Programming Language Concepts CIS 280

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Programming Language Concepts (CIS 280)

• Elsa L Gunter
• NJIT
• Spring 2001

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• Compiler is on the AFS system at
• /usr/local/sml/bin/sml
• A (possibly better, non-PowerPoint) text version
  of this lecture can be found at
• http//www.cis.njit.edu/elsa/280/SML_Lect/sml-she
  ll
• For the SML code for todays lecture see
• http//www.cis.njit.edu/elsa/280/SML_Lect/intro.s
  ml

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WWW Addresses for SML

• http//www.cis.njit.edu/elsa/280/110-smlnj.exe
• ftp//ftp.research.bell-labs.com/dist/smlnj/releas
  e/110/110-smlnj.exe
• http//cm.bell-labs.com/cm/cs/what/smlnj/index.htm
  l
• http//cm.bell-labs.com/cm/cs/what/smlnj/doc/basis
  /pages/sml-std-basis.html

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Books on SML

• Textbook, Appendix A, Section A 7
• Additional texts (not required)
• Elements of ML Programming, by Jeffrey D. Ullman,
  on Prentice Hall
• ML for the Working Programmer, by Lawrence C.
  Paulson, on Cambridge University Press

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Session in SML

• norfolk sml
• Standard ML of New Jersey, Version 110.0.6,
  October 31, 1999 CM autoload enabled
• ( Read-eval-print loop expressions and
  declarations )
• - 2 3
• val it 5 int
• - val test 3 lt 2
• val test false bool

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SML Expressions and Declarations

• - (
• At top-level, an expression
• ltexpgt
• is treated as an abbreviation for the
  declaration
• val it ltexpgt
• )
• - print "Hello world\n"
• Hello world
• val it () unit

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SML Top-level Expressions and Declarations

• - "Hi there"
• val it "Hi there" string
• - ( is string concatenation )
• - it " my good friend"
• val it "Hi there my good friend" string

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Overloading for Basic Arithmetic

• - val x 5 7
• val x 12 int
• - val y x 2
• val y 24 int
• - val z 1.35 0.23
• val z 1.58 real

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Overloading but No Coercion

• - val w y z
• stdIn51.1-51.14 Error operator and operand
  don't agree tycon mismatch
• operator domain int int
• operand int real
• in expression
• y z

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Using SML Code From a File

• File named test.sml contains
• 3 2
• val x 5 7
• val y x 2
• val z 1.35 0.23
• val w y z

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• - use "A\\SML_Lect\\test.sml"
• opening A\SML_Lect\test.sml
• val it 5 int
• val x 12 int
• val y 24 int
• val z 1.58 real
• A\SML_Lect\test.sml5.1-5.14 Error operator and
  operand don't agree tycon mismatch
• operator domain int int
• operand int real
• in expression
• y z

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Booleans (aka Truth Values)

• - true
• val it true bool
• - false
• val it false bool
• - if y gt x then 25 else 0
• val it 25 int

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Booleans

• - 3 gt 1 andalso 4 gt 6
• val it false bool
• - 3 gt 1 orelse 4 gt 6
• val it true bool
• - not (4 gt 6)
• val it true bool

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Functions

• - fun plus_two n n 2
• val plus_two fn int -gt int
• - plus_two 17
• val it 19 int
• - val plus_two fn n gt n 2
• val plus_two fn int -gt int
• - val plus_two fn n gt n 2
• val plus_two fn int -gt int
• - plus_two 14
• val it 16 int

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Values fixed at declaration time

• - val x 12
• val x 12 int
• - fun plus_x y y x
• val plus_x fn int -gt int
• - plus_x 3
• val it 15 int
• - val x 7
• val x 7 int
• - plus_x 3
• val it 15 int

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Functions as arguments and results

• - fun thrice f x f (f (f x))
• val thrice fn ('a -gt 'a) -gt 'a -gt 'a
• - thrice plus_two
• val it fn int -gt int
• - it 4
• val it 10 int
• - thrice (fn s gt "Hi! " s) "Good-bye!"
• val it "Hi! Hi! Hi! Good-bye!" string

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Recursive Functions

• - fun factorial 0 1
• factorial n n factorial (n - 1)
• val factorial fn int -gt int
• - factorial 5
• val it 120 int
• fun is needed for recursion function
  declarations

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Tuples

• - val s (5,"hi",3.2)
• val s (5,"hi",3.2) int string real
• - val (a,b,c) s
• val a 5 int
• val b "hi" string
• val c 3.2 real

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Tuples

• - val d ((1,4,62),("bye",15),73.95)
• val d ((1,4,62),("bye",15),73.95) (int int
  int) (string int) real
• - fun fst_of_3 (x,_,_) x
• val fst_of_3 fn 'a 'b 'c -gt 'a
• - fst_of_3 s
• val it 5 int
• - fst_of_3 d
• val it (1,4,62) int int int

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Records

• - val teacher Name "Elsa L. Gunter", ss
  (119,73,6244), age 102
• val teacher Name"Elsa L. Gunter",age102,ss(1
  19,73,6244)
• Namestring, ageint, ssint int int
• - val ss SocSec, Name elsa, age years
  teacher
• val elsa "Elsa L. Gunter" string
• val years 102 int
• val SocSec (119,73,6244) int int int

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Records

• - val r ss teacher
• val r (119,73,6244) int int int
• - val Name,... teacher
• val Name "Elsa L. Gunter" string

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Records

• - val q (280,student Name "Joseph
  Martins",
• ss (325,40,1276),
• age 19,
• instructor teacher)

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Records

• val q
• (280,
• instructorName"Elsa L. Gunter",age102,ss(
  119,73,6244),
• studentName"Joseph Martins",age19,ss(325,
  40,1276))
• int
• instructorNamestring, ageint, ssint
  int int,
• studentNamestring, ageint, ssint
  int int

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Tuples are Records

• - val strange (1,"f",2) 3 2, 2 "f", 1
  1
• val strange true bool
• - val h 2 s
• val h "hi" string

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Records and Pattern-matching

• - ( All field names must be knowable when
  pattern-matching against a record )
• - fun name Name n,... n
• stdIn94.1-94.28 Error unresolved flex record
• (can't tell what fields there are besides
  Name)
• - fun name Name n, age _, ss _ n
• val name fn Name'a, age'b, ss'c -gt 'a
• - name (student(2 q))
• val it "Joseph Martins" string

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Recursive Types - Lists

• - val fib5 8,5,3,2,1,1
• val fib5 8,5,3,2,1,1 int list
• - val fib6 13 fib5
• val fib6 13,8,5,3,2,1,1 int list
• - (853211) fib5
• val it true bool
• - fib5 _at_ fib6
• val it 8,5,3,2,1,1,13,8,5,3,2,1,... int list

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Functions Over Lists

• - fun double_up
• double_up (x xs) (x x double_up
  xs)
• val double_up fn 'a list -gt 'a list
• - val fib5_2 double_up fib5
• val fib5_2 8,8,5,5,3,3,2,2,1,1,1,1 int list
• - val silly double_up "hi", "there"
• val silly "hi","hi","there","there" string
  list
• - fun rev
• rev (xxs) rev xs _at_ x
• val rev fn 'a list -gt 'a list

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Functions Over Lists

• - fun map f
• map f (ht) (f h) (map f t)
• val map fn ('a -gt 'b) -gt 'a list -gt 'b list
• - map plus_two fib5
• val it 10,7,5,4,3,3 int list
• - map (fn x gt x - 1) fib6
• val it 12,7,4,2,1,0,0 int list

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Scoped Bindings

• - fun fib_seq 0 1
• fib_seq 1 1,1
• fib_seq n
• let val result as (m1m2_) fib_seq (n
  - 1)
• in (m1 m2)result
• end

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Scoped Bindings

• stdIn121.9-121.52 Warning binding not
  exhaustive
• result as m1 m2 _ ...
• val fib_seq fn int -gt int list
• - fib_seq 6
• val it 13,8,5,3,2,1,1 int list

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User-defined Datatypes

• - datatype weekday Monday Tuesday Wednesday
• Thursday Friday Saturday Sunday
• datatype weekday
• Friday Monday Saturday Sunday
  Thursday Tuesday Wednesday

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Functions Over Datatypes

• - fun day_after Monday Tuesday
• day_after Tuesday Wednesday
• day_after Wednesday Thursday
• day_after Thursday Friday
• day_after Friday Saturday
• day_after Saturday Sunday
• day_after Sunday Monday
• val day_after fn weekday -gt weekday

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Functions Over Datatypes

• - fun days_later 0 day day
• days_later n day
• if n gt 0
• then day_after (days_later (n - 1) day)
• else days_later (n 7) day
• val days_later fn int -gt weekday -gt weekday
• - days_later 1 Wednesday
• val it Tuesday weekday
• - days_later 4 Monday
• val it Thursday weekday

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More Datatypes

• - datatype sum_tokens
• Int_token of int Plus Lparen Rparen
• datatype sum_tokens Int_token of int Lparen
  Plus Rparen
• - datatype 'a option NONE SOME of 'a
• datatype 'a option NONE SOME of 'a

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Functions Over Datatypes

• - fun first_int NONE
• first_int (Int_token n _) SOME n
• first_int (_ rest) first_int rest
• val first_int fn sum_tokens list -gt int option

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Functions Over Datatypes

• - val expression Lparen, Int_token 5, Plus,
  Int_token 0, Rparen,
• Plus, Int_token 3, Plus, Int_token 7
• val expression
• Lparen,Int_token 5,Plus,Int_token
  0,Rparen,Plus,Int_token 3,Plus,
• Int_token 7 sum_tokens list
• - first_int expression
• val it SOME 5 int option

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Functions Over Datatypes

• - fun stream_of_list list
• let val token_list ref list
• in fn () gt (case !token_list of
• gt NONE
• tok rest gt (token_list
  rest SOME tok))
• end
• val stream_of_list fn 'a list -gt unit -gt 'a
  option
• - val token_stream stream_of_list expression
• val token_stream fn unit -gt sum_tokens option

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Exceptions

• - exception Cant_handle of string
• exception Cant_handle of string
• - fun head raise Cant_handle "head"
• head (htl) h
• fun tail raise Cant_handle "tail"
• tail (_tl) tl
• val head fn 'a list -gt 'a
• val tail fn 'a list -gt 'a list

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Exceptions

• - fun fib_seq 0 1
• fib_seq 1 1,1
• fib_seq n
• if n lt 0 then raise Cant_handle "fib_seq"
• else
• let val result as (m1m2_) fib_seq
  (n - 1)
• in (m1 m2)result
• end

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Exceptions

• stdIn130.6-130.49 Warning binding not
  exhaustive
• result as m1 m2 _ ...
• val fib_seq fn int -gt int list
• - fib_seq (15)
• uncaught exception Cant_handle
• raised at stdIn128.25-128.46

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Exceptions

• - fib_seq (2) handle Cant_handle _ gt
• val it int list
• - fun new_fib_seq n fib_seq n handle
  Cant_handle "fib_seq" gt
• val new_fib_seq fn int -gt int list
• - new_fib_seq (3)
• val it int list

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Exceptions

• - exception Cant_happen
• exception Cant_happen
• - fun fib_seq 0 1
• fib_seq 1 1,1
• fib_seq n
• if n lt 0 then raise Cant_handle "fib_seq"
• else
• (case fib_seq (n - 1) of
• result as (r1 r2 _) gt r1 r2
  result
• _ gt raise Cant_happen)
• val fib_seq fn int -gt int list

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Side Effects and Imperative Programming

• - local
• val cell ref 0
• in
• fun get_count ()
• let val count !cell
• val _ cell count 1
• in count
• end
• end

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Side Effects and Imperative Programming

• val get_count fn unit -gt int
• - get_count()
• val it 0 int
• - get_count()
• val it 1 int
• - get_count()
• val it 2 int
• ( Example of an integer stream )

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Side Effects and Imperative Programming

• - local
• val cell ref (1)
• in
• fun get_count ()
• (cell ((!cell) 1)
• !cell)
• end
• val get_count fn unit -gt int

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Side Effects and Imperative Programming

• - get_count()
• val it 0 int
• - get_count()
• val it 1 int
• - get_count()
• val it 2 int

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Side Effects and Imperative Programming

• - local
• val cell ref (1)
• in
• fun get_count ()
• let val _ cell !cell 1
• in !cell
• end
• end
• val get_count fn unit -gt int