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Demystifying
Functional Programming
tamizhvendan
tamizhvendan
Lead Consultant
www.ajira.tech
Tamizhvendan S
Passionate, Pragmatic and Polyglot Programmer
https://www.demystifyfp.com
λ
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Why
Functional
Programming?
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Solution
Problem
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Solution
Problem
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Vehicle Influences
The Journey
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Solution
Problem
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Language Influences
The Thinking
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int numbers[3] = {2,3,4}, squaredNumbers[3];
for (int i = 0; i < 3; i!++) {
squaredNumbers[i] = numbers[i] * numbers[i];
}
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let numbers = [1,2,3];
let squaredNumbers =
numbers.map(n !=> n * n);
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“A language that doesn’t affect the way you
think about programming, is not worth
knowing.”
- Alan Perlis
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Solution
Problem
Challenges
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Domain Complexity
Requirement Change
Time To Market
Reliability
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Solution
Problem
Abstractions
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Inheritance
Encapsulation
Polymorphism
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GoF Patterns!
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Inheritance
Encapsulation
Polymorphism
GoF Patterns!
Better
Abstractions
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Functional
Programming
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Solution
Problem
Better
Abstractions
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FP
Three Step Approach
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Step-1
Model The Domain
(Data)
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“Data dominates. If you’ve chose the right data
structures & organise things well, the algorithms
will almost be self-evident. Data structures, not
algorithms, are central to programming”
- Rob Pike
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Step-2
Define The Behaviour
(Algorithm)
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“Make each program do one thing well. To do a
new job, build afresh rather than complicate the
old programs by adding new “features”.
Expect the output of every program to become
the input of another, as yet unknown, program”
- Unix Philosophy
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Step-3
Define The System
(Composition)
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“Design is to take things apart in a such a way
that they can be put back together”
- Rich Hickey
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Separation of Concerns
Composition
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Dynamic
HTML Generation
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Step-1
Model The Domain
(Data)
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type Attribute = {
Name : string
Value : string
}
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type Element = {
Name : string
Attributes : Attribute list
}
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This is a paragraph
type Element = { Name : string Attributes : Attribute list Content : Content } and Content = PlainText of stringSlide 32
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type Element = {
Name : string
Attributes : Attribute list
Content : Content option
} and Content =
PlainText of string
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- Coffee
- Tea
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Algebraic Data Types
type Element = {
Name : string
Attributes : Attribute list
Content : Content option
} and Content =
| PlainText of string
| ChildElements of Element list
Sum (or Choice) Type
Product Type
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Step-2
Define The Behaviour
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Element !-> string
let beveragesList =
let content =
childElements [
element "li" [] (plainText "Tea")
element "li" [] (plainText "Coffee")]
element "ul" [("class", "ulist")] content
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Step-3
Divide &
Conquer (Compose)
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Attribute !-> string
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// Attribute string
let attrToHtmlStr attr =
sprintf @"%s=""%s""" attr.Name attr.Value
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List !-> string
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List !-> string
List !-> List
List !-> string
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What we have
What we want
List !-> List
Attribute !-> string
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Map
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// List List
let attrsToHtmlStr attrs =
List.map attrToHtmlStr attrs
(A !-> B) !-> List !-> List
List.map
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What we have
What we want
List !-> string
string !-> List !-> string
String.concat
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Partial Application
partially providing the argument(s)
string !-> List !-> string
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string !-> List !-> string
String.concat
String.concat " "
string !-> List !-> string
String.concat " "
string !-> List !-> string
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What we have
List !-> List
List !-> string
What we want
List !-> string
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Expect the output of every program to become
the input of another, as yet unknown, program”
- Unix Philosophy
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A !-> B B !-> C
A !-> C
Function Composition
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Function Composition
// List string
let attrsToHtmlStr =
List.map attrToHtmlStr
String.concat " "
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Function Composition
Using Piping
// List string
let attrsToHtmlStr attrs =
List.map attrToHtmlStr attrs
String.concat " "
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// List string
let attrsToHtmlStr attrs =
attrs
List.map attrToHtmlStr
String.concat " "
partial application
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let attrs = [
{Name = "src"; Value = "cat.jpg"}
{Name = "alt"; Value = "cat"}
]
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Option !-> string
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Content !-> string
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Pattern Matching
// Content string
let contentToHtmlStr content =
match content with
| PlainText x x
| ChildElements xs
xs
List.map elementToHtmlStr
String.concat ""
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What we have
What we want
Option !-> string
Content !-> string
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Option !-> string
Option
!-> Option
Option !-> string
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What we have
What we want
Content !-> string
Option
!-> Option
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map
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map
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// Option
let contentOpt =
element.Content
Option.map contentToHtmlStr
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let content =
defaultArg contentOpt ""
Option !-> string
core library function
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What we have
What we want
Element !-> string
Option !-> string
List !-> string
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let elementToHtmlStr element =
let attrs =
attrsToHtmlStr element.Attributes
let contentOpt =
element.Content
Option.map contentToHtmlStr
let content =
defaultArg contentOpt “"
let tag = element.Name
sprintf "<%s %s> %s "
tag attrs content tag
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let beveragesList =
let content =
childElements [
element "li" [] (plainText "Tea")
element "li" [] (plainText "Coffee")]
element "ul" [("class", "ulist")] content
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Error Handling
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Result (Either) Type
int !-> Result
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// int Result
let getElementById id =
if id = 1 then
Ok beveragesList
else
Error "unable to fetch"
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Transform Element to Html String
after fetching
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What we have
What we want
Result
!-> Result
Element !-> string
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map
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map
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// int Result
let getHtmlStrById id =
getElementById id
Result.map elementToHtmlStr
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map Preserves
The Structure
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map Transforms
The Value
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No content
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Merging Elements!
Element !-> Element !-> Element
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List List
List
Merging Attributes
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let mergeAttrs attrs1 attrs2 =
attrs1 @ attrs2
List.groupBy (fun (a : Attribute) a.Name)
List.map (fun (name, attrs)
let value =
attrs
List.map (fun a a.Value)
String.concat " "
{Name = name; Value = value}
)
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Content Content Content
Merging Content
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let mergeContent content1 content2 =
match content1, content2 with
| PlainText x, PlainText y
sprintf "%s\n%s" x y
PlainText
| ChildElements xs, ChildElements ys
ChildElements (xs @ ys)
| _ content1
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What we have
What we want
Element !-> Element !-> Element
List List
List
Content Content Content
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type Element = {
Name : string
Attributes : Attribute list
Content : Content option
}
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What we have
What we want
Content Content Content
Option !-> Option
!-> Option
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aka map2
lift2
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let mergeElement e1 e2 =
let attrs =
mergeAttrs e1.Attributes e2.Attributes
let content =
Option.lift2 mergeContent e1.Content e2.Content
{ Name = e1.Name
Attributes = attrs
Content = content}
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Merge Elements after fetching
Result
!-> Result
!-> Result
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What we have
What we want
Element Element Element
Result
!-> Result
!-> Result
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lift2
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let getAndMergeElements e1Id e2Id =
let e1R = getElementById e1Id
let e2R = getElementById e2Id
Result.lift2 mergeElement e1R e2R
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lift2
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No content
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Each Configuration Has Own
Element Id
type Configuration = {
Id : int
ElementId : int
}
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Fetch Configuration And Then
Fetch It’s Element
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Fetch Configuration
int !-> Result
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// int Result
let getConfigurationById id =
if id = 1 then
Ok {Id = 1; ElementId = 1}
else
Error "unable to fetch"
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What we have
What we want
int !-> Result
int !-> Result
int !-> Result
configuration id
element id
configuration id
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bind
aka flatMap
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let getElementByConfigId cId =
getConfigurationById cId
Result.bind (fun c
getElementById c.ElementId)
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bind
aka flatMap
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Prompt
let namePrompt : Prompt =
stringPrompt "What's Your name?”
> namePrompt();;
What’s your name?
Tamizhvendan
val it : Result = Ok “Tamizhvendan”
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Prompt
let agePrompt : Prompt =
intPrompt "What's Your age?”
> agePrompt();;
What’s your age?
29
val it : Result = Ok 29
> agePrompt();;
What’s your age?
foobar
val it : Result = Error ParserError
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let greeting name age =
sprintf "Hey %s, you are %d years old!” name age
let greetingWizard =
lift2 greeting namePrompt agePrompt
> greetingWizard();;
What’s your name?
Tamizhvendan
What’s your age?
29
val it : Result = Ok “Hey Tamizhvendan, you are 29 years old!”
lift2
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let addAgeBy10Years age = age + 10
let prankWizard =
agePrompt
|> map addAgeBy10Years
|> lift2 greeting namePrompt
> prankWizard();;
What’s your name?
Tamizhvendan
What’s your age?
29
val it : Result = Ok “Hey Tamizhvendan, you are 39 years old!”
map
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By The Way!
M
AP
F Functor
Applicative
Monad
* There are some laws!
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FP Demystified
λ Algebraic Data Types
λ Separate Pure & Impure Functions
λ Function Signature Matters!
λ map, liftN, bind, etc.,
λ Have Fun!!
https://www.demystifyfp.com