blue-print-solution.md

// P46 (**) Truth tables for logical expressions.
//     Define functions and, or, nand, nor, xor, impl, and equ (for logical
//     equivalence) which return true or false according to the result of their
//     respective operations; e.g. and(A, B) is true if and only if both A and B
//     are true.
//
//     scala> and(true, true)
//     res0: Boolean = true
//
//     scala> xor(true. true)
//     res1: Boolean = false
//
//     A logical expression in two variables can then be written as a function of
//     two variables, e.g: (a: Boolean, b: Boolean) => and(or(a, b), nand(a, b))
//    
//     Now, write a function called table2 which prints the truth table of a
//     given logical expression in two variables.
//
//     scala> table2((a: Boolean, b: Boolean) => and(a, or(a, b)))
//     A     B     result
//     true  true  true
//     true  false true
//     false true  false
//     false false false

// The trick here is not using builtins.  We'll define `not`, `and`, and `or`
// directly (using pattern matching), and the other functions in terms of those
// three.

object S99Logic {
  def not(a: Boolean) = a match {
    case true  => false
    case false => true
  }
  def and(a: Boolean, b: Boolean): Boolean = (a, b) match {
    case (true, true) => true
    case _            => false
  }
  def or(a: Boolean, b: Boolean): Boolean = (a, b) match {
    case (true, _) => true
    case (_, true) => true
    case _         => false
  }
  def equ(a: Boolean, b: Boolean): Boolean = or(and(a, b), and(not(a), not(b)))
  def xor(a: Boolean, b: Boolean): Boolean = not(equ(a, b))
  def nor(a: Boolean, b: Boolean): Boolean = not(or(a, b))
  def nand(a: Boolean, b: Boolean): Boolean = not(and(a, b))
  def impl(a: Boolean, b: Boolean): Boolean = or(not(a), b)
  
  def table2(f: (Boolean, Boolean) => Boolean) {
    println("A     B     result")
    for {a <- List(true, false);
         b <- List(true, false)} {
     printf("%-5s %-5s %-5s\n", a, b, f(a, b))
    }
  }
}