview: Advantages of functional programming
\end_layout
\end_inset
\end_layout
\begin_deeper
\begin_layout Standard
Features of functional programming are being added to many languages
\end_layout
\begin_layout Itemize
What are these features?
\end_layout
\begin_layout Itemize
What advantages do they give programmers?
\end_layout
\begin_layout Standard
Programmer-facing features listed in the increasing order of complexity
\end_layout
\begin_layout Standard
\begin_inset space ~
\end_inset
\begin_inset space ~
\end_inset
(and decreasing order of advantage-to-cost ratio)
\end_layout
\begin_layout Enumerate
Write iterative code without loops
\end_layout
\begin_layout Enumerate
Use functions parameterized by types, checked by compiler
\end_layout
\begin_layout Enumerate
Use disjunctive types to model special cases, errors, etc.
\end_layout
\begin_layout Enumerate
Use special syntax for chaining effectful computations
\end_layout
\end_deeper
\begin_layout Standard
\begin_inset Separator parbreak
\end_inset
\end_layout
\begin_layout Frame
\begin_inset Argument 4
status open
\begin_layout Plain Layout
Feature 1: Loop-free iterative programs.
Transformation
\end_layout
\end_inset
\begin_inset Separator latexpar
\end_inset
\end_layout
\begin_deeper
\begin_layout Standard
An easy interview question:
\end_layout
\begin_layout Standard
\begin_inset space ~
\end_inset
\begin_inset space ~
\end_inset
Given an array of integers, find all pairs that sum to a given number
\end_layout
\begin_layout Itemize
Solutions using loops:
\begin_inset CommandInset href
LatexCommand href
name "10-20 lines of code"
target "https://javarevisited.blogspot.com/2014/08/how-to-find-all-pairs-in-array-of-integers-whose-sum-equal-given-number-java.html"
literal "false"
\end_inset
\end_layout
\begin_layout Itemize
FP solution with
\begin_inset Formula $O(n^{2})$
\end_inset
complexity:
\end_layout
\begin_layout LyX-Code
\size footnotesize
\color darkgray
# Python
\end_layout
\begin_layout LyX-Code
\family typewriter
\size footnotesize
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[ (x, y) for x in array for y in array if x + y == n ]
\end_layout
\begin_layout LyX-Code
\size footnotesize
\color darkgray
// Scala
\end_layout
\begin_layout LyX-Code
\family typewriter
\size footnotesize
\color blue
for { x <- array; y <- array; if x + y == n } yield (x, y)
\end_layout
\begin_layout LyX-Code
\size footnotesize
\color darkgray
-- Haskell
\end_layout
\begin_layout LyX-Code
\family typewriter
\size footnotesize
\color blue
do; x <- array; y <- array; guard (x + y == n); return (x, y)
\end_layout
\begin_layout Itemize
FP solution with
\begin_inset Formula $O(n\log n)$
\end_inset
complexity:
\end_layout
\begin_layout LyX-Code
\size footnotesize
\color darkgray
// Scala
\end_layout
\begin_layout LyX-Code
\family typewriter
\size footnotesize
\color blue
val hash = array.toSet
\end_layout
\begin_layout LyX-Code
\family typewriter
\size footnotesize
\color blue
for { x <- hash; if hash contains (n - x) } yield (x, n - x)
\end_layout
\end_deeper
\begin_layout Standard
\begin_inset Separator parbreak
\end_inset
\end_layout
\begin_layout Frame
\begin_inset Argument 4
status open
\begin_layout Plain Layout
Feature 1: Loop-free iterative programs.
Aggregation
\end_layout
\end_inset
\begin_inset Separator latexpar
\end_inset
\end_layout
\begin_deeper
\begin_layout Itemize
Given an array of integers, compute the sum of square roots of all elements
except negative ones
\end_layout
\begin_layout LyX-Code
\size footnotesize
\color darkgray
# Python
\end_layout
\begin_layout LyX-Code
\family typewriter
\size footnotesize
\color blue
sum( [ sqrt(x) for x in givenArray if x >= 0 ] )
\end_layout
\begin_layout LyX-Code
\size footnotesize
\color darkgray
// Scala
\end_layout
\begin_layout LyX-Code
\family typewriter
\size footnotesize
\color blue
givenArray.filter(_ >= 0).map(math.sqrt).sum
\end_layout
\begin_layout Itemize
Given a set of integers, compute the sum of those integers which are non-negativ
e and whose square root is also present in the set
\end_layout
\begin_deeper
\begin_layout Itemize
Solution using loops:
\begin_inset CommandInset href
LatexCommand href
name "15-20 lines of code"
target "https://www.geeksforgeeks.org/sum-of-elements-whose-square-root-is-present-in-the-array/"
literal "false"
\end_inset
\end_layout
\begin_layout Itemize
FP solution:
\end_layout
\end_deeper
\begin_layout LyX-Code
\family typewriter
\size footnotesize
\color blue
givenSet
\family default
\color darkgray
// Scala
\end_layout
\begin_layout LyX-Code
\family typewriter
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\color blue
.filter { x => x >= 0 && givenSet.contains(math.sqrt(x)) }
\end_layout
\begin_layout LyX-Code
\family typewriter
\size footnotesize
\color blue
.sum
\end_layout
\begin_layout Itemize
Compute a positive integer from a given array of its decimal digits
\end_layout
\begin_layout LyX-Code
\family typewriter
\size footnotesize
\color blue
digits.reverse.zipWithIndex
\family default
\color darkgray
// Scala
\end_layout
\begin_layout LyX-Code
\family typewriter
\size footnotesize
\color blue
.map { case (d, i) => d * math.pow(10, i).toInt } .sum
\end_layout
\end_deeper
\begin_layout Standard
\begin_inset Separator parbreak
\end_inset
\end_layout
\begin_layout Frame
\begin_inset Argument 4
status open
\begin_layout Plain Layout
Feature 1: Loop-free iterative programs.
Induction
\end_layout
\end_inset
\begin_inset Separator latexpar
\end_inset
\end_layout
\begin_deeper
\begin_layout Itemize
Compute the mean value of a given sequence in single pass
\end_layout
\begin_layout LyX-Code
\family typewriter
\size footnotesize
\color blue
scala> Seq(4, 5, 6, 7).foldLeft((0.0, 0.0)) {
\end_layout
\begin_layout LyX-Code
\family typewriter
\size footnotesize
\color blue
| case ((sum, count), x) => (sum + x, count + 1)
\end_layout
\begin_layout LyX-Code
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| }.pipe { case (sum, count) => sum / count }
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\begin_layout LyX-Code
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res1: Double = 5.5
\end_layout
\begin_layout Itemize
Any inductive definition can be converted to a
\begin_inset Quotes eld
\end_inset
fold
\begin_inset Quotes erd
\end_inset
\end_layout
\begin_deeper
\begin_layout Itemize
Base case is the initial value
\end_layout
\begin_layout Itemize
Inductive step is the updater function
\end_layout
\end_deeper
\end_deeper
\begin_layout Standard
\begin_inset Separator parbreak
\end_inset
\end_layout
\begin_layout Frame
\begin_inset Argument 4
status open
\begin_layout Plain Layout
Feature 1: Loop-free iterative programs.
Other applications
\end_layout
\end_inset
\begin_inset Separator latexpar
\end_inset
\end_layout
\begin_deeper
\begin_layout Itemize
The implementation of
\family typewriter
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\color blue
map
\family default
\size default
\color inherit
,
\family typewriter
\size footnotesize
\color blue
filter
\family default
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,
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\color blue
fold
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,
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\color blue
flatMap
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,
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\color blue
groupBy
\family default
\size default
\color inherit
,
\family typewriter
\size footnotesize
\color blue
sum
\family default
\size default
\color inherit
, etc., may be asynchronous (Akka Streams), parallel, and/or distributed
(Spark, Flink)
\end_layout
\begin_deeper
\begin_layout Itemize
The programmer writes loop-free code in the map/filter/reduce style
\end_layout
\begin_layout Itemize
The runtime engine implements parallelism, fault tolerance, etc.
\end_layout
\begin_layout Itemize
Many types of programmer errors are avoided automatically
\end_layout
\end_deeper
\begin_layout Itemize
What we need to support programming in the map/filter/reduce style:
\end_layout
\begin_deeper
\begin_layout Itemize
Collections with user-definable methods
\end_layout
\begin_layout Itemize
Functions (
\begin_inset Quotes eld
\end_inset
lambdas
\begin_inset Quotes erd
\end_inset
) passed as parameters to other functions
\end_layout
\begin_layout Itemize
Easy work with tuple types
\end_layout
\end_deeper
\begin_layout Itemize
Lambdas were added to most programming languages by 2015
\end_layout
\end_deeper
\begin_layout Standard
\begin_inset Separator parbreak
\end_inset
\end_layout
\begin_layout Frame
\begin_inset Argument 4
status open
\begin_layout Plain Layout
Feature 2: Type parameters.
Usage
\end_layout
\end_inset
\begin_inset Separator latexpar
\end_inset
\end_layout
\begin_deeper
\begin_layout Itemize
Collections can have types parameterized by element type
\end_layout
\begin_deeper
\begin_layout Itemize
Array of integers:
\family typewriter
\size footnotesize
\color blue
Array[Int]
\family default
\size default
\color inherit
\end_layout
\begin_layout Itemize
Array of strings:
\family typewriter
\size footnotesize
\color blue
Array[String]
\family default
\size default
\color inherit
\end_layout
\begin_layout Itemize
Array of arrays of pairs of integers and strings:
\family typewriter
\size footnotesize
\color blue
Array[Array[(Int, String)]]
\family default
\size default
\color inherit
\end_layout
\end_deeper
\begin_layout Itemize
Methods such as
\family typewriter
\size footnotesize
\color blue
map
\family default
\size default
\color inherit
,
\family typewriter
\size footnotesize
\color blue
zip
\family default
\size default
\color inherit
,
\family typewriter
\size footnotesize
\color blue
flatMap
\family default
\size default
\color inherit
,
\family typewriter
\size footnotesize
\color blue
groupBy
\family default
\size default
\color inherit
change the type parameters
\end_layout
\begin_layout LyX-Code
\family typewriter
\size footnotesize
\color blue
Seq(4, 5, 6, 7)
\family default
\color darkgray
// Seq[Int]
\end_layout
\begin_layout LyX-Code
\family typewriter
\size footnotesize
\color blue
.zip(Seq("a", "b", "c", "d"))
\family default
\color darkgray
// Seq[(Int, String)]
\end_layout
\begin_layout LyX-Code
\family typewriter
\size footnotesize
\color blue
.map { case (i, s) => s"$s : $i" }
\family default
\color darkgray
// Seq[String]
\end_layout
\begin_layout Itemize
The compiler prevents using incorrect type parameters anywhere
\end_layout
\end_deeper
\begin_layout Standard
\begin_inset Separator parbreak
\end_inset
\end_layout
\begin_layout Frame
\begin_inset Argument 4
status open
\begin_layout Plain Layout
Feature 2: Type parameters.
Language features
\end_layout
\end_inset
\begin_inset Separator latexpar
\end_inset
\end_layout
\begin_deeper
\begin_layout Itemize
Code can be written once, then used with different type parameters
\end_layout
\begin_layout Itemize
Example (Scala):
\end_layout
\begin_layout LyX-Code
\family typewriter
\size footnotesize
\color blue
def map[A, B](xs: Seq[A])(f: A => B): Seq[B] = ...
\end_layout
\begin_layout Itemize
Collections (
\family typewriter
\size footnotesize
\color blue
Array
\family default
\size default
\color inherit
,
\family typewriter
\size footnotesize
\color blue
Set
\family default
\size default
\color inherit
, etc.) with type parameters support such methods
\end_layout
\begin_layout Itemize
Many programming languages have type parameters
\end_layout
\begin_deeper
\begin_layout Itemize
Functions with type parameters were added to Java in 2006
\end_layout
\begin_layout Itemize
C++ can imitate this functionality with templates
\end_layout
\begin_layout Itemize
Go-lang might get type parameters in the future
\end_layout
\end_deeper
\end_deeper
\begin_layout Standard
\begin_inset Separator parbreak
\end_inset
\end_layout
\begin_layout Frame
\begin_inset Argument 4
status open
\begin_layout Plain Layout
Feature 3: Disjunctive types
\end_layout
\end_inset
\begin_inset Separator latexpar
\end_inset
\end_layout
\begin_deeper
\begin_layout Itemize
Enumeration type (
\family typewriter
\size footnotesize
\color blue
enum
\family default
\size default
\color inherit
) describes a set of disjoint possibilities:
\end_layout
\begin_layout LyX-Code