The document discusses the principles and practices of functional programming, emphasizing concepts like referential transparency, side effects, and the separation of effects from pure values. It explores various techniques such as monads and functional reactive programming that help manage effects while maintaining program integrity. Overall, it highlights the importance of avoiding side effects for safer and more efficient code execution.

1. EFFECTING PURE CHANGE
HOW ANYTHING EVER GETS DONE IN FP
ANUPAM JAIN

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Hello!

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❑ MEETUP: https://www.meetup.com/DelhiNCR-Haskell-And-
Functional-Programming-Languages-Group
❑ TELEGRAM: https://t.me/fpncr
❑ GITHUB: https://github.com/fpncr
❑ SLACK: https://functionalprogramming.slack.com #FPNCR
FPNCR

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Effects

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❑ Programming with Mathematical Functions.
❑ A Function has Input and Output.
❑ Referentially Transparent
❑ A complete Program is just a function!
Functional Programming

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❑ No Global State
❑ No Assignments
❑ No Statements
❑ No Input-Output
Referential Transparency

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❑ Easier to Reason About
❑ Easier to Optimize
❑ Easier to Parallelize
❑ Easier to Test
❑ ???
Why?

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SideEffects!!

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What Happens If we
Ignore the Danger

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write :: String -> ()
read :: () -> String
What could go wrong

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write "Do you want a pizza?"
if (read() == "Yes") orderPizza()
write "Should I launch missiles?"
if (read() == "Yes") launchMissiles()
What could go wrong
PSEUDO-CODE

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write "Do you want a pizza?"
if (read() == "Yes") orderPizza()
write "Should I launch missiles?"
if (read() == "Yes") launchMissiles()
What could go wrong
May be optimized away

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write "Do you want a pizza?"
if (read() == "Yes") orderPizza()
write "Should I launch missiles?"
if (read() == "Yes") launchMissiles()
What could go wrong
May reuse value from
previous read()

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OCAML
let val ha = (print "ha") in ha; ha end
HASKELL
let ha = putStr “ha” in ha >> ha
Referential Transparency is Vital
ha
haha

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Taming Side Effects

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❑ Separate “Effects” from “Values”
(Hint: Strong Types are great for this)
❑ Effects are forever. No way to recover a “pure” Value.
❑ As much as possible, tag the flavor of side effects. “Print”
cannot launch nukes.
Contain. Not Prohibit.

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Effectful Logic
Pure Logic
Outside World

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❑ React – Functional Views
❑ The Elm Architecture
❑ Functional Reactive Programming
❑ Monads and Algebraic Effects
Examples

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render() {
let n = this.state.count
return <button onClick = {setState({count:n+1})}>{n}</button>
}
React – Functional Views

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view address model =
button [onClick address Increment] [text (toString model)]
update action model = case action of
Increment -> model + 1
The Elm Architecture

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❑ Interface between things that are static and those that vary
❑ Forms a graph of relationships between varying things
❑ The program creates the graph, and then lets things run. Akin
to cellular automata.
Functional Reactive
Programming

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❑ Event a – e.g. Button Clicks
❑ Behaviour a – e.g. System Time
❑ toBehaviour :: Event a -> Behaviour a
❑ mergeEvents :: Event a -> Event a -> Event a
❑ zipBehaviour :: (a -> b -> c) -> Behaviour a -> Behaviour b ->
Behaviour c
FRP Primitives

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❑ Event a – e.g. Button Clicks
❑ Behaviour a – e.g. System Time
❑ hold :: Event a -> Behaviour a
❑ sample :: Behaviour a -> Event b -> Event (a,b)
❑ merge :: Event a -> Event a -> Event a
❑ zip :: Behaviour a -> Behaviour b -> Behaviour (a,b)
FRP Primitives

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❑ text :: Behaviour String -> IO ()
❑ button :: Event ()
❑ gui = do
clicks <- button
text hold “Not Clicked” (map (_ -> “Clicked!”) clicks)
FRP GUIs

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Monads

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❑ Tagged values
❑ Provide explicit control over sequencing
Monads
IO String

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❑ Values can be evaluated in any order (or left unevaluated).
❑ Effects need explicit control over sequencing and sharing.
❑ The only way sequencing is possible in FP is through nested
functions. i.e. Callbacks.
Controlling Sequencing
write "Hello" (() -> write "World")

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write "Do you want a pizza?" (
() -> read (
ans -> if (ans == "Yes") orderPizza (
() -> write "Should I launch missiles?" (
() -> read (
ans -> if (ans == "Yes") launchNukes () ) ) ) ) )
Continuation Passing Style

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x <- foo
…
Sprinkle Some Syntactic Sugar
foo (x -> …)

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() <- write "Do you want a pizza?“
ans <- read
if(ans == "Yes") () <- orderPizza
() <- write "Should I launch missiles?“
ans <- read
if (ans == "Yes") () <- launchNukes
Sprinkle Some Syntactic Sugar
write "Do you want a pizza?" (() ->
read (ans ->
if (ans == "Yes") orderPizza (() ->
write "Should I launch missiles?" (() ->
read (ans ->
if (ans == "Yes") launchNukes () ) ) ) ) )

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do write "Do you want a pizza?“
ans <- read
if(ans == "Yes") orderPizza
write "Should I launch missiles?“
ans <- read
if (ans == "Yes") launchNukes
Do notation

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❑ Callbacks are generally associated with Asynchronous code
❑ Do notation avoids “Callback Hell”
Asynchronous
Computations are Effects
ajax GET “google.com" (response -> …)

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do
post <- ajax GET “/post/1“
map post.comments (cid -> do
comment <- ajax GET “/comments/{cid}“
…
)
Avoiding Callback Hell
ajax GET “/post/1" (post ->
map post.comments (cid ->
ajax GET “/comments/{cid}" (comment ->
…
) ) )

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if(ans == "Yes") orderPizza
else ???
Where is the Else block?

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if(ans == "Yes") orderPizza
else return ()
Where is the Else block?

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Type: IO a
Bind: IO a -> (a -> IO b) -> IO b
Return: a -> IO a
The Monad

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❑ Reader
❑ Logger
❑ State
❑ Exceptions
❑ Random
❑ …
Bring Your Own Monad

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Type: Reader e a :: e -> a
Bind: Reader e a -> (a -> Reader e b) -> Reader e b
Return: a -> Reader e a
ask: Reader e e
runReader: e -> Reader e a -> a
Reader Monad

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main = runReader myConfig do
res <- foo
bar res
foo = do config <- ask; …
bar res = do config <- ask; …
Reader Monad

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“Haskell” is the world’s finest
imperative programming
language.
~Simon Peyton Jones

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“Haskell” is the world’s finest
imperative programming
language.
~Simon Peyton Jones
(Creator of Haskell)

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❑ Like Monads, you can define your own Effects
❑ But you can define the usage and handling of the effects
separately
❑ And effects compose freely (pun intended)
Algebraic Effects

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data Console callback
= Read (String -> callback)
| Write String callback
handle (Read cb) = …
handle (Write s cb) = …
Console Effect PSEUDO-CODE

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data Console callback
= Read (String -> callback)
| Write String callback
handle (Read cb) = s = do readLine(); cb(s)
handle (Write s cb) = do console.log(s); cb()
Console Effect PSEUDO-CODE

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handle (Write s cb) = do console.log(s); console.log(s); cb();
handle (Write s cb) = cb();
handle (Write s cb) = do cb(); console.log(s);
handle (Write s cb) = do if(test(s)) console.log(s); cb();
You can do more things
PSEUDO-CODE

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handle (Return x) = return (x,””);
handle (Write s cb) = (x,rest) = do cb(); return (x, s:rest);
Returning Values from
Handlers PSEUDO-CODE

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SideEffects!!

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BestFriends!!

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Thank You