Transcript Getting Functional

Getting Functional
What is Functional Programming (FP)?
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In FP,
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Functions are first-class objects. That is, they are values, just like other
objects are values, and can be treated as such
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Functions should only transform their inputs into their outputs
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A function should have no side effects
 It should not do any input/output
 It should not change any state (any external data)
Given the same inputs, a function should produce the same outputs, every
time
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Functions can be assigned to variables, passed as parameters to higher-order
functions, returned as results of functions
There is some way to write function literals
But we need random numbers, date and time, etc.
Functions have referential transparency—any given function call could
safely be replaced by the function’s result
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Creating Lists
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scala> List('a', 'b', 'c')
res0: List[Char] = List(a, b, c)
scala> "abc" toList
res1: List[Char] = List(a, b, c)
scala> "Welcome to Scala" split(" ")
res2: Array[java.lang.String] = Array(Welcome, to, Scala)
scala> val scala = "Scala" toList
scala: List[Char] = List(S, c, a, l, a)
scala> "Hello" :: scala
res3: List[Any] = List(Hello, S, c, a, l, a)
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Nil and ::
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scala> List()
res4: List[Nothing] = List()
scala> Nil
res5: scala.collection.immutable.Nil.type = List()
scala> List() == Nil
res6: Boolean = true
scala> List[String]()
res7: List[String] = List()
scala> "xyz" :: Nil
res8: List[java.lang.String] = List(xyz)
scala> "abc" :: "xyz" :: Nil
res9: List[java.lang.String] = List(abc, xyz)
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head, tail, and isEmpty
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scala> val penn = "Pennsylvania" toList
penn: List[Char] = List(P, e, n, n, s, y, l, v, a, n, i, a)
scala> penn head
res10: Char = P
scala> penn tail
res11: List[Char] = List(e, n, n, s, y, l, v, a, n, i, a)
scala> penn isEmpty
res12: Boolean = false
scala> Nil isEmpty
res13: Boolean = true
scala> Nil head
java.util.NoSuchElementException: head of empty list (plus many more lines!)
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take, drop, and splitAt
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scala> penn
res16: List[Char] = List(P, e, n, n, s, y, l, v, a, n, i, a)
scala> penn take 4
res17: List[Char] = List(P, e, n, n)
scala> penn drop 4
res18: List[Char] = List(s, y, l, v, a, n, i, a)
scala> penn splitAt 4
res19: (List[Char], List[Char]) = (List(P, e, n, n),List(s, y, l, v, a, n, i, a))
scala> penn.splitAt(4)
res20: (List[Char], List[Char]) = (List(P, e, n, n),List(s, y, l, v, a, n, i, a))
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toString and mkString
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scala> List(1, 2, 3).toString
res25: String = List(1, 2, 3)
scala> List(1, 2, 3).toString == "List(1, 2, 3)"
res26: Boolean = true
scala> List(1, 2, 3) mkString(" is less than ")
res27: String = 1 is less than 2 is less than 3
scala> List(1, 2, 3) mkString("*")
res28: String = 1*2*3
scala> List(1, 2, 3) mkString("<: ", "--", " :>")
res29: String = <: 1--2--3 :>
scala> List(1, 2, 3) mkString("(", ", ", ")")
res30: String = (1, 2, 3)
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zip and unzip
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scala> val words = "one two three" split " "
words: Array[java.lang.String] = Array(one, two, three)
scala> val numbers = List(1, 2, 3, 4, 5)
numbers: List[Int] = List(1, 2, 3, 4, 5)
scala> val z = words zip numbers
z: Array[(java.lang.String, Int)] = Array((one,1), (two,2), (three,3))
scala> val zz = numbers zip words
zz: List[(Int, java.lang.String)] = List((1,one), (2,two), (3,three))
scala> z toMap
res31: scala.collection.immutable.Map[java.lang.String,Int] = Map((one,1), (two,2),
(three,3))
scala> zz unzip
res32: (List[Int], List[java.lang.String]) = (List(1, 2, 3),List(one, two, three))
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Higher-order functions
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The basic syntax of a function literal is
parameter_list => function_body
A higher-order function is one that takes a function as a parameter, or returns a function as a result
scala> val brag = "Scala is great!" toList
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brag: List[Char] = List(S, c, a, l, a, , i, s, , g, r, e, a, t, !)
scala> brag count((ch: Char) => ch == 'a')
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res34: Int = 3
scala> brag count((ch: Char) => !(ch isLetter))
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res35: Int = 3
scala> "Scala is great!".toList.count((ch: Char) => ch < 'f')
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res36: Int = 9
scala> "aeiou" contains 'e'
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res37: Boolean = true
scala> "Scala is great!".toList.count((ch: Char) => "aeiou" contains ch)
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res38: Int = 5
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Abbreviations
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In a literal function, you can usually omit the type (and, if there’s only one parameter,
the parentheses)
scala> brag
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res40: List[Char] = List(S, c, a, l, a, , i, s, , g, r, e, a, t, !)
scala> brag count((ch: Char) => ch == 'a')
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res41: Int = 3
scala> brag count (ch => ch == 'a')
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res42: Int = 3
In fact, if there is only one parameter, used once, you can omit the parameter and the
=> and just use _ to stand in for the parameter
scala> brag count (_ == 'a')
res44: Int = 3
Let me repeat that: Used once
scala> brag count (_ == 'a' || _ == 'e')
<console>:7: error: wrong number of parameters; expected = 1
brag count (_ == 'a' || _ == 'e')
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sortWith
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scala> brag sortWith((x, y) => x < y)
res49: List[Char] = List( , , !, S, a, a, a, c, e, g, i, l, r, s, t)
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Since there are two parameters, we can use two underscores
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scala> brag sortWith (_ < _)
res50: List[Char] = List( , , !, S, a, a, a, c, e, g, i, l, r, s, t)
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Order matters!
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scala> brag sortWith (_ > _)
res52: List[Char] = List(t, s, r, l, i, g, e, c, a, a, a, S, !, , )
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forall and exists
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Whereas count returns an Int, forall and exists return a Boolean
scala> val n = List(3, 1, 4, 1, 6)
n: List[Int] = List(3, 1, 4, 1, 6)
scala> n forall(x => x < 8)
res53: Boolean = true
scala> n forall(x => x < 5)
res54: Boolean = false
scala> n exists(_ < 5)
res55: Boolean = true
scala> n exists(_ > 8)
res56: Boolean = false
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foreach
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foreach returns the (uninteresting) Unit value
scala> val brag = List("Scala", "is", "great!")
brag: List[java.lang.String] = List(Scala, is, great!)
scala> brag foreach (println(_))
Scala
is
great!
scala> brag foreach(println)
Scala
is
great!
scala> List(3, 1, 4, 1, 6) foreach (x => if (x > 1) println(x))
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Least upper bound
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The following list contains only integers:
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Now let’s add a non-integer to it
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scala> val list = List(1, 2, 3)
list: List[Int] = List(1, 2, 3)
scala> "hello" :: list
res0: List[Any] = List(hello, 1, 2, 3)
The type of this new list is the lowest class in the hierarchy that is
a superclass of all the elements of the list
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Closures
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scala> var c = 5
c: Int = 5
scala> val mult = (x: Int) => c * x
mult: (Int) => Int = <function1>
scala> mult(3)
res63: Int = 15
scala> c = 7
c: Int = 7
scala> mult(10)
res64: Int = 70
What will happen if we pass the mult function into another function in another context?
It continues to “link to” (enclose?) the original variable c, not it’s value
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map
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map produces a new list by applying the given function to each element of the
given list
scala> val n = List(1, 2, 3, 4, 5)
n: List[Int] = List(1, 2, 3, 4, 5)
scala> n map(x => x * x)
res65: List[Int] = List(1, 4, 9, 16, 25)
scala> n map (_ >= 3)
res66: List[Boolean] = List(false, false, true, true, true)
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flatMap
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flatMap produces a new list by applying the given function to each element of the
given list, and “flattening” the result
scala> n
res70: List[Int] = List(1, 2, 3, 4, 5)
scala> n map (x => List(x, x*x))
res72: List[List[Int]] = List(List(1, 1), List(2, 4), List(3, 9), List(4, 16), List(5,
25))
scala> n flatMap(x => List(x, x*x))
res73: List[Int] = List(1, 1, 2, 4, 3, 9, 4, 16, 5, 25)
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filter
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filter produces a new list consisting of the values that pass the given test
scala> n
res67: List[Int] = List(1, 2, 3, 4, 5)
scala> n filter (_ < 4)
res68: List[Int] = List(1, 2, 3)
scala> n filter (_ % 2 == 1)
res69: List[Int] = List(1, 3, 5)
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Some Map methods
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These methods have little to do with “being functional,” so think of them as a bonus :-)
We’re going to use a map that takes small numbers (1..5) to their squares:
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scala> val m = Map(1 -> 1, 2 -> 4, 3 -> 9, 4 -> 16, 5 -> 25)
m: scala.collection.immutable.Map[Int,Int] = Map((5,25), (1,1), (2,4),
(3,9), (4,16))
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scala> m(4)
res1: Int = 16
scala> m(10)
java.util.NoSuchElementException: key not found: 10
...and many more lines of error message
scala> m.get(4)
res3: Option[Int] = Some(16)
scala> m.get(10)
res4: Option[Int] = None
scala> m.get(4) match {
| case Some(x) => x
| case None => -999
|}
res5: Int = 16
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Another Map method
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scala> m
res6: scala.collection.immutable.Map[Int,Int] = Map((5,25), (1,1),
(2,4), (3,9), (4,16))
scala> m.getOrElse(4, -999)
res7: Int = 16
scala> m.getOrElse(10, -999)
res8: Int = -999
Now consider the following carefully: Given that
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res9: List[Any] = List(hello, 1, 2, 3)
What do you think the type and value of the following will be?
 scala> val x = m.getOrElse(4, "Hello")
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“Houston, we have a problem.”
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scala> val x = m.getOrElse(4, "Hello")
x: Any = 16
That’s pretty scary--but it gets worse:
scala> x + 1
<console>:8: error: type mismatch;
found : Int(1)
required: String
x+1
^
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The End
If I were to pick a language to use today
other than Java, it would be Scala.
--James Gosling, creator of Java
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