Walk through a solution to Advent of Code 2016 Day 8 problem, done in F# with immutable data structures and pure functions

1. THE TASTE OF
F#I am still Mikhail Shilkov
You can nd me at @MikhailShilkov and http://mikhail.io

2. FACTS ABOUT F#
.NET language
Functional First
Open Source

4. 0 1 2 3 4 5 6 7 8 9 x
0 # # # #
1 # # #
2 # # # # # # #
3 # # #
4 #
y

5. // rect AxB
//
// turns on all of the pixels in a rectangle
// at the top‐left which is A wide and B tall
//
// e.g. rect 3x2:
0 1 2 3 4 5 6 7 8 9 x
0
1
2
3
4
y
0 1 2 3 4 5 6 7 8 9 x
0 # # #
1 # # #
2
3
4
y

6. // rotate row y=A by B
//
// shifts (with cycle) all of the pixels
// in row A right by B pixels
//
// e.g. rotate row y=1 by 2
0 1 2 3 4 5 6 7 8 9 x
0 # # #
1 # # #
2
3
4
y
0 1 2 3 4 5 6 7 8 9 x
0 # # #
1 # # #
2
3
4
y

7. // rotate column x=A by B
//
// shifts (with cycle) all of the pixels
// in column A down by B pixels
//
// e.g. rotate column x=2 by 4
0 1 2 3 4 5 6 7 8 9 x
0 # # #
1 # # #
2
3
4
y
0 1 2 3 4 5 6 7 8 9 x
0 # # #
1 # #
2
3
4 #
y

8. rect 1x1
rotate row y=0 by 5
rect 1x1
rotate row y=0 by 6
rect 1x1
rotate row y=0 by 5
rect 1x1
rotate row y=0 by 2
rect 1x1
rotate row y=0 by 5
rect 2x1
rotate row y=0 by 2
rect 1x1
...

9. let Width = 10
// val Width : int = 10
let Height = 5
// val Height : int = 5

10. type Pixel = int * int
let pixel = (5, 2)
// val pixel : int * int = (5, 2)
0 1 2 3 4 5 6 7 8 9 x
0
1
2 #
3
4
y

11. type Screen = Set<Pixel>
let screen = set [ (2, 1); (5, 2); (8, 3) ]
// val screen : Set<int * int> = set [(2, 1); ...]
0 1 2 3 4 5 6 7 8 9 x
0
1 #
2 #
3 #
4
y

12. Set.empty
Set.add element set
Set.filter predicate set
Set.union set1 set2
Set.map f set

13. let emptyScreen = Set.empty
let fullScreen =
seq { for x in 0..Width‐1 do
for y in 0..Height‐1 ‐> x, y }
|> Set.ofSeq
0 1 2 3 4 5 6 7 8 9 x
0
1
2
3
4
y
0 1 2 3 4 5 6 7 8 9 x
0 # # # # # # # # # #
1 # # # # # # # # # #
2 # # # # # # # # # #
3 # # # # # # # # # #
4 # # # # # # # # # #
y

14. let turnOnOne (x, y) screen =
Set.add (x, y) screen
// val turnOnOne :
// x:'a * y:'b ‐> screen:Set<'a * 'b> ‐> Set<'a * 'b>
// when 'a : comparison and 'b : comparison
let screen2 = turnOnOne (1, 4) screen
0 1 2 3 4 5 6 7 8 9 x
0
1 #
2 #
3 #
4
y
0 1 2 3 4 5 6 7 8 9 x
0
1 #
2 #
3 #
4 #
y

15. let turnOn predicate screen =
let filtered = Set.filter predicate fullScreen
Set.union screen filtered
// val turnOn : f:(int * int ‐> bool) ‐>
// screen:Set<int * int> ‐> Set<int * int>

16. let turnOn predicate screen =
fullScreen
|> Set.filter predicate
|> Set.union screen
// val turnOn : f:(int * int ‐> bool) ‐>
// screen:Set<int * int> ‐> Set<int * int>

17. let turnOnRect (xmin, ymin) (xmax, ymax) screen =
let isInRect (x, y) =
x >= xmin && x <= xmax
&& y >= ymin && y <= ymax
turnOn isInRect screen
// val turnOnRect :
// xmin:int * ymin:int ‐> xmax:int * ymax:int ‐>
// screen:Set<int * int> ‐> Set<int * int>

18. let screen3 = turnOnRect (2, 1) (3, 4) screen2
// val screen3 : Set<int * int> = set [ ... ]
0 1 2 3 4 5 6 7 8 9 x
0
1 #
2 #
3 #
4 #
y
0 1 2 3 4 5 6 7 8 9 x
0
1 # #
2 # # #
3 # # #
4 # # #
y

19. let move f screen =
Set.map f screen
// val move :
// f:('a ‐> 'b) ‐> screen:Set<'a> ‐> Set<'b>
// when 'a : comparison and 'b : comparison

20. let rotateRow index by screen =
let mapAt (x, y) =
if y = index then (x + by) % Width, y
else x, y
move mapAt screen
// val rotateRow :
// index:'a ‐> by:int ‐>
// screen:Set<int * 'a> ‐> Set<int * 'a>
// when 'a : comparison

21. let screen4 = rotateRow 3 2 screen3
// val screen4 : Set<int * int> = set [ ... ]
0 1 2 3 4 5 6 7 8 9 x
0
1 # #
2 # # #
3 # # #
4 # # #
y
0 1 2 3 4 5 6 7 8 9 x
0
1 # #
2 # # #
3 # # #
4 # # #
y

22. let rotateColumn index by screen =
let mapAt (x, y) =
if x = index then x, (y + by) % Height
else x, y
move mapAt screen
// val rotateColumn :
// index:'a ‐> by:int ‐>
// screen:Set<int * 'a> ‐> Set<int * 'a>
// when 'a : comparison

23. type Instruction =
| Rect of width: int * height: int
| RotateRow of index: int * by: int
| RotateColumn of index: int * by: int

24. let execute screen instruction =
match instruction with
| Rect (w, h) ‐> turnOnRect (0, 0) (w‐1, h‐1) screen
| RotateRow (i, by) ‐> rotateRow i by screen
| RotateColumn (i, by) ‐> rotateColumn i by screen
// val execute :
// screen:Set<int * int> ‐> instruction:Instruction
// ‐> Set<int * int>

25. let rec step screen instructions =
match instructions with
| [] ‐> screen
| instruction :: others ‐>
let newScreen = execute screen instruction
step newScreen others
let solve1 instructions =
step Set.empty instructions

26. let solve2 instructions =
List.fold execute Set.empty instructions

27. DEMO

28. WHAT WE SAW
Strong types, cheap to create
Low overhead, no boilerplate
Reusable patterns and data structures
"The pit of functional success"

29. THANKS!Mikhail Shilkov
You can nd me at @MikhailShilkov and http://mikhail.io