Effecting Pure Change - How anything ever gets done in functional programming

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

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2 Hello!

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3 ❑ 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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4 Effects

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5 ❑ 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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6 ❑ No Global State ❑ No Assignments ❑ No Statements ❑ No Input-Output Referential Transparency

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

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8 Side Effects !!

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9 What Happens If we Ignore the Danger

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

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11 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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12 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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13 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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14 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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15 Taming Side Effects

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16 ❑ 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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17 Effectful Logic Pure Logic Outside World

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

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19 render() { let n = this.state.count return {n} } React – Functional Views

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20 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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21 ❑ 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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22 ❑ 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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23 ❑ 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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24 ❑ text :: Behaviour String -> IO () ❑ button :: Event () ❑ gui = do clicks <- button text hold “Not Clicked” (map (_ -> “Clicked!”) clicks) FRP GUIs

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25 Monads

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

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27 ❑ 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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28 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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29 x <- foo … Sprinkle Some Syntactic Sugar foo (x -> …)

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30 () <- 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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31 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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32 ❑ 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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33 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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34 if(ans == "Yes") orderPizza else ??? Where is the Else block?

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

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

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

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38 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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39 main = runReader myConfig do res <- foo bar res foo = do config <- ask; … bar res = do config <- ask; … Reader Monad

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

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

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42 ❑ 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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43 data Console callback = Read (String -> callback) | Write String callback handle (Read cb) = … handle (Write s cb) = … Console Effect PSEUDO-CODE

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44 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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45 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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46 handle (Return x) = return (x,””); handle (Write s cb) = (x,rest) = do cb(); return (x, s:rest); Returning Values from Handlers PSEUDO-CODE