Lecture 3

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Administrative

1. check the class bulletin bboard regularly for announcements, and
   Q&As about assignments.
  - 110.07 version of SML is fine
  - logon to mlb.acad.ece.udel.edu to use sml
  - email the intructor to get your class id

2. any feedback so for?


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ML (continued)

> Define your own datatype

 e.g., 
  - datatype IntOrBool = Int of int | Bool of bool

  - [Int 5, Bool true]
  val it = ... : IntOrBool list

 e.g., 
  - datatype RegExp = Empty
                    | Char of char
                    | Alt of RegExp * RegExp
                    | Seq of RegExp * RegExp
                    | Repeat of RegExp

 Naming conventions: Capitalize value constructors to distinguish
   them from value variables.

 Write a function "sample" to take a regular expression exp and 
 return a string matching that regular exp

 - fun sample Empty = ""
     | sample (Char c) = str c
     | sample (Alt (r1,r2)) = sample r1
     | sample (Seq (r1,r2)) = sample r1 ^ sample r2
     | sample (Repeat r) = "";
  or using wildcard
     | sample (Repeat _) = ""; 

 Note: pattern matching is used extensively on datatype
                
 An application of function "sample":

 - sample (Seq(Char #"a", Char #"b"))

   val it = "ab"

 Datatypes are recursive by default.

 e.g.,

 datatype 'a tree = Lf
                  | Br of 'a * 'a tree * 'a tree;


> Exception

  - fun f m n = 1 + m div n
        handle Div => 0
       | Overflow => 1

> How to define your own Exception?
 An exception name in SML is a constructor of the built-in type exn.

 e.g., exception Foo

 Contructors can also be functions:
 e.g., exception Badvalue of int;

 Whenever possible, declare exceptions at top level.

> Raising exceptions

 - fun f m n = (1 + m div n)
       handle Div => raise Foo

  1 + raise Foo

 any raise Exception is treated as 'a when ML does type checking.

 Infinite loop

 e.g., 
  - fun f x = f x
     'a -> 'b
> Standard exceptions

  Overflow
  Div
  Size
  Fail

> Handling exceptions

 E handle P1 => E1 | ... | Pn => En

  - fun f m n = 1 + m div n
        handle Div => 0
       | Overflow => 1