Meap Scala Notes



functional traits

  • interfaces for functions
  • we introduce data types with [traits]
  • sealed trait
    • sealed means that all implentations must be in this file, kinda the same as [dinal]

other syntax quirks

  • each anonymous function has a implicit [apply] method that exposes the interface or [trait] of the function
  • the function apply means that a function object can be called implicitly
  • square brackets with capitals signify [generics]/[polymorphic] functions
  • remember that anonymous functions should be wrapped in [brackets] and can name arguments only at [top] level
  • note that applying 1 function after another requires repeated use of [brackets]
def uncurry[A,B,C](f: A => B=> C): (A,B)=>C =
(a:A,b:B) => f (a) (b)

data constructors

  • varying forms use the case statement
    • think overloading constructors
  • are made by [extend]ing the trait
  • each data constructor introduces a pattern


  • lists
    • lists can be made [polymorphic] with square brackets
    • adding a + makes the list [covariant]
    • We defined Nil to extend List[Nothing]
      • Nothing mathes all types
      • Therefore because list is covarient
        • Nil has a type [List[A]] for any A
    • Variadic functions
      what does _* mean?
      //recursively applies cons
      def apply[A](as: A*): List[A] =
        if (as.isEmpty) Nil
        else Cons(as.head, apply(as.tail: _*))
      • The Answer
        • * is just syntatic sugar for a Seq which are lists or array like structures passed around instead of variable arguments
        • as is bound to Seq[A]
        • _~* allows us to pass a Seq to a varargs method,
          • a bit like pythonic **
    • covariance
      • generally if X is a subtype of Y then
        • List[X] is a subtupe of List[Y]
    • companion objects
      • has the same [name] as our data type
      • also has [convience] functions
  • constructor quirks
    why is Nil defined with object a and list defined via class
    • The Answer
  • pattern matching
    • _ matches any value
    def sum(ints:List[Int]):Int = ints match {
    case Nil => 0
    case Cons(x,xs) => x + sum(xs)
    • syntatical quirks
      • need the ints for the match clause -called the [target] or [scrutinee]
      • case statements
      • currying
        • is done by sepearting out curried terms using [brackets]
        • apparently currying makes type infrerence work better
        call curried items via
        g(a) (b)
  • foldRight
    • replaces cons with f and Nil with its intialiser
    • foldRight must traverse all the way to the end of the list (pushing frames onto the call stack as we go) before it can begin collapsing it
    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))
  • foldLeft
    • pretty much sampe as fold right
    • word on the street is that foldl is broken
    • but moves computation inside recurusion
      • forcing early evaluation
      • use initialiser as an accumulator
    def foldLeft[A,B](l: List[A], z: B)(f: (B, A) => B): B = 
    l match {
      case Nil => z
      case Cons(x,xs) => foldLeft( xs, f(z, x)) (f)

algebraeic data types

  • definition
    • a datatype with one or more constructors
    • a type is the sum of its construstors
    • each constructor is the product of its arguments

notes on exercises

  • Exercise 23
    was unable to define addlists in terms of map foldmap perhaps there is another way?
    def addLists(a:List[Int], b:List[Int]): List[Int] = a match { 
      case Nil => b
      case Cons(x,xs) => Cons(x+ head(b), addLists(xs, tail(b)))
    • The Answer
  • Exercise 24
    compiler cant find scanRight
    def hasSubsequence[A](l: List[A], sub: List[A]): Boolean  =  scanRight (l,Nil:List[A])  ((a:A, b:List[B]) => Cons(a,b)) exists ((c:List[A]) => c == l)
  • The Answer


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