Cse321, Programming Languages and Compilers 1 7/14/2015 Lecture #3b, Jan. 22, 2007 References While Loops Accumulating parameter functions Regular Expressions as programs RE to NFA Patterns for RE’s

Cse321, Programming Languages and Compilers 2 7/14/2015 Libraries There are lots of predefined types and functions in SML when it starts up. You can find out about them at: Many more can be found in the other libraries. – Libraries are encapsulated in Structures which are classified by Signatures (a list of what is in the structure).

Cse321, Programming Languages and Compilers 3 7/14/2015 Peeking inside a Library To see what is inside a Structure you can open it. This is somewhat of a hack, but it is useful. Standard ML of New Jersey v [built: Mon Nov 21 21:46: ] - open Int; opening Int type int = ?.int val precision : Int31.int option val minInt : int option val maxInt : int option val toLarge : int -> IntInf.int val fromLarge : IntInf.int -> int val toInt : int -> Int31.int val fromInt : Int31.int -> int val div : int * int -> int val mod : int * int -> int val quot : int * int -> int val rem : int * int -> int val min : int * int -> int val max : int * int -> int

Cse321, Programming Languages and Compilers 4 7/14/2015 The List library - open List; opening List datatype 'a list = :: of 'a * 'a list | nil exception Empty val null : 'a list -> bool val hd : 'a list -> 'a val tl : 'a list -> 'a list val last : 'a list -> 'a val getItem : 'a list -> ('a * 'a list) option val nth : 'a list * int -> 'a val take : 'a list * int -> 'a list val drop : 'a list * int -> 'a list val length : 'a list -> int val rev : 'a list -> 'a list : 'a list * 'a list -> 'a list val concat : 'a list list -> 'a list val revAppend : 'a list * 'a list -> 'a list val app : ('a -> unit) -> 'a list -> unit val map : ('a -> 'b) -> 'a list -> 'b list val mapPartial : ('a -> 'b option) -> 'a list -> 'b list val find : ('a -> bool) -> 'a list -> 'a option val filter : ('a -> bool) -> 'a list -> 'a list val partition : ('a -> bool) -> 'a list -> 'a list * 'a list val foldr : ('a * 'b -> 'b) -> 'b -> 'a list -> 'b val foldl : ('a * 'b -> 'b) -> 'b -> 'a list -> 'b val exists : ('a -> bool) -> 'a list -> bool val all : ('a -> bool) -> 'a list -> bool val tabulate : int * (int -> 'a) -> 'a list val collate : ('a * 'a -> order) -> 'a list * 'a list -> order -

Cse321, Programming Languages and Compilers 5 7/14/2015 References References allow one to write programs with mutable variables. The interface to assignment and update is slightly different from other languages –val r = (ref 5) Create a new reference that can be updated –!r Get the value stored in the reference –(ref n) => … Pattern match to get value »fun ! (ref n) = n –r := 6 + z Update a reference with a new value Later today we will use the following: val next = ref 0; fun new () = let val ref n = next in (next := n+1; n) end; Alternatively fun new () = let val n = !next in (next := n+1; n) end;

Cse321, Programming Languages and Compilers 6 7/14/2015 While Loops While loops are similar to other languages, they usually require use of references (to get the condition to eventually change). Statements are inside ()’s and separated by “;” val n = ref 4; val w1 = while (!n > 0) do (print (Int.toString (!n) ^ "\n"); n := (!n) – 1 ); Semicolon to separate statements Semicolon to end the “val w1 = …” declaration

Cse321, Programming Languages and Compilers 7 7/14/2015 Factorial as a while loop Factorial fun fact3 n = let val ans = ref 1 val count = ref n in while (!count > 0) do (ans := !ans * !count ; count := !count - 1); !ans end; fun fact1 n = if n=0 then 1 else n * (fact1 (n-1)); fun fact2 0 = 1 | fact2 n = n * (fact2 (n-1)); Inside a let between “in” and “end” we don’t need to surround statements with ()s, but we still separate with “;” Return the value stored in the reference ans compare with the recursive versions

Cse321, Programming Languages and Compilers 8 7/14/2015 Accumulating parameters Many loops look like this { ans = init; While test do stuff; Return ans} There is a pattern that mimics this in ML called functions with accumulating parameter. The pattern consists of a recursive function with two (or more) parameters. The first parameter drives the loop (usually by pattern matching), the second accumulates an answer (like ans in example above). We call the function with init as the value of the second argument to get started. We return the second argument when the function is done recurring.

Cse321, Programming Languages and Compilers 9 7/14/2015 Fact as an accumulating function fun fact4 n = let fun loop 0 ans = ans | loop n ans = loop (n-1) (n*ans) in loop n 1 end; { ans = init; While test do stuff; Return ans}

Cse321, Programming Languages and Compilers 10 7/14/2015 Flat as an accumulating function datatype Tree = Tip | Node of Tree * int * Tree; fun flat3 x = let fun help Tip ans = ans | help (Node(x,y,z)) ans = help x (y::(help z ans)) in help x [] end; { ans = init; While test do stuff; Return ans}

Cse321, Programming Languages and Compilers 11 7/14/2015 Regular Expressions Regular Languages and Regular expressions are used to describe the patterns which describe lexemes. Regular expressions are composed of empty-string, concatenation, union, and closure. Examples: A(A | D)* where A is alphabetic and D is a digit (+ | - | ε ) D D* closure union Empty-string Concatenation is implicit

Cse321, Programming Languages and Compilers 12 7/14/2015 Meaning of Regular Expressions Let A,B be sets of strings: The empty string: "" ε= { "" } (sometimes ) Concatenation by juxtaposition: AB = a^b where a in A and b in B A = {"x", "qw"} and B = {"v", "A"} then AB = { "xv", "xA", "qwv", "qwA"}

Cse321, Programming Languages and Compilers 13 7/14/2015 Meaning of Regular Expressions (cont.) Union by | (or other symbols like U etc) A = {"x", "qw"} and B = {"v", "A"} then A|B = {"x", "qw", "v", "A"} Closure by * Thus A* = {""} | A | AA | AAA |... = A 0 | A 1 | A 2 | A 3 |... A = {"x", "qw"} then A* = { "" } | {"x", "qw"} | {"xqw", "qwx","xx", "qwqw"} |...

Cse321, Programming Languages and Compilers 14 7/14/2015 Regular Expressions as a language We can treat regular expressions as a programming language. Each expression is a new program. Programs can be compiled. How do we represent the regular expression language? By using a datatype. datatype RE = Empty | Union of RE * RE | Concat of RE * RE | Star of RE | C of char;

Cse321, Programming Languages and Compilers 15 7/14/2015 Example RE program (+ | - | ε ) D D* val re1 = Concat(Union(C #”+”,Union(C #”-”,Empty)),Concat(C #”D”,Star (C #”D”)))

Cse321, Programming Languages and Compilers 16 7/14/2015 R.E.’s and FSA’s Algorithm that constructs a FSA from a regular expression. FSA –alphabet, A –set of states, S –a transition function, A x S -> S –a start state, S 0 –a set of accepting states, S F subset of S Defined by cases over the structure of regular expressions Let A,B be R.E.’s, “x” in A, then – ε is a R.E. –“x” is a R.E. –AB is a R.E. –A|B is a R.E. –A* is a R.E. 1 Rule for each case

Cse321, Programming Languages and Compilers 17 7/14/2015 Rules ε “x” AB A|B A* ε x B A A B ε ε ε ε A ε ε ε ε

Cse321, Programming Languages and Compilers 18 7/14/2015 Example: (a|b)*abb ε ε ε ε 8 a b ε a b b 1 Note the many ε transitions Loops caused by the * Non-Determinism, many paths out of a state on “a” ε ε ε

Cse321, Programming Languages and Compilers 19 7/14/2015 Building an NFA from a RE datatype Label = Epsilon | Char of char; type Start = int; type Finish = int; datatype Edge = Edge of Start * Label * Finish; val next = ref 0; fun new () = let val ref n = next in (next := n+1; n) end; Ref makes a mutable variable Semi colon separates commands (inside parenthesis)

Cse321, Programming Languages and Compilers 20 7/14/2015 fun nfa Empty = let val s = new() val f = new() in (s,f,[Edge(s,Epsilon,f)]):Nfa end | nfa (C x) = let val s = new() val f = new() in (s,f,[Edge(s,Char x,f)]) end | nfa (Union(x,y)) = let val (sx,fx,xes) = nfa x val (sy,fy,yes) = nfa y val s = new() val f = new() val newes = [Edge(s,Epsilon,sx),Edge(s,Epsilon,sy),Edge(fx,Epsilon,f),Edge(fy,Epsilon,f)] in yes) end ε x A B ε ε ε ε

Cse321, Programming Languages and Compilers 21 7/14/2015 | nfa (Concat(x,y)) = let val (sx,fx,xes) = nfa x val (sy,fy,yes) = nfa y in (sx,fy,(Edge(fx,Epsilon,sy)):: yes)) end | nfa (Star r) = let val (sr,fr,res) = nfa r val s = new() val f = new() val newes = [Edge(s,Epsilon,sr),Edge(fr,Epsilon,f),Edge(s,Epsilon,f),Edge(f,Epsilon,s)] in res) end A ε ε ε ε B A

Cse321, Programming Languages and Compilers 22 7/14/2015 Example use val re1 = Concat(Union(C #”+”,Union(C #”-”,Empty)),Concat(C #”D”,Star (C #”D”))) Val ex6 = nfa re1; val ex6 = (8,15, [Edge (9,Epsilon,10),Edge (8,Epsilon,0),Edge (8,Epsilon,6),Edge (1,Epsilon,9),Edge (7,Epsilon,9),Edge (0,Char #,1),Edge (6,Epsilon,2),Edge (6,Epsilon,4),Edge (3,Epsilon,7),Edge (5,Epsilon,7),Edge (2,Char #,3),Edge (4,Epsilon,5),...]) : Nfa

Cse321, Programming Languages and Compilers 23 7/14/2015 Assignment #3 CS321 Prog Lang & Compilers Assignment # 3 Assigned: Jan 22, 2007 Due: Wed. Jan 24, 2007 Turn in a listing, and a transcript that shows you have tested your code. A minimum of 3 tests is necessary. Some functions may require more than 3 tests to receive full credit. 1) Write the following functions over lists. You must use pattern matching and recursion. A. reverse a list so that its elements appear in the oposite order. reverse [1,2,3,4] ----> [4,3,2,1] B. Count the number of occurrences of an element in a list count 4 [1,2,3,4,5,4] ---> 2 count 4 [1,2,3,2,1] ---> 0 C. concatenate together a list of lists concat [[1,2],[],[5,6]] ----> [1,2,5,6] 2) Using the datatype for Regular Expressions we defined in class datatype RE = Empty | Union of RE * RE | Concat of RE * RE | Star of RE | C of char; Write a function that turns a RE into a string, so that it can be printed. Minimize the number of parenthesis, but keep the string unambigouous by using the following rules. 1) Star has highest precedence so: ab* means a(b*) 2) Concat has the next highest precedence so: a+bc means a+(bc) 3) Union has lowest precedence so: a+bc+c* means a+(bc)+(c*) 4) Use the hash mark (#) as the empty string. 5) Special characters *+()\ should be escaped by using a preceeding backslash. So (Concat (C #"+") (C #"a")) should be "\+a" Hints: 1) The string concatenation operator is usefull: "abc" ^ "zx" -----> "abczx" 2) Write this is two steps. First, fully paranethesize every RE Second, Change the function to not add the parenthesis which the rules don't require.