5 // Polymorphic Types

     5 // Polymorphic Types

     6 //===================

     6 //===================

     7 

     7 

     8 // You do not want to write functions like contains, first, 

     8 // You do not want to write functions like contains, first, 

     9 // length and so on for every type of lists.

     9 // length and so on for every type of lists.

    10 

    25 

    11 

    26 

    12 def length_string_list(lst: List[String]): Int = lst match {

    27 def length_string_list(lst: List[String]): Int = lst match {

    13   case Nil => 0

    28   case Nil => 0

    14   case x::xs => 1 + length_string_list(xs)

    29   case x::xs => 1 + length_string_list(xs)

   331 nfa.accepts("aaaaabbbaaa".toList)    // false

   346 nfa.accepts("aaaaabbbaaa".toList)    // false

   332 nfa.accepts("ac".toList)             // false

   347 nfa.accepts("ac".toList)             // false

   333 

   348 

   334 

   349 

   335 // Q: Why the kerfuffle about the polymorphic types in DFAs/NFAs?

   350 // Q: Why the kerfuffle about the polymorphic types in DFAs/NFAs?

   337 

   352 

   338 def subset[A, C](nfa: NFA[A, C]) : DFA[Set[A], C] = {

   353 def subset[A, C](nfa: NFA[A, C]) : DFA[Set[A], C] = {

   339   DFA(nfa.starts, 

   354   DFA(nfa.starts, 

   340       { case (qs, c) => nfa.nexts(qs, c) }, 

   355       { case (qs, c) => nfa.nexts(qs, c) }, 

   341       _.exists(nfa.fins))

   356       _.exists(nfa.fins))

   403 def charlist2rexp(s: List[Char]): Rexp = s match {

   418 def charlist2rexp(s: List[Char]): Rexp = s match {

   404   case Nil => ONE

   419   case Nil => ONE

   405   case c::Nil => CHAR(c)

   420   case c::Nil => CHAR(c)

   406   case c::s => SEQ(CHAR(c), charlist2rexp(s))

   421   case c::s => SEQ(CHAR(c), charlist2rexp(s))

   407 }

   422 }

   409 

   425 

   410 

   426 

   411 val r1 = STAR("ab")

   427 val r1 = STAR("ab")

   412 val r2 = STAR(ALT("ab", "baa baa black sheep"))

   428 val r2 = STAR(ALT("ab", "baa baa black sheep"))

   413 val r3 = STAR(SEQ("ab", ALT("a", "b")))

   429 val r3 = STAR(SEQ("ab", ALT("a", "b")))