Remaining Features of SML97

1
Remaining Features of SML97

• Records
• Exceptions
• Reference Types
• Arrays

2
Records

• type component
• key int, info string
• val part key 3, info nail
• ( val part info"nail",key3
  infostring, keyint )
• key part 3
• part info na il, key 3-0
• ( val it true bool )
• Order of fields is irrelevant when matching.
• (2, two) 1 2, 2 two
• Tuples are special records with numbers as field
  labels.

3
Pattern Matching for Records

• fun info_compare (p1component) (p2component)
• String.compare (info p1, info p2)
• fun key_compare (keyk1,info_component)
• (keyk2, ... component)
• Int.compare (k1, k2)
• ( Wildcards Type name mandatory with
  ellipses. )
• fun check_info (c as key,infocomponent)
• (info c info)
• ( Using a field name to refer to value
  )
• ( maps a component to true
  )

4
Exceptions

• An exception name is a constructor of the
  built-in type exn. This datatype is special in
  that the set of constructors can be extended.
• - exception Overflow
• - exception message of string
• - exception outOfRange of intint
• - Overflow ( val it Overflow(-) exn
  )
• - message ( val it fn string -gt exn
  )
• - outOfRange ( val it fn int int -gt
  exn )
• Predefined exceptions Div, Empty, Match,... etc.

5
Operations on exception

• raise exn-name
• expression handle exn_name1 gt
• exn_name2 gt

Organization
CLIENT
SERVER
Define exception
Trigger exception
Raise exception
Handle exception
6
Motivation for incorporating exception

• Robustness
• Language/application specified behavior on error
• Context-sensitive handling
• Retry
• to overcome transient errors
• Attempt
• another solution strategy (heuristic) on failure
• Rescue
• reinstate any required invariant (consistency)
• Dynamic Handling via run-time call stack

7
Examples

• exception fail of string
• fun search name raise (fail name)
• search name ((x,v)xvs)
• if (name x) then v
• else search name xvs
• datatype employers
• wsu wpafb ncr lexis_nexis
• val name_emp (john,wsu),(jane,ncr)
• (search jill name_emp)
• handle (fail elem) gt wpafb

Server
Client
8
(contd)

• val name_car
• (john, acura),(jane, lexus)
• (search jill name_car)
• handle (fail elem) gt
• elem owns Dodge Stealth!
• val name_tel
• (john,8501),(jane,1359)
• (search jill name_tel)
• handle (fail elem) gt
• elem s phone number is unlisted.
• ( type error )

9
References and Assignments
10

• (ML) Redefinition
• val x 5 val x 2
• ( The first list is inaccessible (garbage).
  )
• (ML) Shadowing
• let val x 5 in
• (let val x 2 in (hd x) end)
• x
• end
• (Pascal) Assignment
• var i int
• begin i 0 i i 1 end
• Redefinition and shadowing are different
  from assignment.

11
References

• ML supports a separate sub-language to deal with
  variables and assignments (that can change value
  bound to a variable).
• val x ref 5
• x 6
• x 1 !x
• l-value r-value
• (address of location) (contents
  of location)

5
x
12
Iteration Pascal vs ML

• var i, sum int 0
• var n int 5
• while i lt n do
• begin
• i i 1
• sum sum i
• end
• The l-value of a variable on the rhs of an
  assignment is automatically coerced to its
  r-value.

• val i ref 0
• val sum ref 0
• val n 5
• while !i lt n do
• ( i !i 1
• sum !sum !i
• )
• A reference variable on the rhs of an
  assignment must be explicitly dereferenced, to
  get its r-value.

13
Aliasing problem

• fun rot3 (a,b,c)
• let val t !a in
• a !b b !c c t
• end
• val i ref 0 val j ref 1 val k ref 2
• (!i,!j,!k) rot3 (i,j,k) (!i,!j,!k)
• ( (0,1,2) , () unit,
  (1,2,0) )
• val i ref 0 val j ref 1 val k ref 2
• (!i,!j,!i) rot3 (i,j,i) (!i,!j,!i)
• ( (0,1,0) , () unit,
  (0,1,0) )

14
Encapsulating State Objects

• fun new_counter ()
• let
• val cnt ref 0
• fun tick () (
• cnt !cnt 1
• !cnt
• )
• fun reset () (
• cnt 0
• )
• in
• tick tick,
• reset reset
• end

• val c1 new_counter()
• val c2 new_counter()
• (tick c1) () ( 1 )
• (tick c2) () ( 1 )
• (tick c1) () ( 2 )
• (tick c2) () ( 2 )
• (reset c1) ()
• (tick c1) () ( 1 )
• (tick c2) () ( 3 )
• ( Object-based programming )

15
Mutable Data Structure structure Array

• open Array
• val n 3
• val M
• array(n, array(n,0))
• ( Constructs array whose top-level elements
  share the same inner array. )
• val i ref 0
• while (!i lt n) do (
• update(M,!i,array(n,0))
• i !i 1
• )
• ( creates n x n locations initialized to 0 )

• i 0
• val j ref 0
• while (!i lt n) do (
• while (!j lt n) do (
• update(sub(M,!i),!j,!i !j)
• j !j 1
• )
• j 0
• i !i 1
• )
• ( initializes Mi,j to ij. )
• ( M 0,1,2,1,2,3,2,3,4 int
  array array )

16
Interaction with polymorphism

• Normally, variables/functions can be
  polymorphic, but not values.
• fun id x x
• id 5
• ( (fn x gt (id x)) 5 )
• ( (fn f gt (f 5)) id )
• ( (fn f gt (f 5)) (int -gt 'a) -gt 'a
  )
• (id 5, id true)
• ( (fn f gt (f 5, f true)) id )
• ( Type error )

17
Value Restriction for polymorphism

• Polymorphic references are banned.
• val fp ref id
• ( potential type
• a-gta ref )
• fp not
• !fp 5

• Polymorphic exceptions are banned.
• exception
• fail of a
• (raise fail true) handle
• (fail x) gt 0 x