It’s time for a new chapter in the Seven Languages in Seven Weeks series: today, I take a crack at Scala.
Scala, Day 1: Thoughts
After using Java for years, I was curious to try out Scala, which has often been described as the next step in the evolution of Java. Scala’s feature list is impressive: object oriented, functional, type inferencing, traits/mixins, currying, pattern matching, concise syntax, interoperability with Java code, an active community, and so on. My previous experiences with Scala had been very shallow/short, so I was excited to take a slightly deeper dive.
The first chapter introduced the imperative and object-oriented features of Scala and walked through the basic syntax. On the surface, the language looks like Java and uses many of the same keywords, so I was able to jump right in. However, I was quickly slowed down by some unexpected differences, including types specified after the variable name instead of before, “static” methods and fields separated into “companion classes” (confusingly named “objects”), and methods definitions sometimes including or omitting an equals sign and or parentheses depending on whether they return values or take parameters as inputs.
I slowed down even more once I started looking at the functional programming concepts and, worst of all, trying to make sense of the API docs. Although they look thorough, the docs are astonishingly bad when you actually try to use them. For example, here is all the documentation provided for the “reduce” method of Scala’s List:
def reduce [A1 >: A] (op: (A1, A1) ⇒ A1): A1
Reduces the elements of this sequence using the specified associative binary
operator.
The order in which operations are performed on elements is unspecified and may
be nondeterministic.
Note this method has a different signature than the reduceLeft and reduceRight
methods of the trait Traversable. The result of reducing may only be a
supertype of this parallel collection's type parameter T.If you’re new to Scala’s syntax, functional programming, or just a hacker trying to get something done, this sort of documentation is almost useless. Plain, human English or an example would be an order of magnitude more useful. The reduce concept isn’t actually that hard to understand, but parsing the dense syntax of the method signature and phrases like “associative binary operator” makes it seem like a PhD is necessary to use this language. Compare this to the reduce documentation for Ruby and underscore.js to see a world of a difference.
The type hierarchy also proved tough to navigate. For example, how do I find the closest common ancestor between List and MutableList? I thought it might be LinearSeq, but there seem to be separate mutable and immutable versions of it. Other classes/traits further up the hierarchy overlap, but are missing common methods I need, such as “collect” or “foldLeft”. Overall, this basic search was much harder than, for example, finding the common ancestor between Java libraries like ArrayList and Vector: a glance at the top of the API doc and you’re done.
I also ran into difficulties with type inferencing. It definitely saved me some typing and looked beautiful for simple cases and closure parameters. However, type inference couldn’t handle many cases that seemed obvious. This was compounded by the sub-par IDE support, at least from IntelliJ 11, which took a while to get working in the first place. I routinely found code that IntelliJ accepted wouldn’t actually compile. Oh, and the compiler is slow. Ridiculously slow, given the tiny snippets of code I was testing.
Having said that, I’m still a newbie to the language, and shouldn’t complain too much. I’m sure I’ll get used to the code, API, and Scala idioms. Still, there is value in being “hacker friendly”: one of the reasons Ruby, PHP, and JavaScript have such huge user bases is because you can get started with them in minutes. And there’s also something to say about complexity: Scala has a lot of features, syntax, and complicated concepts. I hope that these make the language more powerful and expressive rather than bloated and incomprehensible.
Scala, Day 1: Problems
Build a two player Tic Tac Toe Game
The code:
import collection.mutable.MutableList
object Marker extends Enumeration {
type Marker = Value
val EMPTY = Value("_")
val X = Value("x")
val O = Value("o")
}
import Marker._
class Board(boardSize: Int) {
val board = MutableList.fill(boardSize * boardSize)(EMPTY)
def spacer(value: String) = " " * (boardSize.toString.length() - value.length() + 2)
override def toString = printBoard ((marker, index) => marker.toString)
def printMoves() = printBoard ((marker, index) => if (marker == EMPTY) index.toString else EMPTY.toString)
def printBoard(printMarker: (Marker, Int) => String): String = board.zipWithIndex.foldLeft(""){(acc, marker) =>
val printedMarker = printMarker(marker._1, marker._2)
acc + (if (marker._2 > 0 && marker._2 % boardSize == 0) "\n" else "") + printedMarker + spacer(printedMarker)
}
def placeMarker(index: Int, marker: Marker): Boolean = {
if (index >= 0 && index < board.length && board(index) == EMPTY) {
board(index) = marker
return true
}
false
}
def allEqual(elements: MutableList[Marker]) = !elements.contains(EMPTY) && elements.distinct.size == 1
def anyListAllEqual(lists: TraversableOnce[MutableList[Marker]]): Boolean = lists.foldLeft(false)(_ || allEqual(_))
def anyListAllEqual(lists: MutableList[MutableList[Marker]]): Boolean = anyListAllEqual(lists.toIterator)
def rows() = board.grouped(boardSize)
def cols() = board.zipWithIndex.groupBy(_._2 % boardSize).map(_._2.map(_._1))
def diags() = MutableList(
board.zipWithIndex.filter(_._2 % (boardSize + 1) == 0).map(_._1),
board.zipWithIndex.filter(value => (value._2 % (boardSize - 1) == 0) && (value._2 > 0) && (value._2 < board.length - 1)).map(_._1)
)
def boardWon() = anyListAllEqual(rows()) || anyListAllEqual(cols()) || anyListAllEqual(diags())
def boardFull() = board.count(_ == EMPTY) == 0
}
val board = new Board(3)
var currentMove = O
while (!board.boardFull() && !board.boardWon()) {
currentMove = if (currentMove == X) O else X
println("\nThe board:\n")
println(board)
println("\nAvailable locations on the board:\n")
println(board.printMoves())
print("\nIt's " + currentMove + "'s turn! Enter a location: ")
val index = Console.readInt()
if (!board.placeMarker(index, currentMove)) {
println("Invalid location!")
}
}
println()
if (board.boardWon()) {
println("Player " + currentMove + " wins!")
} else {
println("It's a draw!")
}
println()
println(board)Sample output:
The board:
_ _ _
_ _ _
_ _ _
Available locations on the board:
0 1 2
3 4 5
6 7 8
It's x's turn! Enter a location: 4
The board:
_ _ _
_ x _
_ _ _
Available locations on the board:
0 1 2
3 _ 5
6 7 8
It's o's turn! Enter a location: 0
The board:
o _ _
_ x _
_ _ _
Available locations on the board:
_ 1 2
3 _ 5
6 7 8
It's x's turn! Enter a location: 6
The board:
o _ _
_ x _
x _ _
Available locations on the board:
_ 1 2
3 _ 5
_ 7 8
It's o's turn! Enter a location: 2
The board:
o _ o
_ x _
x _ _
Available locations on the board:
_ 1 _
3 _ 5
_ 7 8
It's x's turn! Enter a location: 1
The board:
o x o
_ x _
x _ _
Available locations on the board:
_ _ _
3 _ 5
_ 7 8
It's o's turn! Enter a location: 8
The board:
o x o
_ x _
x _ o
Available locations on the board:
_ _ _
3 _ 5
_ 7 _
It's x's turn! Enter a location: 5
The board:
o x o
_ x x
x _ o
Available locations on the board:
_ _ _
3 _ _
_ 7 _
It's o's turn! Enter a location: 3
The board:
o x o
o x x
x _ o
Available locations on the board:
_ _ _
_ _ _
_ 7 _
It's x's turn! Enter a location: 7
Player x wins!
o x o
o x x
x x o I tried to keep the code fairly generic, so it should work for any NxN tic tac toe board. I also used this as an opportunity to play with some functional programming, so I intentionally stuffed everything into a List (albeit a mutable one), avoided for loops, too many objects, and so on. To be honest, I’m not thrilled with the result.
I was able to use lots of one-liners, but many are hard to read. I got familiar with the fold, map, and filter methods, but in some cases, a for-loop would’ve been much cleaner (and faster). Overall, I just get the feeling that the code doesn’t communicate its intent very well. I’d love some feedback from how a more seasoned Scala user would’ve tackled this problem. Would pattern matching be useful? Recursive calls on the head/tail of the List? Or is the imperative style with loops and a 2D array the best way to go?
On to day 2
Continue on to Scala, Day 2.
Herman van der Veer
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