This document discusses functional programming and the web. It begins with a brief history of functional programming concepts like the Entscheidungsproblem and the work of Hilbert, Gödel, Turing, and Church. It then presents some basic lambda calculus expressions and fixed point combinators. The rest of the document focuses on aspects of functional programming that are important for web development, such as immutability, declarative programming, concurrency, and correctness. It concludes with questions and invites the reader to learn more about purely functional data structures and functional programming languages that can be used for web development.

1. F U N C T I O N A L
P R O G R A M M I N G
(A N D T H E W E B )
BY @ B O B J LO N G

2. DAV I D H I L B E RT
1 8 3 2 - 1 94 3
E N T S C H E I D U N G S P RO B L E M
( D E C IS I O N P RO B L E M)

3. Gödel / Turing / Church

4. <expr> = <variable>
<expr> = (λ<identifier>.<expr>)
<expr> = (<expr><expr>)

5. λf.λx.x
λf.λx.f x
λf.λx.f (f x)
λf.λx.f (f (f x))
λn.λf.λx.f (n f x)

6. λn.λf.λx.f (n f x)

7. λf.λx.x <- 0
λf.λx.f x <- 1
λn.λf.λx.f (n f x)

8. λn.λf.λx.f (n f x)
n = λf.λx.x

9. λf.λx.f (λf.λx.x f x)

10. λf.λx.f x
= 1

11. (let ((a 1)(b 2))
(+ b a))

12. ((lambda (a b)
(+ b a)) 1 2)

13. T H I N G S W E
C A R E A B O U T
immutability
declarative programming
concurrency/parallelism
correctness

14. immutability: avoids direct DOM mutation,
+ general immutable helpers
declarative: avoids imperative manipulation
correctness / performance: sum of parts

15. {
“name”: “Foo”,
“position”: [-1, 30]
}

18. CO N C L US I O N S …
Q U E S T I O N S ?
@ B O B J LO N G

19. purely functional data structures
Links!
The zen of stateless state
Faylang Purescript elm
little example app using Purescript + a virtual DOM