Lecture 2: Basic Control and Procedures

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Lecture 2 Basic Control and Procedures

• Per Brand

2
Basic Control Structures

• Sequence of statements
• S1 S2
• The thread executes statements in a sequential
  order. However a thread, contrary to conventional
  languages, may suspend in some statement, so
  above a thread has to complete execution of S1,
  before starting S2.
• Empty statement
• skip

3
If Statement

• Oz provides a simple form of conditional
  statement having the following form
• if B then S1 else S2 end
• B should be a Boolean value.
• Semantics
• If B is bound to true S1 is executed
• if B is bound to false S2 is executed
• if B is bound to an non-Boolean value, an
  exception is raised
• otherwise if B is unbound the thread suspends
  until one of the cases above applies

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Comparison Procedures

• Oz provides a number of built-in tertiary
  procedures These include that we have seen
  earlier as well as \, lt, lt, gt, and gt.
• Used as Boolean functions in an infix notation.
• In this example Z is bound to the maximum of X
  and Y, i.e. to Y
• declare X Y ZX  5 Y  10  if X gt Y then Z  X
   else Z  Y end
•  

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Abbreviations

• A statement using the keyword elseif
• if B1 then S1 elseif B2 then S2 else S3 end
• is shorthand for nested if-statements
• if B1 then S1 
• else if B2 then S2     else S3 end
• end
• An if-statement missing the else part
• if B1 then S1 end
• is equivalent to
• if B1 then S1 else skip end

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Procedure Abstractions

• A procedure in Oz is a first class value
• can be defined.
• declare
• proc P X1 Xn S end
• can be applied
• P Y1 Yn
• passed around as argument to another procedure,
  e.g..
• Q P 1
• and, stored in an arbitrary data-structure, e.g.
  record.
• Xrec(P)

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Procedure Definition Application

• By example
• declare Max X Y Z proc Max X Y Z if X
  gt Y then Z X else Z Y end
  end X 5 Y 10 Max X Y Z Show Z
  shows 10

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Procedures contd

• Procedures dont return anything directly
• Return achieved by using single-assignment
  variables as parameters
• e.g. Max In1 In2 Return
• More general as procedures can return multiple
  values
• declare Two F S proc Two In First Second
  FirstIn.1 Second In.2 end Two f(a b) F
  S F is bound to a, S to b

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Procedures contd

• Observe convention
• put the output parameter(s) at the end
• Very often there is one and exactly one return
  value
• Oz supports functional declaration as syntactic
  sugar
• Recommendation use it for pure functions
• fun Max X Y procMax X Y Aux if X gt Y
  then X if X gt Y then AuxX else Y else
  AuxY end end end end

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1st class

• Procedures and everything else in Oz are first
  class
• declare Max Max proc X Y Z if X gt
  Y then Z X else Z Y end
  end
• P is bound to the procedure
• The procedure is in the store just like other
  data structures
• What can you do with P??
• Usual stuff
• Apply it

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Equality

• Equality and equality test - token or pointer
  equality
• records, numbers etc. have structural equality
• declare proc Max X Y Z if X gt Y then Z
  X else Z Y end end proc Max2 X Y
  Z if X gt Y then Z X else Z Y end
  end proc Min X Y Z if X gt Y then Z
  Y else Z X end end AliasMax Show
  MaxMin shows false Show MaxMax2
  shows false Show MaxAlias shows true

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Example procedures as parameters

• declare proc Twice Inc X Out local Aux
  in Max A B Aux Max Aux C Out end
  end Show Max3 1 5 4 shows 5

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Example procedures in data structure

• declare fun Max A B
• end fun Min A B
• end MyMathModulemath(maxMax minMin)
• declare Max Min MyMathModule.max 1 3 Max
  Max3 MyMathModule.min 1 3 Min Min1

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Example procedures returning procedures

• declare fun GenMax Less fun A B
• if Less A B then B
• else A end
• end
• end PGenMax fun A B AltB end Show
  P 5 3 Shows 5

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Lexical Scoping

• In the statement S some variable occurrences are
  syntactically bound while others are free.
• A variable occurrence X in S is bound
• If X is in the scope of the procedure
  formal-parameter X.
• Or is in the scope of a variable introduction
  statement, or other variable-binding construct,
  that introduces X.
• Otherwise, the identifier occurrence is free.
• Each free variable occurrence in an program is
  eventually bound by the closest textually
  surrounding identifier-binding construct.
• Lexical (static) scoping when free variables in
  procedure, function, or a class are bound at
  their definition position by the nearest
  enclosing definition
• Dynamic scoping otherwise, at the context of
  call, or location of a call

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Semantics proc P X1 Xn S end

• The variable P should be already introduced.
• Executing the above statement will create a
  procedure (closure) an abstraction ?(X1
  Xn).S.
• P is bound to the closure.
• A closure contains
• code
• references to those entities referenced by the
  free variables in the procedure.

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Closures

• ?(X1 Xn).S.
• Assume Y1 Yn occures free in S.

Y1 Yn
Y1 Yn
Environment
Code for S
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Example procedures returning procedures revisited

• declare procGenMax Less Aux1
  Aux1proc A B Aux2
• if Less A B then Aux2B in closure
• else Aux2A end
• end
• end
• PGenMax fun A B AltB end
• QGenMax fun A B end

Closure of GenMax contains the procedure Less
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Example closure containing data

• declare Mod procMyMath Out
• local Pi A C in
• Pi3.14159
• Outmath(areaA circC)
• Afun Rad PiRadRad
• Bfun Rad 2.0PiRad end end
• MyMath Mod
• Show Mod.area 3.0Mod.circ 3.0

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Closures

• Closures contain code arbitrary entites
• Most common
• other procedures
• stateful entities (dealt with in a later lecture)

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Anonymous Procedures and Variable Initialization

• Why is a variable bound to a procedure in a way
  that is different from the variable being bound
  to record
• X Value?
• What you see is just a syntactic variant of the
  equivalent form
• P proc X1 Xn S end
• The R.H.S. defines an anonymous procedural value.
  
• ?(X1 Xn).S

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variable-initialization equality

• Variables could be introduced and initialized
  simultaneously by
• X OzValue or Record OzValue
• between local and in, in a statement of the form
  local ... in ... end.
• local Max proc X Y Z if X gt Y
  then Z X else Z Y end end X 5 Y
  10 Zin Max X Y Z Show Zend

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Variable-initialization equality

• Only the variables occurring on the L.H.S. of the
  equality are the ones being introduced.
• local Y 1in local M f(M Y)
  X1 Y L L 1 2 in Show M L
  endend
• The outer variable Y is invisible in the
  innermost local ... end statement.

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Variable-initialization equality

• local Y 1in local M f(M Y)
  X1 Y L L 1 2 in Show M L
  endend

• local Y in Y 1 local M X1 Y L in M
  f(M Y) L X1 Y L 1 2
  Show M L endend

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The exclamation mark !

• Used to suppress variable introduction.
• An exclamation mark ! is only meaningful in the
  L.H.S. of an initializing equality and in pattern
  matching constructs.
• local Y 1in local M f(M Y)
  X1 !Y L L 1 2 in Show M L
  endend

• local Y in Y 1 local M X1 L in M
  f(M Y) L X1 Y L 1 2
  Show M L endend

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Variable declaration abbreviation

• local X in S end often abbreviated
• X in S
• procMyMath Out
• Pi A C in
• Pi3.14159
• Outmath(areaA circC)
• Afun Rad PiRadRad
• Bfun Rad 2.0PiRad
• implicit end here
• end

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Pattern Matching

• Case Statement
• case E of  Pattern1 then S1  Pattern2 then S
  2  ... else S end
• All variables introduced in Patterni are
  implicitly declared, and have a scope stretching
  over the corresponding Si.

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Pattern Matching Semantics

• E is evaluated to V.
• Executing the case statement will sequentially
  match V against the patterns Pattern1, Pattern2,
  ...,Patternn in this order.
• Matching V against Patterni is done in
  left-to-right depth-first manner.
• If V matches Patterni without binding any
  variable occuring in V, the corresponding Si
  statement is executed.
• If V matches Patterni but binds some variables
  occuring in V, the thread suspends
• If the matching of V and Patterni fails, V is
  tried against the next pattern Patterni1,
  otherwise the else statement S is executed.
• The else part may be omitted, in which case an
  exception is raised if all matches fail.
• Suppressing the introduction of a new local
  variable by using !. For example, in the
  following example
• case f(X1 X2) of f(!Y Z) then ... else ... end

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Case for pattern matching

• proc Insert Key Value TreeIn ?TreeOut case
  TreeIn of nil then TreeOut tree(Key Value
  nil nil) tree(K1 V1 T1 T2) then if
  Key K1 then TreeOut tree(Key Value
  T1 T2) elseif Key lt K1 then T in
  TreeOut tree(K1 V1 T T2) Insert Key
  Value T1 T else T in TreeOut
  tree(K1 V1 T1 T) Insert Key Value T2 T
  end endend

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Nesting

• Intermediate variables are introduced
• local T0 T1 T2 T3 in Insert a 3 nil T0
  Insert b 15 T0 T1 Insert c 10 T1 T2
  Insert d 7 T2 T3 end
• Oz provides syntactic support for nesting one
  procedure calls inside another
• local Y in P ... Y ... Q Y ... end
• could be written as
• Q P ... ... ...
• using as a nesting marker.

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Functional Nesting

• A procedure P X R is a function its result
  is the argument R.
• P X is a function call that can be inserted
  in any expression instead of the result argument.
  Q P X is equivalent to local R in
  P X ... R Q R ... endOur example, a
  more concise form using functional nesting is
• Insert d 7 Insert c 10 Insert
  b 15 Insert a 3 nil

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Nested Application

• Nested application inside a record or a tupleZs
  XSMerge Xr Ys
• The nested application goes after the record
  construction statement.local Zr in Zs XZr
  SMerge Xr Ys Zrend

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Example - merging two sorted lists

• proc SMerge Xs Ys Zs   case XsYs   of nilYs
   then ZsYs    Xsnil then ZsXs    (XXr) 
   (YYr) then       if XltY then          Zs  X
  SMerge Xr Ys       else Zr in          Zs  Y
  SMerge Xs Yr       end    end end

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Procedures as Values

• The procedure BinaryTree checks a structure to
  verify whether it is a binary tree or not, and
  accordingly returns true or false in its result
  argument B. What is a binary tree?local
  proc And B1 B2 ?B if B1 then B B2 else
  B false end endin proc BinaryTree T
  ?B case T of nil then B true
  tree(K V T1 T2) then And
  BinaryTree T1 BinaryTree T2
  B else B false end endend

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Problem and Fix

• The call
• And BinaryTree T1BinaryTree T2 B
• is certainly doing unnecessary work.
• According to our nesting rules, it evaluates its
• 2nd argument even if the first is false.

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Problem Fix

• The call And BinaryTree T1BinaryTree T2 B
  may be doing unnecessary work.
• According to our nesting rules, it evaluates its
  2nd argument even if the first is false.
• Define new procedure AndThen
• It takes as its first two arguments two
  procedures, and calls the second procedure only
  if the first returns false.
• The procedure is another example of a
  higher-order procedure

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Control Abstractions

• Higher-order procedures are used in Oz to define
  various control abstractions. In modules Control
  and List as well as many others, you will find
  many control abstractions.
• Syntactic support for iterators
• for ltiteratorsgt do ltSgt end
• where iterators are
• X in L iterates over the list L
• X in I..J iterates over the integers I to
  J, inclusive
• X in I..J K iterates over the integers I to
  J, inclusive with increments of K

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More higher order procedures

• proc ForAll Xs P case Xs of nil then
  skip XXr then P X ForAll Xr
  P endend
• ForAll 1 2 3 Show
• or use the below
• for X in 1 2 3 do Show end

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Example - iteration

• local proc HelpPlus C To Step P if
  CltTo then P C HelpPlus CStep To
  Step P end end proc HelpMinus C To
  Step P if CgtTo then P C
  HelpMinus CStep To Step P end endin
  proc For From To Step P if Stepgt0
  then HelpPlus From To Step P else
  HelpMinus From To Step P end endend For 3
  7 2 proc X Show X end orfor X in 3..72
  do Show X end

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Note about documentation

• Iterators are not mentioned in the Tutorial
• See changes under documentation