1. Introduction to ML – Part 1 Frances Spalding

2. Assignment 1 http://www.cs.princeton.edu/courses/ar chive/fall05/cos441/assignments/a1.ht m http://www.cs.princeton.edu/courses/ar chive/fall05/cos441/assignments/a1.ht m Due next Monday (Oct 3 rd ) Make sure you’ve signed up for the mailing list

3. Standard ML Standard ML is a domain-specific language for building compilers Support for Complex data structures (abstract syntax, compiler intermediate forms) Memory management like Java Large projects with many modules Advanced type system for error detection

4. Introduction to ML You will be responsible for learning ML on your own. Today I will cover some basics Aquinas will do a second class on ML on Thursday Resources: Jeffrey Ullman “Elements of ML Programming” Robert Harper’s “an introduction to ML” See course webpage for pointers and info about how to get the software

5. Intro to ML Highlights Data Structures for compilers Data type definitions Pattern matching Strongly-typed language Every expression has a type Certain errors cannot occur Polymorphic types provide flexibility Flexible Module System Abstract Types Higher-order modules (functors)

6. Intro to ML Interactive Language Type in expressions Evaluate and print type and result Compiler as well High-level programming features Data types Pattern matching Exceptions Mutable data discouraged

7. Preliminaries Read – Eval – Print – Loop - 3 + 2;

8. Preliminaries Read – Eval – Print – Loop - 3 + 2; > 5: int

9. Preliminaries Read – Eval – Print – Loop - 3 + 2; > 5: int - it + 7; > 12 : int

10. Preliminaries Read – Eval – Print – Loop - 3 + 2; > 5: int - it + 7; > 12 : int - it – 3; > 9 : int - 4 + true; stdIn:17.1-17.9 Error: operator and operand don't agree [literal] operator domain: int * int operand: int * bool in expression: 4 + true

11. Preliminaries Read – Eval – Print – Loop - 3 div 0; Failure : Div- run-time error

12. Basic Values - (); > () : unit=> like “void” in C (sort of) => the uninteresting value/type - true; > true : bool - false; > false : bool - if it then 3+2 else 7;“else” clause is always necessary > 7 : int - false andalso loop_Forever; > false : booland also, or else short-circuit eval

13. Basic Values Integers - 3 + 2 > 5 : int - 3 + (if not true then 5 else 7); > 10 : intNo division between expressions and statements Strings - “Dave” ^ “ “ ^ “Walker”; > “Dave Walker” : string - print “foo\n”; foo > 3 : int Reals - 3.14; > 3.14 : real

14. Using SML/NJ Interactive mode is a good way to start learning and to debug programs, but… Type in a series of declarations into a “.sml” file - use “foo.sml” [opening foo.sml] … list of declarations with their types

15. Larger Projects SML has its own built in interactive “make” Pros: It automatically does the dependency analysis for you No crazy makefile syntax to learn Cons: May be more difficult to interact with other languages or tools

16. Compilation Manager % sml - OS.FileSys.chDir “ ~/courses/510/a2 ” ; - CM.make(); looks for “ sources.cm ”, analyzes dependencies [compiling … ] compiles files in group [wrote … ] saves binaries in./CM/ - CM.make ’ “ myproj/ ” (); specify directory sources.cm c.smlb.smla.sig Group is a.sig b.sml c.sml

17. What is next? ML has a rich set of structured values Tuples: (17, true, “stuff”) Records: {name = “Dave”, ssn = 332177} Lists: 3::4::5::nil or [3,4]@[5] Datatypes Functions And more! Rather than list all the details, we will write a couple of programs

18. An interpreter Interpreters are usually implemented as a series of transformers: stream of characters abstract syntax lexing/ parsing evaluate abstract value print stream of characters

19. A little language (LL) An arithmetic expression e is a boolean value an if statement (if e1 then e2 else e3) an integer an add operation a test for zero (isZero e)

20. LL abstract syntax in ML datatype term = Bool of bool | If of term * term * term | Num of int | Add of term * term | IsZero of term -- constructors are capitalized -- constructors can take a single argument of a particular type type of a tuple another eg: string * char vertical bar separates alternatives

21. LL abstract syntax in ML Add (Num 2, Num 3) represents the expression “2 + 3” Add Num 23

22. LL abstract syntax in ML If (Bool true, Num 0, Add (Num 2, Num 3)) represents “if true then 0 else 2 + 3” Add Num 2 3 true BoolNum 0 If

23. Function declarations fun isValue t = case t of Num n => true | Bool b => true | _ => false function name function parameter default pattern matches anything

24. What is the type of the parameter t? Of the function? fun isValue t = case t of Num n => true | Bool b => true | _ => false function name function parameter default pattern matches anything

25. What is the type of the parameter t? Of the function? fun isValue (t:term) : bool = case t of Num n => true | Bool b => true | _ => false val isValue : term -> bool ML does type inference => you need not annotate functions yourself (but it can be helpful)

26. A type error fun isValue t = case t of Num _ => true | _ => false ex.sml:22.3-24.15 Error: types of rules don't agree [literal] earlier rule(s): term -> int this rule: term -> bool in rule: _ => false

27. A type error Actually, ML will give you several errors in a row: ex.sml:22.3-25.15 Error: types of rules don't agree [literal] earlier rule(s): term -> int this rule: term -> bool in rule: Successor t2 => true ex.sml:22.3-25.15 Error: types of rules don't agree [literal] earlier rule(s): term -> int this rule: term -> bool in rule: _ => false

28. A very subtle error fun isValue t = case t of num => true | _ => false The code above type checks. But when we test it refined the function always returns “true.” What has gone wrong?

29. A very subtle error fun isValue t = case t of Num 0 => 1 | Add(Num t1,Num t2) => t1 + t2 | _ => 0 The code above type checks. But when we test it refined the function always returns “true.” What has gone wrong? -- num is not capitalized (and has no argument) -- ML treats it like a variable pattern (matches anything!)

30. Exceptions exception Error of string fun debug s : unit = raise (Error s)

31. Exceptions exception Error of string fun debug s : unit = raise (Error s) - debug "hello"; uncaught exception Error raised at: ex.sml:15.28-15.35 in SML interpreter:

32. Evaluator fun isValue t =... exception NoRule fun eval t = case t of Bool _ | Num _ => t |...

33. Evaluator... fun eval t = case t of Bool _ | Num _ => t | If(t1,t2,t3) => let val v = eval t1 in case v of Bool b => if b then (eval t2) else (eval t3) | _ => raise NoRule end let statement for remembering temporary results

34. Evaluator exception NoRule fun eval1 t = case t of Bool _ | Num _ =>... |... | Add (t1,t2) => case (eval v1, eval v2) of (Num n1, Num n2) => Num (n1 + n2) | (_,_) => raise NoRule

35. Finishing the Evaluator fun eval1 t = case t of... |... | Add (t1,t2) =>... | IsZero t =>... be sure your case is exhaustive

36. Finishing the Evaluator fun eval1 t = case t of... |... | Add (t1,t2) =>... What if we forgot a case?

37. Finishing the Evaluator ex.sml:25.2-35.12 Warning: match nonexhaustive (Bool _ | Zero) =>... If (t1,t2,t3) =>... Add (t1,t2) =>... fun eval1 t = case t of... |... | Add (t1,t2) =>... What if we forgot a case?