Explores how to write a tic-tac-toe API that meets some interesting static typing constraints. Specifically, programs using the API may fail to compile, depending on the state of play in the game, such as trying to call move() with an already completed game board. The real theme of the presentation is not so much solving the tic-tac-toe problem but, rather, pushing static typing to its limits (and some might argue beyond its useful limits—you will have to judge for yourself).

1. ©ASERT2006-2013
Dr Paul King @paulk_asert
Director, ASERT, Brisbane, Australia
http:/slideshare.net/paulk_asert/tictactoe-groovy
https://github.com/paulk-asert/tictactoe-groovy
Coding TicTacToe:
A coding style
challenge!
** Coming
soon!

2. Topics
Introduction
• Dynamic solution
• Options for increasing type safety
• Groovy challenge solution
• Other language implementations
• Interlude: solving tic tac toe
• Going beyond the challenge
©ASERT2006-2013

3. Tic Tac Toe
• Players take it in turn to
place “marks” on the
board
• By convention, Player 1
uses “X” and starts
first; Player 2 uses “O”
• The game finishes once
one of the players has
three of their marks in a
row
• Also called noughts
and crosses (and other
names) with 4 x 4, 3D
and other variants

4. Challenge: Tic Tac Toe API
• move: given a board and position returns a
board with the current player at that position
– Can only be called on a board that is in-play; calling move on a
game board that is finished is a compile-time type error
• whoWon: given a finished board indicates if the
game was a draw or which player won
– Can only be called on a board that is finished; calling whoWon
on a game board that is in-play is a compile-time type error
• takeBack: takes either a finished board or a
board in-play that has had at least one move
and returns a board in-play
– It is a compile-time type error when used on an empty board
• playerAt: takes any board and position and
returns the (possible) player at the position
The main example for this talk is based on and inspired by:
https://github.com/tonymorris/course-answers/blob/master/src/TicTacToe/TicTacToe.md

5. But what’s this talk really about?
• Not really about solving/coding TicTacToe
• Looking at the Dynamic <-> Static typing
spectrum
• Looking at the OO <-> functional style
spectrum
• Looking at the pros and cons of different
programming styles

6. But what’s this talk really about?
• We all have the same goal
–Productively writing “bug-free”
maintainable code that “solves” some
real world users problem
–Types are one trick in my toolkit but I
also have tests, invariants
–Numerous ways to benefit from types
–Types are a burden on the programmer
–When writing DSLs, certain scripts,
certain test code, types may not justify
the burden

7. …DSL example...
©ASERT2006-2012
class FluentApi {
def action, what
def the(what) { this.what = what; this }
def of(arg) { action(what(arg)) }
}
show = { arg -> println arg }
square_root = { Math.sqrt(it) }
please = { new FluentApi(action: it) }
please show the square_root of 100 // => 10.0
DSL usage

8. …DSL example...
©ASERT2006-2012
please show the square_root of 100
please(show).the(square_root).of(100)
Inspiration for this example came from …

9. ...DSL example
©ASERT2006-2012
// Japanese DSL using GEP3 rules
Object.metaClass.を =
Object.metaClass.の =
{ clos -> clos(delegate) }
まず = { it }
表示する = { println it }
平方根 = { Math.sqrt(it) }
まず 100 の 平方根 を 表示する
// First, show the square root of 100
// => 10.0
source: http://d.hatena.ne.jp/uehaj/20100919/1284906117
also: http://groovyconsole.appspot.com/edit/241001

10. Topics
• Introduction
Dynamic solution
• Options for increasing type safety
• Groovy challenge solution
• Other language implementations
• Interlude: solving tic tac toe
• Going beyond the challenge
©ASERT2006-2013

11. Show me the code
DynamicTTT.groovy

12. Topics
• Introduction
• Dynamic solution
Options for increasing type safety
• Groovy challenge solution
• Other language implementations
• Interlude: solving tic tac toe
• Going beyond the challenge
©ASERT2006-2013

13. Java Typing limitations
long freezingC = 0 // 0 °C
long boilingF = 212 // 212 °F
long delta = boilingF - freezingC
long heavy = 100 // 100 kg

14. Using JScience
@GrabResolver('http://maven.obiba.org/maven2')
@Grab('org.jscience:jscience:4.3.1')
import ...
//@TypeChecked
def main() {
Amount<Temperature> freezingC = valueOf(0L, CELSIUS)
def boilingF = valueOf(212L, FAHRENHEIT)
printDifference(boilingF, freezingC)
def heavy = valueOf(100L, KILO(GRAM))
printDifference(heavy, boilingF)
}
def <T> void printDifference(Amount<T> arg1, Amount<T> arg2) {
println arg1 - arg2
}
(180.00000000000006 ± 1.4E-14) °F
javax.measure.converter.ConversionException: °F is
not compatible with kg

15. Using JScience & @TypeChecked
...
@TypeChecked
def main() {
Amount<Temperature> freezingC = valueOf(0L, CELSIUS)
def boilingF = valueOf(212L, FAHRENHEIT)
printDifference(boilingF, freezingC)
def heavy = valueOf(100L, KILO(GRAM))
printDifference(heavy, boilingF)
}
def <T> void printDifference(Amount<T> arg1, Amount<T> arg2) {
println arg1 - arg2
}
[Static type checking] - Cannot call
ConsoleScript46#printDifference(org.jscience.physics.amount.Amount
<T>, org.jscience.physics.amount.Amount <T>) with arguments
[org.jscience.physics.amount.Amount <javax.measure.quantity.Mass>,
org.jscience.physics.amount.Amount
<javax.measure.quantity.Temperature>]

16. Mars Orbiter (artists impression)

17. …Typing…
import groovy.transform.TypeChecked
import experimental.SprintfTypeCheckingVisitor
@TypeChecked(visitor=SprintfTypeCheckingVisitor)
void main() {
sprintf('%s will turn %d on %tF', 'John', new Date(), 21)
}
[Static type checking] - Parameter types didn't match types
expected from the format String:
For placeholder 2 [%d] expected 'int' but was 'java.util.Date'
For placeholder 3 [%tF] expected 'java.util.Date' but was 'int'
sprintf has an Object varargs
parameter, hence not normally
amenable to further static checking
but for constant Strings we can do
better using a custom type checking
plugin.

18. Show me the code
Type safe builder, phantom types, dependent types: Rocket, HList

19. ©ASERT2006-2012
GContracts
@Grab('org.gcontracts:gcontracts-core:1.2.10')
import org.gcontracts.annotations.*
@Invariant({ speed >= 0 })
class Rocket {
@Requires({ !started })
@Ensures({ started })
def start() { /* ... */ }
@Requires({ started })
@Ensures({ old.speed < speed })
def accelerate() { /* ... */ }
/* ... */
}
def r = new Rocket()
r.start()
r.accelerate()

20. Topics
• Introduction
• Dynamic solution
• Options for increasing type safety
Groovy challenge solution
• Other language implementations
• Interlude: solving tic tac toe
• Going beyond the challenge
©ASERT2006-2013

21. Show me the code
TicTacToe

22. Topics
• Introduction
• Dynamic solution
• Options for increasing type safety
• Groovy challenge solution
Other language implementations
• Interlude: solving tic tac toe
• Going beyond the challenge
©ASERT2006-2013

23. Show me the code
Haskell, Scala

24. Topics
• Introduction
• Dynamic solution
• Options for increasing type safety
• Groovy challenge solution
• Other language implementations
Interlude: solving tic tac toe
• Going beyond the challenge
©ASERT2006-2013

25. Interlude: Solving Tic Tac Toe
• Use a game tree to
map out all possible
moves
• Solve the game tree
using brute force or
with various
optimisation
algorithms

26. Interlude: Tic Tac Toe game tree
Source: http://en.wikipedia.org/wiki/Game_tree

27. Interlude: Tic Tac Toe game tree
• Brute force
• Minimax
– Reduced
lookahead, e.g.
2 layers/plies
• Alpha beta pruning
– Improved efficiency
• Iterative deepening
– Often used with
alpha beta pruning
• But for Tic Tac Toe only 100s of
end states and 10s of thousands
of paths to get there
Source: http://en.wikipedia.org/wiki/Game_tree

28. Topics
• Introduction
• Dynamic solution
• Options for increasing type safety
• Groovy challenge solution
• Other language implementations
• Interlude: solving tic tac toe
Going beyond the challenge
©ASERT2006-2013

29. Static Type Checking: Pluggable type system…
import groovy.transform.TypeChecked
import checker.BoringNameEliminator
@TypeChecked(visitor=BoringNameEliminator)
class Foo {
def method1() { 1 }
}
import groovy.transform.TypeChecked
import checker.BoringNameEliminator
@TypeChecked(visitor=BoringNameEliminator)
class Foo {
def method() { 1 }
}
[Static type checking] - Your method name is boring, I cannot allow it!
Groovy 2.1+

30. …Static Type Checking: Pluggable type system
package checker
import org.codehaus.groovy.ast.*
import org.codehaus.groovy.control.SourceUnit
import org.codehaus.groovy.transform.stc.*
class BoringNameEliminator extends StaticTypeCheckingVisitor {
BoringNameEliminator(SourceUnit source, ClassNode cn,
TypeCheckerPluginFactory pluginFactory) {
super(source, cn, pluginFactory)
}
final message = "Your method name is boring, I cannot allow it!"
@Override void visitMethod(MethodNode node) {
super.visitMethod(node)
if ("method".equals(node.name) || "bar".equals(node.name)) {
addStaticTypeError(message, node)
}
}
}
Groovy 2.1+

31. …Typing…
import groovy.transform.TypeChecked
import tictactoe.*
Import static tictactoe.Position.*
@TypeChecked(visitor=TicTacToeTypeVisitor)
void main() {
Board.empty().move(NW).move(C).move(W).move(SW).move(SE)
}
package tictactoe
enum Position {
NW, N, NE, W, C, E, SW, S, SE
}
class Board {
static Board empty() { new Board() }
Board move(Position p) { this }
}

32. …Typing…
package tictactoe
import fj.*
import fj.data.List
import fj.data.Option
import fj.data.TreeMap
import static fj.P.p
import static fj.data.List.list
import static fj.data.List.nil
import static fj.data.Option.none
import static tictactoe.GameResult.Draw
import static tictactoe.Player.Player1
import static tictactoe.Player.toSymbol
import static tictactoe.Position.*
final class Board extends BoardLike {
private final List<P2<Position, Player>> moves
private final TreeMap<Position, Player> m
private static final Ord<Position> positionOrder = Ord.comparableOrd()
private Board(final List<P2<Position, Player>> moves, final TreeMap<Position, Player> m) {
this.moves = moves
this.m = m
}
Player whoseTurn() { moves.head()._2().alternate() }
boolean isEmpty() { false }
List<Position> occupiedPositions() { m.keys() }
int nmoves() { m.size() }
Option<Player> playerAt(Position pos) { m.get(pos) }
TakenBack takeBack() {
moves.isEmpty() ?
TakenBack.isEmpty() :
TakenBack.isBoard(new Board(moves.tail(), m.delete(moves.head()._1())))
}
@SuppressWarnings("unchecked")
MoveResult moveTo(final Position pos) {
final Player wt = whoseTurn()
final Option<Player> j = m.get(pos)
final TreeMap<Position, Player> mm = m.set(pos, wt)
final Board bb = new Board(moves.cons(p(pos, wt)), mm)
final List<P3<Position, Position, Position>> wins =
list(
p(NW, W, SW), p(N, C, S), p(NE, E, SE), p(NW, N, NE),
p(W, C, E), p(SW, S, SE), p(NW, C, SE), p(SW, C, NE)
)
final boolean isWin = wins.exists(new F<P3<Position, Position, Position>, Boolean>() {
public Boolean f(final P3<Position, Position, Position> abc) {
return list(abc._1(), abc._2(), abc._3()).mapMOption(mm.get()).exists(new F<List<Player>, Boolean>() {
public Boolean f(final List<Player> ps) {
return ps.allEqual(Equal.<Player> anyEqual())
}
})
}
})
final boolean isDraw = Position.positions().forall(new F<Position, Boolean>() {
Boolean f(final Position pos2) {
mm.contains(pos2)
}
})
j.isSome() ?
MoveResult.positionAlreadyOccupied() :
isWin ?
MoveResult.gameOver(new FinishedBoard(bb, GameResult.win(wt))) :
isDraw ?
MoveResult.gameOver(new FinishedBoard(bb, Draw)) :
MoveResult.keepPlaying(bb)
}
// …
// …
@Override
String toString() {
toString(new F2<Option<Player>, Position, Character>() {
Character f(final Option<Player> pl, final Position _) {
pl.option(p(' '), toSymbol)
}
}) + "n[ " + whoseTurn().toString() + " to move ]"
}
static final class EmptyBoard extends BoardLike {
private EmptyBoard() {}
@SuppressWarnings("unchecked")
Board moveTo(final Position pos) {
new Board(list(p(pos, Player1)), TreeMap.<Position, Player> empty(positionOrder).set(pos, Player1))
}
private static final EmptyBoard e = new EmptyBoard()
static EmptyBoard empty() { e }
Player whoseTurn() { Player1 }
boolean isEmpty() { true }
List<Position> occupiedPositions() { nil() }
int nmoves() { 0 }
Option<Player> playerAt(Position pos) { none() }
}
static final class FinishedBoard extends BoardLike {
private final Board b
private final GameResult r
private FinishedBoard(final Board b, final GameResult r) {
this.b = b
this.r = r
}
Board takeBack() {
b.takeBack().fold(
Bottom.<Board> error_("Broken invariant: board in-play with empty move list. This is a program bug"),
Function.<Board> identity()
)
}
Player whoseTurn() { b.whoseTurn() }
boolean isEmpty() { false }
List<Position> occupiedPositions() { b.occupiedPositions() }
int nmoves() { b.nmoves() }
Option<Player> playerAt(final Position pos) { b.playerAt(pos) }
GameResult result() { r }
@Override
String toString() {
b.toString() + "n[[" + r.toString() + " ]]"
}
}
}

33. …Typing
import groovy.transform.TypeChecked
import tictactoe.*
Import static tictactoe.Position.*
@TypeChecked(visitor=TicTacToeTypeVisitor)
void main() {
Board.empty().move(NW).move(C).move(W).move(SW).move(SE)
}
package tictactoe
enum Position {
NW, N, NE, W, C, E, SW, S, SE
}
[Static type checking] - Attempt to call suboptimal
move SE not allowed [HINT: try NE]
Custom type checker which fails
compilation if programmer attempts
to code a suboptimal solution. Where
suboptimal means doesn’t agree with
what is returned by a minimax,
alpha-beta pruning, iterative
deepening solving engine.

34. Show me the code

35. More Information: Groovy in Action