N-Queens Combinatorial Problem - Polyglot FP for Fun and Profit – Haskell and Scala - Part 2

Graham Hutton
@haskellhutt
Miran Lipovača
Paul Chiusano
Runar Bjarnason
@pchiusano
@runarorama
Gloss
Doodle
N-Queens Combinatorial Problem
Polyglot FP for Fun and Profit – Haskell and Scala
see how the guard function has migrated from MonadPlus to Alternative and learn something about the latter
learn how to write a Scala program that draws an N-Queens solution board using the Doodle graphics library
see how to write the equivalent Haskell program using the Gloss graphics library
learn how to use Monoid and Foldable to compose images both in Haskell and in Scala
Part 2
with excerpts from
@philip_schwarz
slides by https://www.slideshare.net/pjschwarz
Cats Effect

After I finished working on Part 1, I realised that since Miran Lipovača wrote his book, the definition of the
guard function has changed a bit.
For the purposes of this series, the behaviour of the function is unchanged, but it can be interesting to know
what did change.
If you are not interested, then skip the next 14 slides.
The next slide reminds us of how Miran Lipovača explained that the guard function is defined for
MonadPlus instances. In fact, due to some odd omission on my part, Part 1 says that the type of the guard
function is this
Bool -> m ()
whereas the correct type includes a constraint, i.e. it is this
(MonadPlus m) => Bool -> m ()
That omission is another good reason for having the next slide.
@philip_schwarz

MonadPlus and the guard Function
…
The MonadPlus type class is for monads that can also act as monoids. Here is its definition:
class Monad m => MonadPlus m where
mzero :: ma
mplus :: ma -> ma -> ma
mzero is synonymous with mempty from the Monoid type class, and mplus corresponds to mappend. Because lists are monoids as well as
monads, they can be made an instance of this type class:
instance MonadPlus [] where
mzero = []
mplus = (++)
For lists, mzero represents a nondeterministic computation that has no results at all – a failed computation. mplus joins two
nondeterministic values into one. The guard function is defined like this:
guard :: (MonadPlus m) => Bool -> m ()
guard True = return ()
guard False = mzero
guard takes a Boolean value. If that value is True, guard takes a () and puts it in a minimal default context that succeeds. If the
Boolean value is False, guard makes a failed monadic value. Here it is in action:
ghci> guard (5 > 2) :: Maybe ()
Just ()
ghci> guard (1 > 2) :: Maybe ()
Nothing
ghci> guard (5 > 2) :: [()]
[()]
ghci> guard (1 > 2) :: [()]
[]
Miran Lipovača

https://hackage.haskell.org/package/base-4.15.0.0/docs/Control-Monad.html
It looks like currently it is not MonadPlus instances that
provide a guard function, but Alternative instances.
See the next slide for the first part of Graham Hutton’s
introduction to the Alternative type class.

13.5 Making choices
…
Making a choice between two alternatives isn’t specific to parsers but can be generalised to a range of applicative types. This
concept is captured by the following class declaration in the library Control.Applicative:
class Applicative f => Alternative f where
empty :: f a
(<|>) :: f a -> f a -> f a
That is, for an applicative functor to be an instance of the Alternative class, it must support empty and <|> primitives of the
specified types. (The class also provides two further primitives, which will be discussed in the next section.) The intuition is that
empty represents an alternative that has failed, and <|> is an appropriate choice operator for the type. The two primitives are
also required to satisfy the following identity and associativity laws:
empty <|> x = x
x <|> empty = x
x <|> (y <|> z) = (x <|> y) <|> z
The motivating example of an Alternative type is the Maybe type, for which empty is given by the failure value Nothing, and
<|> returns its first argument if this succeeds, and its second argument otherwise:
instance Alternative Maybe where
-- empty :: Maybe a
empty = Nothing
-- (<|>) :: Maybe a -> Maybe a -> Maybe a
Nothing <|> my = my
(Just x) <|> _ = Just x
Graham Hutton
@haskellhutt

instance Alternative [] where
-- empty :: [a]
empty = []
-- (<|>) :: [a] -> [a] -> [a]
(<|>) = (++)
instance Alternative Maybe where
-- empty :: Maybe a
empty = Nothing
-- (<|>) :: Maybe a -> Maybe a -> Maybe a
Nothing <|> my = my
(Just x) <|> _ = Just x
Haskell> [1,2,3] <|> [4,5,6]
[1,2,3,4,5,6]
Haskell> [] <|> [4,5,6]
[4,5,6]
Haskell> [1,2,3] <|> []
[1,2,3]
Haskell> [] <|> []
[]
Haskell> empty <|> [4,5,6]
[4,5,6]
Haskell> [1,2,3] <|> empty
[1,2,3]
Haskell> (empty::[Int]) <|> empty
[]
Haskell> Just 3 <|> Just 4
Just 3
Haskell> Nothing <|> Just 4
Just 4
Haskell> Just 3 <|> Nothing
Just 3
Haskell> Nothing <|> Nothing
Nothing
Haskell> empty <|> Just 4
Just 4
Haskell> Just 3 <|> empty
Just 3
Haskell> (empty::Maybe Int) <|> empty
Nothing
Another instance of Alternative is the list. Here are examples of
using both the Maybe Alternative and the list Alternative.

We have seen how Haskell’s Alternative is an
Applicative that supports empty and <|>.
What about in Scala?
In the Cats library, Alternative’s empty function is
provided by MonoidK, whose supertrait SemigroupK
provides combineK, which is the equivalent of Haskell’s
<|>, but whose operator alias is called <+>.
class Applicative f => Alternative f where
empty :: f a
(<|>) :: f a -> f a -> f a
https://typelevel.org/cats/typeclasses/alternative.html
See next slide for usage
examples of the Alternative
instances for List and Option.

scala> val alt = Alternative[List]
val alt: cats.Alternative[List] = …
scala> alt.combineK(alt.empty[Int], List(4,5,6))
val res0: List[Int] = List(4, 5, 6)
scala> alt.combineK(List(1,2,3), List(4,5,6))
val res1: List[Int] = List(1, 2, 3, 4, 5, 6)
scala> alt.combineK(List(1,2,3), List())
scala> val alt = Alternative[Option]
val alt: cats.Alternative[Option] = …
scala> alt.combineK(alt.empty[Int], 4.some)
val res0: Option[Int] = Some(4)
scala> alt.combineK(3.some, 4.some)
scala> alt.combineK(3.some, None)
scala> List() <+> List(4,5,6)
scala> List(1,2,3) <+> List(4,5,6)
val res1: List[Int] = List(1, 2, 3, 4, 5, 6)
scala> List(1,2,3) <+> List()
scala> none <+> 3.some
scala> 3.some <+> 4.some
scala> 3.some <+> None
Haskell> empty <|> [4,5,6]
[4,5,6]
Haskell> [] <|> [4,5,6]
[4,5,6]
Haskell> [1,2,3] <|> [4,5,6]
[1,2,3,4,5,6]
Haskell> [1,2,3] <|> []
[1,2,3]
Haskell> empty <|> Just 3
Just 3
Haskell> Nothing <|> Just 3
Just 3
Haskell> Just 3 <|> Just 4
Just 3
Haskell> Just 3 <|> Nothing
Just 3

If you are asking yourself what’s with the K in MonoidK, SemigroupK
and combineK, see the first two slides of the following slide deck.
@philip_schwarz

It turns out that the Cats Alternative provides a guard function!
On the next slide we take it for a ride.

[[1,2],[3,4]] >>= nums ->
[10,20] >>= num ->
guard (num > 15) >>
return (num:nums)
[[1,2],[3,4]] >>= nums ->
[10,20] >>= num ->
guard (True) >>
return (num:nums)
[[1,2],[3,4]] >>= nums ->
[10,20] >>= num ->
guard (False) >>
return (num:nums)
guard lets
through some
elements
guard lets
through all
elements
guard lets
through no
elements
List(List(20,1,2),List(20,3,4))
List(List(1,2),List(3,4)).flatMap { nums =>
List(10, 20).flatMap { num =>
alt.guard(true) >>
alt.pure(num::nums)}}
alt.guard(false) >>
alt.guard(num > 15) >>
List(List(10,1,2),List(20,1,2),
List(10,3,4),List(20,3,4))
List()
import cats.Alternative
import cats.implicits
val alt = Alternative[List]
[[10,1,2],[20,1,2],[10,3,4],[20,3,4]]
[]
[[20,1,2],[20,3,4]]
failed non-deterministic computation

In the next five slides we mention the IO monad. If you are not familiar with it, feel free to skip the slides.
If you are interested in an introduction to the IO monad then see the following slide decks

https://stackoverflow.com/questions/48450826/what-is-the-purpose-of-instance-alternative-io
We have seen Alternative instances for Maybe/Option and for lists.
Out of curiosity, is there an Alternative instance for IO?
On the one hand, it looks like it:
On the next slide we have a look at an
example of using the Alternative for IO.
@philip_schwarz

On the LHS of Alternative’s <|> operator we have a program that first asks the user to enter their name, and then tells them what their name is.
On the RHS of <|> we have a second program that we only want to execute if the first program encounters an error.
We are using <|> to ensure that either the first program executes successfully, or it encounters an error, in which case the second program is
then executed.
See below for how the behaviour of the overall program changes depending on which part of the first program fails, if any.
(putStr "Enter Your Name:" >> getLine >>= name -> putStrLn("Your Name is:" ++ name))
<|>
putStrLn("An IO Error Occurred!") Enter Your Name:Fred
Your Name is:Fred
(empty >> getLine >>= name -> putStrLn("Your Name is:" ++ name))
<|>
putStrLn("An IO Error Occurred!")
An IO Error Occurred!
(putStr "Enter Your Name:" >> empty >>= name -> putStrLn("Your Name is:" ++ name))
<|>
putStrLn("An IO Error Occurred!")
Enter Your Name:An IO Error Occurred!
(putStr "Enter Your Name:" >> getLine >>= name -> empty)
<|>
putStrLn("An IO Error Occurred!") Enter Your Name:Fred

But then on the other hand, it looks like the
Alternative instance for IO is unlawful and/or broken.

What about in Cats?
Is there an Alternative instance for IO?
I could not find one.
It looks like there was an attempt to
introduce an instance for some form of
IO, and for fibers, but in the end only
the proposal for fibers succeeded.

import cats.effect.IO
import cats.implicits._
def getLine(): IO[String] = IO{ scala.io.StdIn.readLine }
def putStr(s: String): IO[Unit] = IO{ print(s) }
def putStrLn(s: String): IO[Unit] = IO{ println(s) }
def empty: IO[Unit] = IO.raiseError(RuntimeException("bang"))
putStr("Enter Your Name:") >> getLine().flatMap{ name => putStrLn(s"Your Name is $name") }
<+>
putStrLn("An IO Error occurred!")
empty >> getLine().flatMap{ name => putStrLn(s"Your Name is $name") }
<+>
putStr("Enter Your Name:") >> empty.flatMap{ name => putStrLn(s"Your Name is $name") }
<+>
putStr("Enter Your Name:") >> getLine().flatMap{ name => empty }
<+>
Enter Your Name:Fred
Your Name is:Fred
Enter Your Name:An IO Error Occurred!
Enter Your Name:Fred
FWIW though, using Cats I
was able to reproduce the
same behaviour as in Haskell.
@philip_schwarz

Now that we have seen how the guard function has changed
since Miran Lipovača wrote his book, let’s turn to the task of
displaying a solution to the N-Queens problem using graphics.

In the last slide of Part 1 we saw a show
function for turning a solution into a string.
def show(queens: List[Int]): String =
val lines: List[String] =
for (col <- queens.reverse)
yield Vector.fill(queens.length)("* ")
.updated(col, "X ")
.mkString
"n" + (lines mkString "n")
@main def main =
val solution = List(3, 1, 6, 2, 5, 7, 4, 0)
showQueens(solution)
def showQueens(solution: List[Int]): Unit =
println(show(solution))
X * * * * * * *
* * * * X * * *
* * * * * * * X
* * * * * X * *
* * X * * * * *
* * * * * * X *
* X * * * * * *
* * * X * * * *
If, after printing a solution, we change the
font to Courier, then the resulting board
even takes on a square shape.
Here we wire the function up in a small program.

Principles
A few principles guide the design of Doodle, and differentiate it from other graphics libraries. The section explains these principles.
Pictures are Created by Composition
In Doodle a picture is constructed by combining together smaller pictures. For example, we can create a row by putting pictures beside each other. This
idea of creating complex things from simpler things is known as composition.
There are several implications of this, which means that Doodle operates differently to many other graphics libraries. This first is that Doodle does not draw
anything on the screen until you explicitly ask it to, say by calling the draw method. A picture represents a description of something we want to draw. A
backend turns this description into something we can see (which might be on the screen or in a file). This separation of description and action is known as
the interpreter pattern. The description is a “program” and a backend is an “interpreter” that runs that program. In the graphics world the approach that
Doodle takes is sometimes known as retained mode, while the approach of drawing immediately to the screen is known as immediate mode.
Another implication is that Doodle can allow relative layout of objects. In Doodle we can say that one picture is next to another and Doodle will work out
where on the screen they should be. This requires a retained mode API as you need to keep around information about a picture to work out how much space
it takes up.
A final implication is that pictures have no mutable state. This is needed for composition so you can, for example, put a picture next to itself and have things
render correctly.
All of these ideas are core to functional programming, so you may have seen them in other contexts if you have experienced with functional programming. If
not, don’t worry. You’ll quickly understand them once you start using Doodle, as Doodle makes the ideas very concrete.
We are going to draw a solution board using the Doodle library. Doodle, adopts the concepts of
picture composition and of the separation between, on the one hand, creating a picture, a
description of something to be drawn, and on the other hand, doing the actual drawing, by
processing/interpreting the picture.
https://www.creativescala.org/doodle/
https://github.com/creativescala/doodle
Doodle: Compositional Vector Graphics

Image is based on composition and the interpreter pattern.
Composition basically means that we build big Images out of small Images. For example, if we have an Image
describing a red square and an Image describing a blue square
val redSquare = Image.square(100).fillColor(Color.red)
val blueSquare = Image.square(100).fillColor(Color.blue)
we can create an Image describing a red square next to a blue square by combining them together.
val combination = redSquare.beside(blueSquare)
The Image library is the easiest way to create images using Doodle. The tradeoff the Image library makes is
that it only support a (large but limited) subset of operations that are supported across all the backends.
The interpreter pattern means that we separate describing the Image from rendering it. Writing
Image.square(100)
doesn’t draw anything. To draw an image we need to call the draw() method. This separation is important for
composition; if we were to immediately draw we would lose composition.
…
Image.square(sideLength) creates a square with the given side length.
Just like we can use the beside function to
create an image describing a red square next
to a blue square, we can use the above
function to create an image describing a red
square above a blue square.
Are you thinking what I am thinking? Yes, this
should simplify things: we can can create a
row of squares by combining the squares
using the beside function, and we can create
a board by combining rows using the above
function.
Doodle: Compositional Vector Graphics
red beside blue red above blue

@main def main =
println(show(solution))
val n = solution.length
val frameTitle = s"{n}-Queens Problem – A solution"
val frameWidth = 1000
val frameHeight = 1000
val frameBackgroundColour = Color.white
val frame =
Frame.size(frameWidth,frameHeight)
.title(frameTitle)
.background(frameBackgroundColour)
show(solution).draw
@main def main =
def show(queens: List[Int]): Image =
val square = Image.square(100).strokeColor(Color.black)
val emptySquare: Image = square.fillColor(Color.white)
val fullSquare: Image = square.fillColor(Color.orangeRed)
val squareImageGrid: List[List[Image]] =
for col <- queens.reverse
yield List.fill(queens.length)(emptySquare)
.updated(col,fullSquare)
combine(squareImageGrid)
def show(queens: List[Int]): String =
val lines: List[String] =
for (col <- queens.reverse)
yield Vector.fill(queens.length)("* ")
.updated(col, "X ")
.mkString
"n" + (lines mkString "n")
Here again is the program that prints a
solution board to the console, as text.
X * * * * * * *
* * * * X * * *
* * * * * * * X
* * * * * X * *
* * X * * * * *
* * * * * * X *
* X * * * * * *
* * * X * * * *
And here is a new program that uses
Doodle to draw a solution board.
def combine(imageGrid: List[List[Image]): Image =
imageGrid.map(_.reduce(_ beside _))
.reduce(_ above _)
As suggested on the previous slide, the combine function on the right
• creates the image of a row of squares by combining the images of the
squares using the beside function
• creates the image of a grid of squares by combining the images of the
rows using the above function
Martin
Odersky

List(2, 4, 1, 7, 5, 3, 6, 0)
List(4, 1, 3, 6, 2, 7, 5, 0)
List(3, 1, 6, 2, 5, 7, 4, 0)
The boards drawn by our Scala program for the
first three solutions of the 8-Queens Problem.
@philip_schwarz

The next thing we are going to do is improve a bit the implementation of the combine function used by our program.
To do that, we are going to use the concepts of Monoid and Foldable. If you are new to them then see the next fifteen slides for
a minimal introduction to the concepts, otherwise just skip the slides, which are partly based on the slide decks below.

What is a monoid?
Let’s consider the algebra of string concatenation. We can add "foo" +
"bar" to get "foobar", and the empty string is an identity element for
that operation. That is, if we say (s + "") or ("" + s), the result is
always s.
scala> val s = "foo" + "bar"
s: String = foobar
scala> assert( s == s + "" )
scala> assert( s == "" + s )
scala>
scala> val (r,s,t) = ("foo","bar","baz")
r: String = foo
s: String = bar
t: String = baz
scala> assert( ( ( r + s ) + t ) == ( r + ( s + t ) ) )
scala> assert( ( ( r + s ) + t ) == "foobarbaz" )
scala>
Furthermore, if we combine three strings by saying (r + s + t), the
operation is associative —it doesn’t matter whether we parenthesize it: ((r
+ s) + t) or (r + (s + t)).
The exact same rules govern integer addition. It’s associative, since (x +
y) + z is always equal to x + (y + z)
scala> val (x,y,z) = (1,2,3)
x: Int = 1
y: Int = 2
z: Int = 3
scala> assert( ( ( x + y ) + z ) == ( x + ( y + z ) ) )
scala> assert( ( ( x + y ) + z ) == 6 )
scala>
and it has an identity element, 0 , which “does nothing” when added to
another integer
scala> val s = 3
s: Int = 3
scala> assert( s == s + 0)
scala> assert( s == 0 + s)
scala>

Ditto for integer multiplication
scala> val s = 3
s: Int = 3
scala> assert( s == s * 1)
scala> assert( s == 1 * s)
scala>
scala> val (x,y,z) = (2,3,4)
x: Int = 2
y: Int = 3
z: Int = 4
scala> assert(( ( x * y ) * z ) == ( x * ( y * z ) ))
scala> assert(( ( x * y ) * z ) == 24)
scala>
whose identity element is 1
The Boolean operators && and || are likewise associative
and they have identity elements true and false, respectively
scala> val (p,q,r) = (true,false,true)
p: Boolean = true
q: Boolean = false
r: Boolean = true
scala> assert(( ( p || q ) || r ) == ( p || ( q || r ) ))
scala> assert(( ( p || q ) || r ) == true )
scala> assert(( ( p && q ) && r ) == ( p && ( q && r ) ))
scala> assert(( ( p && q ) && r ) == false )
scala> val s = true
s: Boolean = true
scala> assert( s == ( s && true ) )
scala> assert( s == ( true && s ) )
scala> assert( s == ( s || false ) )
scala> assert( s == ( false || s ) )

These are just a few simple examples, but algebras like this are virtually everywhere. The term for this kind of algebra is monoid.
The laws of associativity and identity are collectively called the monoid laws.
A monoid consists of the following:
• Some type A
• An associative binary operation, op, that takes two values of type A and combines them into one: op(op(x,y), z) == op(x, op(y,z)) for any
choice of x: A, y: A, z: A
• A value, zero: A, that is an identity for that operation: op(x, zero) == x and op(zero, x) == x for any x: A
trait Monoid[A] {
def op(a1: A, a2: A): A
def zero: A
}
val stringMonoid = new Monoid[String] {
def op(a1: String, a2: String) = a1 + a2
val zero = ""
}
An example instance of this trait is the String monoid:
def listMonoid[A] = new Monoid[List[A]] {
def op(a1: List[A], a2: List[A]) = a1 ++ a2
val zero = Nil
}
List concatenation also forms a monoid:
Functional Programming in Scala
(by Paul Chiusano and Runar Bjarnason)
@pchiusano @runarorama
We can express this with a Scala trait:
List function returning a new list containing the elements from the left
hand operand followed by the elements from the right hand operand
String concatenation function

Monoid instances for integer addition and multiplication as well as the Boolean operators
implicit val intAdditionMonoid = new Monoid[Int] {
def op(x: Int, y: Int) = x + y
val zero = 0
}
implicit val intMultiplicationMonoid = new Monoid[Int] {
def op(x: Int, y: Int) = x * y
val zero = 1
}
Just what is a monoid, then? It’s simply a type A and an implementation of Monoid[A] that satisfies the laws.
Stated tersely, a monoid is a type together with a binary operation (op) over that type, satisfying associativity and having an
identity element (zero).
What does this buy us? Just like any abstraction, a monoid is useful to the extent that we can write useful generic code assuming
only the capabilities provided by the abstraction. Can we write any interesting programs, knowing nothing about a type other
than that it forms a monoid? Absolutely!
implicit val booleanOr = new Monoid[Boolean] {
def op(x: Boolean, y: Boolean) = x || y
val zero = false
}
implicit val booleanAnd = new Monoid[Boolean] {
def op(x: Boolean, y: Boolean) = x && y
val zero = true
}
(by Runar Bjarnason)
@runarorama

trait Monoid[A] {
def zero: A
}
trait Semigroup[F] { self =>
def append(f1: F, f2: => F): F
…
}
final class SemigroupOps[F]…(implicit val F: Semigroup[F]) … {
final def |+|(other: => F): F = F. append (self, other)
final def mappend(other: => F): F = F. append (self, other)
final def ⊹(other: => F): F = F. append (self, other)
…
}
trait Monoid[F] extends Semigroup[F] { self =>
def zero: F
…
}
FP in Scala
class Semigroup m where
(<>) :: m -> m -> m
class Semigroup m => Monoid m where
mempty :: m
mappend :: m -> m -> m
mconcat :: [m] -> m
mconcat = foldr mappend mempty
trait Semigroup[A] {
def combine(x: A, y: A): A
…
}
final class SemigroupOps[A: Semigroup](lhs: A) {
def |+|(rhs: A): A = macro Ops.binop[A, A]
def combine(rhs: A): A = macro Ops.binop[A, A]
def combineN(rhs: Int): A = macro Ops.binop[A, A]
}
trait Monoid[A] extends Semigroup[A] {
def empty: A
…
}
The mappend method is redundant and has
the default implementation mappend = '(<>)'
Summary of the naming and location of a Monoid’s associative binary operation and identity element - simplified

def sum(ints: List[Int]): Int =
ints match {
case Nil => 0
case Cons(x, xs) => x + sum(xs)
}
def foldRight[A,B](as: List[A], z: B)(f: (A, B) => B): B =
as match {
case Nil => z
case Cons(x, xs) => f(x, foldRight(xs, z)(f))
}
def sum(ns: List[Int]) =
foldRight(ns, 0)((x,y) => x + y)
def product(ns: List[Double]) =
foldRight(ns, 1.0)(_ * _)
sealed trait List[+A]
case object Nil extends List[Nothing]
case class Cons[+A](head: A, tail: List[A]) extends List[A]
Note how similar these two definitions are. They’re operating on different types (List[Int]versus List[Double]), but aside from this, the only differences are the value to return
in the case that the list is empty (0 in the case of sum, 1.0 in the case of product), and the operation to combine results (+ in the case of sum, * in the case of product).
Whenever you encounter duplication like this, you can generalize it away by pulling subexpressions out into function arguments…
Let’s do that now. Our function will take as arguments the value to return in the case of the empty list, and the function to add an element to the result in the case of a nonempty list.
def foldRightViaFoldLeft[A,B](l: List[A], z: B)(f: (A,B) => B): B =
foldLeft(reverse(l), z)((b,a) => f(a,b))
foldRight is not specific to any one type of
element, and we discover while generalizing
that the value that’s returned doesn’t have to
be of the same type as the elements of the list!
@annotation.tailrec
def foldLeft[A,B](l: List[A], z: B)(f: (B, A) => B): B = l match {
case Nil => z
case Cons(h,t) => foldLeft(t, f(z,h))(f)
}
def product(ds: List[Double]): Double =
ds match {
case Nil => 1.0
case Cons(x, xs) => x * product(xs)
}
scala> sum(Cons(1,Cons(2,Cons(3,Nil))))
res0: Int = 6
scala> product(Cons(1.0,Cons(2.5,Cons(3.0,Nil))))
res1: Double = 7.5
scala>
Implementing foldRight via foldLeft is useful
because it lets us implement foldRight tail-
recursively, which means it works even for large lists
without overflowing the stack.
Our implementation of foldRight is not tail-recursive
and will result in a StackOverflowError for large
lists (we say it’s not stack-safe). Convince yourself that
this is the case, and then write another general list-
recursion function, foldLeft, that is tail-recursive
foldRight(Cons(1, Cons(2, Cons(3, Nil))), 0)((x,y) => x + y)
1 + foldRight(Cons(2, Cons(3, Nil)), 0)((x,y) => x + y)
1 + (2 + foldRight(Cons(3, Nil), 0)((x,y) => x + y))
1 + (2 + (3 + (foldRight(Nil, 0)((x,y) => x + y))))
1 + (2 + (3 + (0)))
6
Folding Right and Left

footnotes
9 In the Scala standard library, foldRight is a method on List and its arguments are curried similarly for better type inference.
10 Again, foldLeft is defined as a method of List in the Scala standard library, and it is curried similarly for better type inference, so you can write
mylist.foldLeft(0.0)(_ + _).
FP in Scala
assert( List(1,2,3,4).foldLeft(0)(_+_) == 10 )
assert( List(1,2,3,4).foldRight(0)(_+_) == 10 )
assert( List(1,2,3,4).foldLeft(1)(_*_) == 24 )
assert( List(1,2,3,4).foldRight(1)(_*_) == 24 )
assert( List("a","b","c","d").foldLeft("")(_+_) == "abcd" )
assert( List("a","b","c","d").foldRight("")(_+_) == "abcd" )

Foldable data structures
In chapter 3, we implemented the data structures List and Tree, both of which could be folded. In chapter 5, we wrote
Stream, a lazy structure that also can be folded much like a List can, and now we’ve just written a fold for IndexedSeq.
When we’re writing code that needs to process data contained in one of these structures, we often don’t care about the shape
of the structure (whether it’s a tree or a list), or whether it’s lazy or not, or provides efficient random access, and so forth.
For example, if we have a structure full of integers and want to calculate their sum, we can use foldRight:
ints.foldRight(0)(_ + _)
Looking at just this code snippet, we shouldn’t have to care about the type of ints. It could be a Vector, a Stream, or a
List, or anything at all with a foldRight method. We can capture this commonality in a trait:
trait Foldable[F[_]] {
def foldRight[A,B](as: F[A])(z: B)(f: (A,B) => B): B
def foldLeft[A,B](as: F[A])(z: B)(f: (B,A) => B): B
def foldMap[A,B](as: F[A])(f: A => B)(mb: Monoid[B]): B
def concatenate[A](as: F[A])(m: Monoid[A]): A =
foldLeft(as)(m.zero)(m.op)
}
Here we’re abstracting over a type constructor F, much like we did with the
Parser type in the previous chapter. We write it as F[_], where the
underscore indicates that F is not a type but a type constructor that takes
one type argument. Just like functions that take other functions as arguments
are called higher-order functions, something like Foldable is a higher-
order type constructor or a higher-kinded type .7
7 Just like values and functions have types, types and type constructors
have kinds. Scala uses kinds to track how many type arguments a type
constructor takes, whether it’s co- or contravariant in those arguments, and
what the kinds of those arguments are.

Don’t spend too much time comparing the Scala and Haskell implementations
of the various folding functions on the next three slides.
There are many different ways of implementing the functions.
While looking at some of the implementations can reinforce our understanding
of the functions, such comparisons are not relevant for our current purposes.

EXERCISE 10.12
Implement Foldable[List], Foldable[IndexedSeq], and Foldable[Stream]. Remember that foldRight, foldLeft,
and foldMap can all be implemented in terms of each other, but that might not be the most efficient implementation.
A Companion booklet to
FP in Scala
FP in Scala
def foldRight[A, B](as: F[A])(z: B)(f: (A, B) => B): B =
foldMap(as)(f.curried)(endoMonoid[B])(z)
def foldLeft[A, B](as: F[A])(z: B)(f: (B, A) => B): B =
foldMap(as)(a => (b: B) => f(b, a))(dual(endoMonoid[B]))(z)
def foldMap[A, B](as: F[A])(f: A => B)(mb: Monoid[B]): B =
foldRight(as)(mb.zero)((a, b) => mb.op(f(a), b))
}
object ListFoldable extends Foldable[List] {
override def foldRight[A, B](as:List[A])(z:B)(f:(A,B)=>B) =
as.foldRight(z)(f)
override def foldLeft[A, B](as:List[A])(z:B)(f:(B,A)=>B) =
as.foldLeft(z)(f)
override def foldMap[A, B](as:List[A])(f:A=>B)(mb:Monoid[B]):B =
foldLeft(as)(mb.zero)((b, a) => mb.op(b, f(a)))
}
object IndexedSeqFoldable extends Foldable[IndexedSeq] {…}
object StreamFoldable extends Foldable[Stream] {
override def foldRight[A, B](as:Stream[A])(z:B)(f:(A,B)=>B) =
as.foldRight(z)(f)
override def foldLeft[A, B](as:Stream[A])(z:B)(f:(B,A)=>B) =
as.foldLeft(z)(f)
}
assert( ListFoldable.foldLeft(List(1,2,3))(0)(_+_) == 6)
assert( ListFoldable.foldRight(List(1,2,3))(0)(_+_) == 6)
assert( ListFoldable.concatenate(List(1,2,3))(intAdditionMonoid) == 6)
assert( ListFoldable.foldMap(List("1","2","3"))(_ toInt)(intAdditionMonoid) == 6)
assert( StreamFoldable.foldLeft(Stream(1,2,3))(0)(_+_) == 6)
assert( StreamFoldable.foldRight(Stream(1,2,3))(0)(_+_) == 6)
assert( StreamFoldable.concatenate(Stream(1,2,3))(intAdditionMonoid) == 6)
assert( StreamFoldable.foldMap(Stream("1","2","3"))(_ toInt)(intAdditionMonoid) == 6)
assert( ListFoldable.foldLeft(List("a","b","c"))("")(_+_) == "abc")
assert( ListFoldable.foldRight(List("a","b","c"))("")(_+_) == "abc")
assert( ListFoldable.concatenate(List("a","b","c"))(stringMonoid) == "abc")
assert( ListFoldable.foldMap(List(1,2,3))(_ toString)(stringMonoid) == "123")
assert( StreamFoldable.foldLeft(Stream("a","b","c"))("")(_+_)) == "abc")
assert( StreamFoldable.foldRight(Stream("a","b","c"))("")(_+_) == "abc")
assert( StreamFoldable.concatenate(Stream("a","b","c"))(stringMonoid) == "abc")
assert( StreamFoldable.foldMap(Stream(1,2,3))(_ toString)(stringMonoid) == "123")
Let’s use the methods of ListFoldable
and StreamFoldable to fold
Lists/Streams of Ints and Strings.
If you are new to monoids, don’t worry about the
implementation of foldRight and foldLeft except for the
fact that it is possible to define them using foldMap.

FPiS def foldMap[A, B](as: F[A])(f: A => B)(mb: Monoid[B]): B =
Scalaz def foldMap[A,B](fa: F[A])(f: A => B)(implicit F: Monoid[B]): B
Cats def foldMap[A, B](fa: F[A])(f: A => B)(implicit B: Monoid[B]): B =
foldLeft(fa, B.empty)((b, a) => B.combine(b, f(a)))
FPiS def foldRight[A, B](as: F[A])(z: B)(f: (A, B) => B): B =
Scalaz def foldRight[A, B](fa: F[A], z: => B)(f: (A, => B) => B): B
Cats def foldRight[A, B](fa: F[A], lb: Eval[B])(f: (A, Eval[B]) => Eval[B]): Eval[B]
FPiS def foldLeft[A, B](as: F[A])(z: B)(f: (B, A) => B): B =
Scalaz def foldLeft[A, B](fa: F[A], z: B)(f: (B, A) => B): B = {
import Dual._, Endo._, syntax.std.all._
Tag.unwrap(foldMap(fa)((a: A) =>
Dual(Endo.endo(f.flip.curried(a))))(dualMonoid)) apply (z)
}
Cats def foldLeft[A, B](fa: F[A], b: B)(f: (B, A) => B): B
FPiS def concatenate[A](as: F[A])(m: Monoid[A]): A =
Scalaz def fold[M:Monoid](t:F[M]):M = def sumr[A](fa:F[A])(implicit A:Monoid[A]):A = def suml[A](fa:F[A])(implicit A: Monoid[A]): A =
foldMap[M, M](t)(x => x) foldRight(fa, A.zero)(A.append) foldLeft(fa, A.zero)(A.append(_, _))
Cats def fold[A](fa: F[A])(implicit A: Monoid[A]): A = def combineAll[A: Monoid](fa: F[A]): A =
foldLeft(fa, A.empty) { (acc, a) => A.combine(acc, a) } fold(fa)
fold
foldMap
foldRight
foldLeft
The four fundamental functions of the Foldable trait in FPiS, Scalaz and Cats
concatenate fold,suml,sumr fold,combineAll
foldMap foldMap foldMap
foldLeft foldLeft foldLeft
foldRight foldRight foldRight
If you are new to monoids, don’t worry about
endoMonoid and the dual function, they are
not important in our context.

object ListFoldable extends Foldable[List] {
override def foldMap[A, B](as:List[A])(f:A=>B)(mb:Monoid[B]):B =
foldLeft(as)(mb.zero)((b, a) => mb.op(b, f(a)))
override def foldRight[A, B](as:List[A])(z:B)(f:(A,B)=>B) =
as match {
case Nil => z
case Cons(h, t) => f(h, foldRight(t, z)(f))
}
override def foldLeft[A, B](as:List[A])(z:B)(f:(B,A)=>B) =
as match {
case Nil => z
case Cons(h, t) => foldLeft(t, f(z,h))(f)
}
}
def foldMap[A, B](as: F[A])(f: A => B)(mb: Monoid[B]): B =
def foldRight[A, B](as: F[A])(z: B)(f: (A, B) => B): B =
def foldLeft[A, B](as: F[A])(z: B)(f: (B, A) => B): B =
def toList[A](as: F[A]): List[A] =
foldRight(as)(List[A]())(_ :: _)
}
class Foldable t where
foldMap :: Monoid b => (a -> b) -> t a -> b
foldr :: (a -> b -> b) -> b -> t a -> b
fold :: Monoid a => t a -> a
foldl :: (a -> b -> a) -> a -> t b -> a
toList :: t a -> [a]
…
trait Monoid[A] {
def zero: A
}
Default Foldable Definitions
foldMap f = foldr (mappend.f) mempty
foldr f v = foldr f v . toList
fold = foldMap id
foldl f v = foldl f v . toList
toList = foldMap (x -> [x])
…
instance Foldable [] where
-- foldMap :: Monoid b => (a -> b) -> [a] -> b
foldMap _ [] = mempty
foldMap f (x:xs) = f x ‘mappend’ foldMap f xs
-- foldr :: (a -> b -> b) -> b -> [a] -> b
foldr _ v [] = v
foldr f v (x:xs) = f x (foldr f v xs)
-- fold :: Monoid a => t a -> a
fold = foldMap id
-- foldl :: (a -> b -> a) -> a -> t b -> a
foldl _ v [] = v
foldl f v (x:xs) = foldl f (f v x) xs
toList :: t a -> [a]
toList = id
…
instance Monoid [a] where
-- mempty :: [a]
mempty = []
-- mappend :: [a] -> [a] -> [a]
mappend = (++)
class Monoid a where
mempty :: a
mappend :: a -> a -> a
mconcat :: [a] -> a
mconcat = foldr mappend mempty

Graham Hutton
@haskellhutt
The next two slides contain
some examples of using fold
and foldMap.
This slide is only here to help
us understand upcoming
examples using the Haskell
Monoids for addition and
multiplication, so feel free to
skip this slide, at least on a
first reading.

(String, +, “”) Monoid
Scala List("a","b","c").combineAll == "abc"
Haskell fold ["a","b","c"] == "abc”
(Integer, +, 0) Monoid
Scala List(1,2,3).combineAll == 6
Haskell getSum (fold (fmap Sum [1,2,3])) == 6
(Integer, *, 1) Monoid
Scala val intProdMonoid = Monoid.instance[Int](1,_*_)
List(1,2,3,4).combineAll(intProdMonoid) == 24
Haskell getProduct (fold (fmap Product [1,2,3,4])) == 24
(List, ++, []) Monoid
Scala List(List(1,2),List(3,4),List(5,6)).combineAll == List(1,2,3,4,5,6)
Haskell fold [[1,2],[3,4],[5,6]] = [1,2,3,4,5,6]
(Option[m.type], m.op, m.zero) Monoid where m = (type=Integer, op=+, zero=0) Monoid
Scala List(Some(1), None, Some(3), Some(4)).combineAll == Some(8)
Haskell fmap getSum (fold [Just (Sum 1), Nothing, Just (Sum 3), Just (Sum 4)]) == Just 8
If we look at the Haskell
definitions of fold, foldMap and
mconcat on the previous slide,
we see that fold and mconcat
are equivalent, so on this slide,
where we use fold, we could
have just as well used mconcat.
In Scala, we are making
the relevant Cats Foldable
and Monoid instances
available by importing
cats._ and cats.implicits._
In Haskell, we are
importing Data.Foldable
and Data.Monoid.
fold Examples
@philip_schwarz

Scala List("a","b","c").foldMap(_ + "x") == "axbxcx"
Haskell foldMap (s -> s ++ "x") ["a","b","c"] == "axbxcx”
Scala List(12,34,56).foldMap(_.toString) == "123456"
Haskell foldMap show [12,34,56] == "123456"
(Integer, +, 0) Monoid
Scala List(1,2,3).foldMap(1 + _) == 9
Haskell getSum (foldMap ((+) 1) (fmap Sum [1,2,3])) == 9
(List, ++, []) Monoid
Scala List(Some(1), Some(2), None, Some(3)).foldMap(_.toList) == List(1, 2, 3)
Haskell foldMap toList [Just 1, Just 2, Nothing, Just 3] == [1,2,3]
(Option[m.type], m.op, m.zero) Monoid where m = (type=Integer, op=+, zero=0) Monoid
Scala List( Some(1), Some(3), None, Some(4) ).foldMap(x => x map (_ + 1 )) == Some(11)
Haskell fmap getSum (foldMap (fmap ((+) 1)) [Just (Sum 1), Just (Sum 2), Nothing, Just (Sum 3)]) == Just 9
foldMap Examples

We can make the combine function even simpler if we
define the two monoids on the right.
Following that refresher on Monoid and Foldable, let’s turn to the task of improving the implementation of our program’s combine function.
See below for the current implementation of the the function.
imageGrid.map(_.reduce(_ beside _))
.reduce(_ above _)
imageGrid.map(_.foldLeft(Image.empty)(_ beside _))
.foldLeft(Image.empty)(_ above _)
That’s a nice way of coalescing all the grid’s images into a
single image, except that if/when the grid has no rows, or has
an empty row, the reduce function will throw an unsupported
operation exception.
That’s better: the foldLeft function now handles
both an empty grid and a grid with empty rows.
def combine(imageGrid: List[List[Image]]): Image =
Foldable[List].foldMap(imageGrid){ row =>
Foldable[List].combineAll(row)(beside)
}(above)
val beside = Monoid.instance[Image](Image.empty, _ beside _)
val above = Monoid.instance[Image](Image.empty, _ above _)
imageGrid.foldMap(_ combineAll beside)(above)
Cats allows us to call xs.foldMap(m) instead
of Foldable[List].foldMap(xs)(m).
Similarly for combineAll.
We can then pass the two monoids to the
foldMap and combineAll functions provided
by the Cats Foldable instance for List.

Having written a Scala program that displays the chess board for an N-Queens solution, it would be nice to
write the equivalent program in Haskell.
We’ll write the program using a graphics library called Gloss.
For our purposes, the main way in which Gloss differs from Doodle is that while the latter makes our life easy
by allowing us to create images without worrying about their position, and then to combine the images using
their beside and above functions, Gloss requires us to position the images ourselves, and then combine them
by stacking one above the other.
Because it is possible to build images with Doodle in the same way that they are built using Gloss, we are
first going to create a modified version of our Scala program that does just that. We are then going to
translate the program into Haskell.

The first thing we need to do is change the combine function so that rather than coalescing a grid of images by
using the images’ beside and above functions, which position an image beside or above another, it does so using
the images’ on function, which simply places one image on top of the other.
In the existing implementation of combine, we are using two monoids, one for composing images horizontally,
and one for composing them vertically. Because of that, even though Foldable’s foldMap and combineAll
functions are able to receive an implicit monoid parameter, we are having to pass the two different monoids into
the functions explicitly.
import cats.Monoid
val beside = Monoid.instance[Image](Image.empty, _ beside _)
val above = Monoid.instance[Image](Image.empty, _ above _)
imageGrid.foldMap(_ combineAll)
import cats.Monoid
given Monoid[Image] = Monoid.instance[Image](Image.empty, _ on _)
In the new implementation however, we only need a single monoid for composing images, i.e. one that superimposes the images,
so if we make the monoid implicit, it is automatically passed to the foldMap and combineAll functions behind the scenes.
@philip_schwarz

def show(queens: List[Int], squareSize: Int): Image =
val n = queens length
val solution = queens reverse
val (emptySquare, fullSquare) = createSquareImages(squareSize)
val squareImages: List[Image] =
for
row <- List.range(0, n)
col <- List.range(0, n)
squareX = col * squareSize
squareY = - row * squareSize
squareImageAtOrigin = if solution(row) == col then fullSquare else emptySquare
squareImage = squareImageAtOrigin.at(squareX,squareY)
yield squareImage
val squareImageGrid = (squareImages grouped n).toList
def show(queens: List[Int]): Image =
val square = Image.square(100).strokeColor(Color.black)
val squareImageGrid: List[List[Image]] =
for col <- queens.reverse
yield List.fill(queens.length)(emptySquare)
.updated(col,fullSquare)
def createSquareImages(squareSize: Int): (Image,Image) =
val square = Image.square(squareSize).strokeColor(Color.black)
(emptySquare, fullSquare)
Now that we have modified the combine function to superimpose images rather than position them beside or above each other, we need to position the
images ourselves before combining them.
To do that, we no longer just create square images, we also use their row and column indices in the grid to compute their desired position in the drawing
and then use Image’s at function to position the images.
On the left we see the current show and combine functions, and on the right, we see the modified ones.

On the left is where the square images end up when we create them, i.e. all at the origin, the last one obscuring all the others, and on the
right is where they end up after we position them relative to the first one, according to their position in the grid.

val n = solution.length
val frameTitle = s"{n}-Queens Problem – A solution"
val frameWidth = 1000
val frameHeight = 1000
val frameBackgroundColour = Color.white
val frame =
Frame.size(frameWidth,frameHeight)
.title(frameTitle)
.background(frameBackgroundColour)
show(solution).draw
@main def main =
val n = solution length
val squareSize = 100
val boardSize = n * squareSize
val boardX = - (boardSize - squareSize) / 2
val boardY = - boardX
val frame = createFrame(n, squareSize)
val boardImageAtOrigin = show(solution, squareSize)
val boardImage = boardImageAtOrigin.at(boardX,boardY)
boardImage.draw(frame)
def createFrame(n: Int, squareSize: Int): Frame =
val title = s"${n}-Queens Problem - A solution"
val backgroundColour = Color.white
val width = boardSize + (squareSize * 2)
val height = width
Frame.size(width, height)
.title(title)
.background(backgroundColour)
@main def main =
Now let’s move on to the remaining functions in the program: main and showQueens. While main doesn’t need to change, showQueens needs to do, for
the whole board, what the show function does for individual squares in the board, i.e. it has to position the whole board image so that it is in the middle of
the Frame, rather than where it is as a result of creating individual squares and positioning them relative to each other, i.e. at the coordinate origin (0,0).
On the left we see the current showQueens function, and on the right, we see the modified one.

On the left we see where the show function places the square images before combining them into a board image, and on the right, we see
that the showQueens function repositions the board image so that it is in the middle of the frame.

@main def main =
for
yield squareImage
given Monoid[Image] = Monoid.instance[Image](Image.empty, _on_)
val width = boardSize + (squareSize * 2)
val height = width
.title(title)
Here is the modified Scala
program in its entirety.
@philip_schwarz

Let’s now turn to the task of translating our Scala program into Haskell.
To do the graphics, we are going to use a library called Gloss, whose documentation says the following:
• Gloss hides the pain of drawing simple vector graphics behind a nice data type and a few display functions.
• Get something cool on the screen in under 10 minutes.
The next slide is a very brief introduction to some of the few concepts that we’ll need to draw a solution board.

First we create a Picture, e.g. a Circle with a radius of 80 pixels.
A picture is drawn on a Display, so we create a Display that consists
of a window that has the desired title and is of the desired size (width
an height) and is located at the desired position (x and y coordinates).
To draw a picture, we call the display function with a Display, the
desired background colour for the Display, and the Picture to be
drawn on the Display.
Just like the drawInWindow function in the SOEGraphics library, the
display function in the Gloss library does not produce any side
effects: it returns an IO action.

createSquareImages :: Int -> (Picture, Picture)
createSquareImages squareSize = (emptySquare, fullSquare)
where square = rectangleSolid (fromIntegral squareSize) (fromIntegral squareSize)
squareBorder = rectangleWire (fromIntegral squareSize) (fromIntegral squareSize)
emptySquare = pictures [color white square, color black squareBorder]
fullSquare = pictures [color red square, color black squareBorder]
We can translate the Scala createSquareImages function into Haskell
by using Gloss functions rectangleWire, rectangleSolid, and pictures.
Gloss

given Monoid[Image] = Monoid.instance[Image](Image.empty, _on_)
instance Monoid Picture where
mempty = Blank
mappend a b = Pictures [a, b]
mconcat = Pictures
combine :: [[Picture]] -> Picture
combine imageGrid = foldMap fold imageGrid
fold and mconcat are equivalent, so we
could have used mconcat if we wanted to.
As for the combine function, it is very easy to translate it into Haskell, because while
when using Doodle we had to define a monoid that combines images by superimposing
them, Gloss already defines such a monoid, so there is even less code to write.
Gloss
@philip_schwarz

show' :: [Int] -> Int -> Picture
show' queens squareSize = combine squareImageGrid
where n = length queens
solution = reverse queens
(emptySquare, fullSquare) = createSquareImages squareSize
squareImages =
do
row <- [0..n-1]
col <- [0..n-1]
let squareX = col * squareSize
squareImageAtOrigin = if (solution !! row) == col then fullSquare else emptySquare
squareImage = translate (fromIntegral squareX) (fromIntegral squareY) squareImageAtOrigin
return squareImage
squareImageGrid = chunksOf n squareImages
Now let’s turn to the heart of the program. Here is the Scala show function, together with its Haskell equivalent.
While in Doodle we position an image by using an image’s at function, in Gloss, we do that using the translate function.
for
yield squareImage
Gloss

showQueens :: [Int] -> IO ()
showQueens solution = display window backgroundColour boardImage
where n = length solution
squareSize = 100
boardSize = n * squareSize
boardX = - fromIntegral (boardSize - squareSize) / 2
boardY = - boardX
window = createWindowDisplay n squareSize
backgroundColour = white
boardImageAtOrigin = show' solution squareSize
boardImage = translate boardX boardY boardImageAtOrigin
@main def main =
val width = boardSize + (2 * squareSize)
val height = width
.title(title)
createWindowDisplay :: Int -> Int -> Display
createWindowDisplay n squareSize = InWindow title (width, height) position
where title = (show n) ++ "-Queens Problem - A solution"
width = boardSize + (2 * squareSize)
height = width
position = (0,0)
main :: IO ()
main = showQueens solution
where solution = [3, 1, 6, 2, 5, 7, 4, 0]
And here is the translation of the rest
of the program from Scala to Haskell.

showQueens :: [Int] -> IO ()
showQueens solution = display window backgroundColour boardImage
where n = length solution
squareSize = 100
boardX = - fromIntegral (boardSize - squareSize) / 2
boardY = - boardX
window = createWindowDisplay n squareSize
backgroundColour = white
boardImageAtOrigin = show' solution squareSize
boardImage = translate boardX boardY boardImageAtOrigin
createWindowDisplay :: Int -> Int -> Display
createWindowDisplay n squareSize = InWindow title (width,height) position
where title = (show n) ++ "-Queens Problem - A solution"
width = boardSize + (2 * squareSize)
height = width
position = (0,0)
main :: IO ()
main = showQueens solution
where solution = [3, 1, 6, 2, 5, 7, 4, 0]
show' :: [Int] -> Int -> Picture
show' queens squareSize = combine squareImageGrid
where n = length queens
solution = reverse queens
(emptySquare, fullSquare) = createSquareImages squareSize
squareImages =
do
row <- [0..n-1]
col <- [0..n-1]
let squareX = col * squareSize
squareImageAtOrigin = if (solution !! row) == col then fullSquare else emptySquare
squareImage = translate (fromIntegral squareX) (fromIntegral squareY) squareImageAtOrigin
return squareImage
squareImageGrid = chunksOf n squareImages
combine :: [[Picture]] -> Picture
combine imageGrid = foldMap fold imageGrid
createSquareImages :: Int -> (Picture, Picture)
createSquareImages squareSize = (emptySquare, fullSquare)
where square = rectangleSolid (fromIntegral squareSize) (fromIntegral squareSize)
squareBorder = rectangleWire (fromIntegral squareSize) (fromIntegral squareSize)
emptySquare = pictures [color white square, color black squareBorder]
fullSquare = pictures [color red square, color black squareBorder]
Here is the Haskell
program in its entirety.

Here are the results of the Haskell program
for the first solution for each of N=4,5,6.
[4, 2, 0, 5, 3, 1]
[3, 1, 4, 2, 0]
[2, 0, 3, 1]
@philip_schwarz

queens n = placeQueens n
where
placeQueens 0 = [[]]
placeQueens k = [queen:queens |
queens <- placeQueens(k-1),
queen <- [1..n],
safe queen queens]
safe queen queens = all safe (zipWithRows queens)
where
safe (r,c) = c /= col && not (onDiagonal col row c r)
row = length queens
col = queen
onDiagonal row column otherRow otherColumn =
abs (row - otherRow) == abs (column - otherColumn)
zipWithRows queens = zip rowNumbers queens
where
rowCount = length queens
rowNumbers = [rowCount-1,rowCount-2..0]
def queens(n: Int): List[List[Int]] =
def placeQueens(k: Int): List[List[Int]] =
if k == 0
then List(List())
else
for
queens <- placeQueens(k - 1)
queen <- 1 to n
if safe(queen, queens)
yield queen :: queens
placeQueens(n)
def onDiagonal(row: Int, column: Int, otherRow: Int, otherColumn: Int) =
math.abs(row - otherRow) == math.abs(column - otherColumn)
def safe(queen: Int, queens: List[Int]): Boolean =
val (row, column) = (queens.length, queen)
val safe: ((Int,Int)) => Boolean = (nextRow, nextColumn) =>
column != nextColumn && !onDiagonal(column, row, nextColumn, nextRow)
zipWithRows(queens) forall safe
def zipWithRows(queens: List[Int]): Iterable[(Int,Int)] =
val rowCount = queens.length
val rowNumbers = rowCount - 1 to 0 by -1
rowNumbers zip queens
To conclude Part 2, here is a reminder, from Part 1, of the
translation of the program for computing the N-Queens Problem.

haskell> queens 4
[[3,1,4,2],[2,4,1,3]]
haskell> queens 5
[[4,2,5,3,1],[3,5,2,4,1],[5,3,1,4,2],[4,1,3,5,2],
[5,2,4,1,3],[1,4,2,5,3],[2,5,3,1,4],[1,3,5,2,4],
[3,1,4,2,5],[2,4,1,3,5]]
haskell> queens 6
[[5,3,1,6,4,2],[4,1,5,2,6,3],
[3,6,2,5,1,4],[2,4,6,1,3,5]]
scala> queens(4)
val res0: List[List[Int]] = List(List(3, 1, 4, 2), List(2, 4, 1, 3))
scala> queens(5)
val res1: List[List[Int]] = List(List(4, 2, 5, 3, 1), List(3, 5, 2, 4, 1), List(5, 3, 1, 4, 2), List(4, 1, 3, 5, 2),
List(5, 2, 4, 1, 3), List(1, 4, 2, 5, 3), List(2, 5, 3, 1, 4), List(1, 3, 5, 2, 4),
List(3, 1, 4, 2, 5), List(2, 4, 1, 3, 5))
scala> queens(6)
val res2: List[List[Int]] = List(List(5, 3, 1, 6, 4, 2), List(4, 1, 5, 2, 6, 3),
List(3, 6, 2, 5, 1, 4), List(2, 4, 6, 1, 3, 5))
And here is a reminder of
the output it produces.

That’s all for Part 2. I hope you found it interesting.
The first thing we are going to do in part 3 is write code that displays, all together, the
results of queens(N) for N = 4, 5, 6, 7, 8.
We are then going to look at some alternative ways of coding the N-Queens Problem.
See you then.
@philip_schwarz

N-Queens Combinatorial Problem - Polyglot FP for Fun and Profit – Haskell and Scala - Part 2

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N-Queens Combinatorial Problem - Polyglot FP for Fun and Profit – Haskell and Scala - Part 2