Tour guide Yan Cui discusses functional programming concepts

tour guide : Yan Cui @theburningmonk

2MONTH
TB
P E R
sec
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25,000

Type Provider Pipes
Statically Resolved TP
Implicit Interface
Implementation
Borrowed Pointers Dependent Types
Uniqueness Types
Bit Syntax
Signals
Macros
Unit-of-Measure
Actor Model

Being literate isn't simply a matter of being
able to put words on the page, it's solidifying
our thoughts such that they can be written.
Interpreting and applying someone else's
thoughts is the equivalent for reading. We call
these composition and comprehension.
And they are what literacy really is.
- Chris Granger

Coding, like writing, is a mechanical act…
Just like writing, we have to know how to solidify
our thoughts and get them out of our head…!
We build mental models of everything… If we
want computers to be able to compute for us,
then we have to accurately extract these
models from our heads and record them.
- Chris Granger

“Programming languages
have a devious influence:
they shape our thinking
habits.”
- Edsger W. Dijkstra

“One of the most disastrous
thing we can learn is the ﬁrst
programming language, even
if it's a good programming
language.”
- Alan Kay

“Just as there are odours that dogs can smell and
we cannot, as well as sounds that dogs can hear
and we cannot, so too there are wavelengths of light
we cannot see and ﬂavours we cannot taste. Why
then, given our brains wired the way they are, does
the remark "Perhaps there are thoughts we cannot
think," surprise you? Evolution, so far, may possibly
have blocked us from being able to think in some
directions; there could be unthinkable thoughts.”
- Richard Hamming

“The limits of my
language means the
limits of my world.”
- Ludwig Wittgenstein

your
app
CSVCSVCSV
CSVCSVXML
some
service

your
app
CSVCSVCSV
CSVCSVXML
some
service
DB

1. define DTO types!
2. I/O!
3. marshal data into DTO!
4. do useful work

compiler
provideexternal
data source typed info

R
FunScript
Azure
Amazon S3
CSVSQLite
SQL Server
WSDL
WorldBank
Regex
ODATA IKVM
Facebook
Apiary
XAMLFreebaseHadoop
Oracle
Minesweeper
Don Syme
Powershell
JSON
Fizzbuzz
Mixin
RSS
Matlab
Dates
NorthPole
XML
Python

“…a clean design is one that
supports visual thinking so
people can meet their
informational needs with a
minimum of conscious effort.”
- Daniel Higginbotham
(www.visualmess.com)

Whilst talking with an ex-colleague, a question came up on how to implement the Stable Marriage
problem using a message passing approach. Naturally, I wanted to answer that question with Erlang!!
!
Let’s first dissect the problem and decide what processes we need and how they need to interact with
one another.!
!
The stable marriage problem is commonly stated as:!
Given n men and n women, where each person has ranked all members of the opposite sex with a
unique number between 1 and n in order of preference, marry the men and women together such that
there are no two people of opposite sex who would both rather have each other than their current
partners. If there are no such people, all the marriages are “stable”. (It is assumed that the participants
are binary gendered and that marriages are not same-sex).!
From the problem description, we can see that we need:!
* a module for man!
* a module for woman!
* a module for orchestrating the experiment!
In terms of interaction between the different modules, I imagined something along the lines of…
how we read ENGLISH
see also http://bit.ly/1KN8cd0

Whilst talking with an ex-colleague, a question came up on how to implement the Stable Marriage
problem using a message passing approach. Naturally, I wanted to answer that question with Erlang!!
!
Let’s first dissect the problem and decide what processes we need and how they need to interact with
one another.!
!
Given n men and n women, where each person has ranked all members of the opposite sex with a
unique number between 1 and n in order of preference, marry the men and women together such that
there are no two people of opposite sex who would both rather have each other than their current
partners. If there are no such people, all the marriages are “stable”. (It is assumed that the participants
* a module for man!
In terms of interaction between the different modules, I imagined something along the lines of…
2.top-to-bottom
1.left-to-right
how we read ENGLISH

how we read CODE
public void DoSomething(int x, int y)!
{!
Foo(y,!
Bar(x,!
Zoo(Monkey())));!
}

how we read CODE
{!
Foo(y,!
Bar(x,!
Zoo(Monkey())));!
}
2.bottom-to-top
1.right-to-left

Whilst talking with an ex-colleague, a question came up on
how to implement the Stable Marriage problem using a
message passing approach. Naturally, I wanted to answer
that question with Erlang!!
!
Let’s first dissect the problem and decide what processes we
need and how they need to interact with one another.!
!
Given n men and n women, where each person has ranked
all members of the opposite sex with a unique number
between 1 and n in order of preference, marry the men and
women together such that there are no two people of
opposite sex who would both rather have each other than
their current partners. If there are no such people, all the
marriages are “stable”. (It is assumed that the participants
* a module for man!
In terms of interaction between the different modules, I
imagined something along the lines of…
2.top-to-bottom
1.left-to-right
how we read ENGLISH
{!
Foo(y,!
Bar(x,!
Zoo(Monkey())));!
}
2.top-to-bottom
1.right-to-left
how we read CODE

“…a clean design is one that
supports visual thinking so
people can meet their
informational needs with a
minimum of conscious effort.”

how we read CODE
let drawCircle x y radius =
radius |> circle
|> ﬁlled (rgb 150 170 150)
|> alpha 0.5
|> move (x, y)

how we read CODE
radius |> circle
|> ﬁlled (rgb 150 170 150)
|> alpha 0.5
|> move (x, y)
2.top-to-bottom
1.left-to-right

circle radius
|> ﬁlled (rgb 150 170 150)
|> alpha 0.5
|> move (x, y)

NASA orbiter crashed
because one engineer
accidentally used miles
instead of kilometres

you’re never too smart
to make mistakes

[<Measure>]!
type Pence
e.g.! 42<Pence>!
! 153<Pence>!
! …

10<Meter> / 2<Second> ! = 5<Meter/Second>!
10<Meter> * 2<Second> ! = 20<Meter Second> !
10<Meter> + 10<Meter> ! = 20<Meter>!
10<Meter> * 10! ! ! = 100<Meter>!
10<Meter> * 10<Meter> ! = 100<Meter2>!
10<Meter> + 2<Second> ! // error!
10<Meter> + 2 ! ! ! // error

Duck Typing
If it looks like a duck
and quacks like a
duck, it's a duck

def say_quack(duck):
duck.quack()

class Duck:
def quack(self):
print("quack quack!”)

class Duck:
def quack(self):
print("quack quack!”)
!
duck = Duck()
say_quack(duck)
!
> quack quack!

class Bird:
def quack(self):
print(“tweet tweet!”)

class Bird:
def quack(self):
print(“tweet tweet!”)
!
bird = Bird()
say_quack(bird)
!
> tweet tweet!

Convenience Safety
see also http://bit.ly/1H2fN9i

statically
resolved!
type parameters

let inline sayQuack (duck : ^a) =
(^a : (member Quack : unit -> unit) duck)

type Duck () =
member __.Quack() =
printfn “quack quack!”
!
let duck = Duck()
sayQuack duck
!
> quack quack!

type Bird () =
member __.Quack() =
printfn “tweet tweet!”
!
let bird = Bird()
sayQuack bird
!
> tweet tweet!

type Dog () =
member __.Bark() =
printfn “woof woof!”
!
let dog = Dog()
sayQuack dog
!
error FS0001: The type ‘Dog’ does not
support the operator ‘Quack’

implicit
interface!
implementation

type Duck interface
{
Quack()
}
see also http://bit.ly/1ER5zVs

func sayQuack(duck Duck) {
duck.Quack()
}

type Donald struct { }
func (d Donald) Quack()
{
fmt.Println(“quack quack!”)
}

type Bird struct { }
func (b Bird) Quack()
{
fmt.Println(“tweet tweet!”)
}

func main() {
donald := Donald{}
sayQuack(donald)
bird := Bird{}
sayQuack(bird)
}

quack quack!
func main() {
donald := Donald{}
sayQuack(donald)
bird := Bird{}
sayQuack(bird)
}

tweet tweet!
func main() {
donald := Donald{}
sayQuack(donald)
bird := Bird{}
sayQuack(bird)
}

type Dog struct { }
func (d Dog) Bark()
{
fmt.Println(“woof woof!”)
}

func main() {
dog := Dog{}
sayQuack(dog)
}
main.go:40: cannot use dog (type Dog) as type
Duck in argument to sayQuack:!
! Dog does not implement Duck (missing
Quack method)

Convenience Safety

patterns are observed
after the fact

system building is also
a process of learning
and discovery

implementation
package
interface package

encourages precise
interface deﬁnitions

Homoiconicity
…homoiconicity is a property of some
programming languages in which the program
structure is similar to its syntax, and therefore
the program’s internal representation can be
inferred by reading the text’s layout…

(let [x 1]
(inc x))
see also http://bit.ly/1PpIrjS

(let [x 1]
(inc x))
!
=> 2

list (1 2 3)
vector [1 2 3]

(let [x 1]
(inc x))
!
list

(let [x 1]
(inc x))
!
symbol

(let [x 1]
(inc x))
!
vector

form :
code as data structure

code data
quote
eval

quote
(+ 1 2)
=> 3

quote
(+ 1 2)
=> 3
(quote (+ 1 2))
=> (+ 1 2)

quote
(+ 1 2)
=> 3
(quote (+ 1 2))
=> (+ 1 2)
‘(+ 1 2)
=> (+ 1 2)

eval
‘(+ 1 2)
=> (+ 1 2)
(eval ‘(+ 1 2))
=> 3

(defmacro assert-equals [actual expected]
‘(let [actual-val# ~actual]
(when-not (= actual-val# ~expected)
(throw
(AssertionError.
(str “FAIL in “ ‘~actual
“n expected: “ ‘~expected
“n actual: “ actual-val#))))))

(assert-equals (inc 1) 2) ; => nil
(assert-equals (inc 1) (+ 0 1))
; => AssertionError FAIL in (inc 1)
; expected: (+ 0 1)
; actual: 2

(assert-equals (inc 1) 2) ; => nil
; => AssertionError FAIL in (inc 1)
; expected: (+ 0 1)
; actual: 2
huh?? where? what? how?

(throw
(AssertionError.

expanded at
compile time

(macroexpand '(assert-equals (inc 1) (+ 0 1)))
; =>
; (let* [actual-value__16087__auto__ (inc 1)]
; (clojure.core/when-not
; (clojure.core/= actual-value__16087__auto__ (+ 0 1))
; (throw (java.lang.AssertionError.
; (clojure.core/str
; "FAIL in " (quote (inc 1))
; "nexpected: " (quote (+ 0 1))
; "n actual: " actual-value__16087__auto__)))))

ownership
see also http://bit.ly/1F6WBVD

memory safety
without GC

ZERO
runtime cost

safety + speed

fn foo() {
// v has ownership of the vector
let v = vec![1, 2, 3];
// mutable binding
let mut v2 = vec![];
}
// vector is deallocated at the
// end of scope,
// this happens deterministically

immutable by default

// take ownership
let v = vec![1, 2, 3];

// take ownership
let v = vec![1, 2, 3];
!
// moved ownership to v2
let v2 = v;

// take ownership
let v = vec![1, 2, 3];
!
// moved ownership to v2
let v2 = v;
!
println!("v[0] is {}", v[0]);
// error: use of moved value: `v`
// println!("v[0] is {}", v[0]);
// ^

fn take(v : Vec<i32>) {
// ownership of vector transferred
// to v in this scope
}

// take ownership
let v = vec![1, 2, 3];
!
// moved ownership
take(v);

// take ownership
let v = vec![1, 2, 3];
!
// moved ownership
take(v);
!
println!("v[0] is {}", v[0]);
// error: use of moved value: `v`
// println!("v[0] is {}", v[0]);
// ^

let me buy
your book

sure thing!

thanks

BURN!!! !
>:D

but I
still need it..!
:’(

borrowing

// note we're taking a reference,
// &Vec<i32>, instead of Vec<i32>
fn take(v : &Vec<i32>) {
// no need to deallocate the vector
// after we go out of scope here
}

// take ownership
let v = vec![1, 2, 3];
!
// notice we're passing a reference,
// &v, instead of v
take(&v); // borrow ownership
!
println!("v[0] is {}", v[0]);
// v[0] is 1

let me
borrow your
book

I’m done,
here you go

immutable by default

fn take(v : &Vec<i32>) {
v.push(5);
}
!
let v = vec![];
take(&v);
// cannot borrow immutable borrowed
// content `*v` as mutable
// v.push(5);
// ^

fn take(v : &mut Vec<i32>) {
v.push(5);
}
!
let mut v = vec![];
take(&mut v);
!
println!("v[0] is {}", v[0]);
// v[0] is 5

borrowing
rules

Rule 1.!
!
the borrower’s scope must not
outlast the owner

Rule 2.!
!
one of the following, but not both:!
2.1 0 or more refs to a resource!
2.2 exactly 1 mutable ref

data race
There is a ‘data race’ when two or more pointers
access the same memory location at the same
time, where at least one of them is writing, and
the operations are not synchronised.

data race
a. two or more pointers to the same resource!
b. at least one is writing!
c. operations are not synchronised

Data Race Conditions!
a. two or more pointers to the same resource!
b. at least one is writing!
c. operations are not synchronised
Borrowing Rules!
one of the following, but not both:!
! 2.1 0 or more refs to a resource!
! 2.2 exactly 1 mutable ref

Dependent Types
Uniqueness Types
Bit Syntax
Borrowed Pointers
Type Provider Pipes
Implicit Interface
Implementation
Signals
Macros
Unit-of-Measure
Actor Model

seen generics?
aka parametric polymorphism

List<T>
List<int> List<Cat>
List<string>

types that depend on
arbitrary values?

Vect n a
vector of n elements of type a

zipWith :
(a -> b -> c)
-> Vect n a
-> Vect n b
-> Vect n c

zipWith f [] [] = []
zipWith f (x :: xs) (y :: ys) =
f x y :: zipWith f xs ys

making invalid state
UNREPRESENTABLE

Tour guide Yan Cui discusses functional programming concepts

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