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Scope Rules. Programming Language Principles Lecture 16. Prepared by Manuel E. Bermúdez, Ph.D. Associate Professor University of Florida. Definition. Scope: the region of program text in which a binding holds. Static binding is prevalent in most modern PLs. New scope in each function.

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scope rules

Scope Rules

Programming Language Principles

Lecture 16

Prepared by

Manuel E. Bermúdez, Ph.D.

Associate Professor

University of Florida

definition
Definition
  • Scope: the region of program text in which a binding holds.
  • Static binding is prevalent in most modern PLs.
    • New scope in each function.
      • Activate bindings for parameters and locals.
      • Deactivate bindings of globals obscured by locals.
      • Deactivate bindings upon function return.
rpal is statically scoped
RPAL Is Statically Scoped

(CSE Rule 4):

CONTROLSTACK

__________________________________________________

ec ...  <λ,x,i,k> R ... ec

ec ... enδk en ...ec

__________________________________________________

where en = [R/x] ei. <--- defines lexical scoping !

rpal is statically scoped cont d
RPAL Is Statically Scoped (cont’d)
  • ei is the environment in effect when the lambda closure was created.
  • Change ei to ec (the current environment at the point of function application), and RPAL becomes dynamically scoped.
example
Example

let Sqr x = x * x

in

let f a b = Sqr a + b

in

Print(f 4 3 + (let Sqr x = x+x in f 4 3)) )

  • In RPAL, this program prints 19 + 19 = 38.
  • If RPAL used dynamic scoping, the result would be 19 + 11 = 30.
suggested exercise
Suggested Exercise
  • Run the CSE by hand, and see the environments.
  • Changing RPAL from lexical to dynamic scoping involves changing ONE letter !

(The power of formal semantic specification ...)

  • Some dynamically scoped languages: SNOBOL, APL, perl (?) early LISP (later changed by J. McCarthy).
terminology
Terminology
  • Referencing environment: the set of active bindings at a given point in a program's execution.
  • Deep binding: the reference environment of a fcn/proc is determined when the function is created.
  • Shallow binding: the reference environment of a fcn/proc is determined when the function is called.
structure of referencing environments
Structure of Referencing Environments
  • Normally, a stack suffices (C, C++, Java).
  • If a procedure (not merely a pointer) can be passed as a parameter, the referencing environments are organized as a tree.
example passing procedures as parameters in pascal
Example: Passing Procedures as Parameters in Pascal:

program test(output);

procedure dummy;

begin end;

procedure Q(procedure S; n:integer);

procedure R;

begin writeln(n) end;

begin { Q }

if n=1 then Q(R,0) else S;

end;

begin {main}

Q(dummy,1)

end.

The output of this program is ???

types of static scope
Types of Static Scope
  • Flat: (BASIC, Cobol)
    • All name sin the same scope, all visible everywhere.
  • Local/global (Fortan, C, Prolog)
    • Only two referencing environments: current and global.
types of static scope cont d
Types of Static Scope (cont’d)
  • Nested procedures (Algol, Pascal, Ada, Modula, RPAL).
    • Each procedure has its own scope, visible to itself and all procedures nested within it.
types of static scope cont d1
Types of Static Scope (cont’d)
  • Modular scope (Modula, Ada, C++, Java).

Scopes defined by modules, which explicitly export names to:

    • The global scope (public)
    • The scopes of subclasses (protected)
    • Nowhere (private)
nested scoping example in pseudo pascal
Nested Scoping Example (in pseudo-Pascal):

proc A;

var a;

proc B;

var b:

proc C;

var c;

begin

c:=b-1;

if b=1 then B

else a:=a-c;

end;{ C }

begin

b:=a-4;

if b<2 then C;

else a:=0;

end;{ B }

begin

a:=5;

B;

end;{ A }

A can only see a.

B can see a and b but not c.

C can see a, b, and c.

C can see many versions of b and a, depending on the calling sequence.

access to non locals
Access to non-locals
  • Needed to handle nested scoping.
  • Two methods:
    • Static links
    • Displays.
  • Will cover displays (and contrast them) later.
static link
Static Link
  • Pointer to the most recent instance of the next statically enclosing procedure (see diagram).
    • Each instance of B points to the most recent version of A
    • Each instance of C points to the most recent version of B
static link cont d
Static Link (cont’d)
    • Compiler keeps track of nesting levels:
      • variableais at level 1,
      • variablebis at level 2,
      • variablecis at level 3.
  • To access a variable, subtract the variable's nesting depth from the current depth, and follow that many static links.
examples
Examples
  • ForBto accessa, depth[B]-depth[a] = 2-1= 1; follow 1 static link.
  • ForCto accessb, depth[C]-depth[b] =

3-2= 1; follow 1 static link.

  • ForCto accessa, depth[C]-depth[a] =

3-1= 2; follow 2 static links.

block scoping
Block Scoping
  • In the C language, execution blocks,delimited with{},define scopes.

{

int t;

t = a;

a = b;

{

float t=3.14;

printf("%f",t);

}

b = t;

}

block scoping cont d
Block Scoping (cont’d)
  • The variable's scope is limited to the{}block that contains its declaration.
  • Handled like ordinary local variables:
    • Space allocated in subroutine prologue (frame setup)
    • Deallocated in subroutine epilogue (frame destruction).
    • Compiler's symbol table keeps track of declarations.
modular scoping
Modular Scoping
  • First languages to incorporate information hiding.
  • Reduces "cognitive load" on programmers.
  • CIA principle: if a code segment doesn't need to know about an object, it shouldn't.
  • Tends to narrow (simplify) interfaces.
  • Aids in abstraction.
modular scoping cont d
Modular Scoping (cont’d)
  • Modules allow encapsulation of objects.
    • Objects inside the module are visible to each other.
    • Objects inside are only visible outside if explicitly exported.
    • Objects outside not visible inside unless imported.
modular scoping cont d1
Modular Scoping (cont’d)
  • Various terms for modules:
    • Clu: clusters
    • Modula, Ada, Java: packages
    • C++: namespaces
example pseudo modula
Example (Pseudo Modula)

module Zip;

import A;

export D;

var t:real;

proc D();

...

end;

end Zip;

module Zap;

import b,D;

export Q;

proc Q();

...

end;

proc R();

...

end;

end Zap;

module Zorch;

export A,b;

var b:integer;

var c:real:

proc A();

...

end

end Zorch;

  • Procedure A can see b and c.
  • Procedure D can see A and t, but not b or c.
  • Procedures Q and R can see b and D, but not A, c, or t.
module types and classes
Module Types and Classes
  • In Modula-2, Turing, and Ada83, a module can define only one instance of the object (module as manager).
  • In Simula, Euclid, and ML, programmer can declare an arbitrary number of module objects (module as type).
module types and classes cont d
Module Types and Classes (cont’d)
  • Historically, this led to classes (Smalltalk, Eiffel, C++, Java), and object-orientation.
    • Classes define types (abstraction).
    • Classes encapsulate object data and methods (encapsulation).
    • Classes hide information (information hiding).
    • Classes facilitate polymorphism and inheritance (polymorphism).
symbol tables
Symbol Tables
  • Compiler must keep track of declared names (type, variables, constants, etc.)
  • Table stores mapping of names to attributes.
  • Must enforce scope rules. Typical operations:
    • Enter a new name, with relevant information (type, components, etc.)
    • Lookup a name. Must return correct instance, in accordance with scope rules.
symbol tables cont d
Symbol Tables (cont’d)
  • Open_scope. New scope must allow visibility of names in outer scopes, and redeclaration of names in new scope.
  • Close_scope. Probably non-destructively, without reclaiming space (need debugging info). Restore mapping of previous scope.
symbol tables cont d1
Symbol Tables (cont’d)
  • Must allow forward declarations (name used before it is declared)
  • Hash coded table, each entry a linked list of hash synonyms.
example1
Example
  • LeBlanc-Cook symbol table (see text book)
    • Intrinsics added first (integer, real).
    • Situation depicts environment inwithstatement.
    • A2, F2andTare synonyms.
example cont d
Example (cont’d)
  • Other mechanisms: Central Reference Tables.
    • Used to keep track of run-time environments.
    • Equivalent to RPAL's environment trees.
binding of reference environments
Binding of Reference Environments
  • Needed when passing a function/proc as a parameter.
  • Need subroutine closures: 4 items.
    • The fact that it is a function.
    • Functionality (prototype, parameters, etc.)
    • Pointer to body of fcn/proc.
    • Referencing environment (environment link in RPAL).
binding of reference environments cont d
Binding of Reference Environments (cont’d)
  • In C, functions are second-class.
    • Can only pass function pointer (item 3) as a parameter.
  • Fcns/procs are first class in Lisp, ML, Haskell, RPAL, but not in Modula, Ada, C, C++, or Java. Some would argue they are, but ...
overloading
Overloading
  • Multiple meanings for the same name, context dependent.
  • Most languages allow it in some limited fashion, e.g. arithmetic operators

(a+bfor ints and reals).

  • In C++, one can overload both functions and operators. Can only overload fcns in Java (may change, more later).
pitfalls in language design
Pitfalls in Language Design
  • In Pascal, functions return values by assigning to the function name (no return statement). If the function name is redeclared, it becomes impossible to return a value.
  • Example:

function f: integer;

var f:integer;

begin

f := 10; // won't return 10

end;

pitfalls in language design cont d
Pitfalls in Language Design (cont’d)
  • Using an outer scope name, and then declaring it (maybe much later), is illegal in Pascal. Very expensive to check.

const m = 0;

procedure f;

const n = m; { illegal use of m, }

{ m not yet declared.}

...

m = 0; { scope of m is all }

{ of f }

end;

pitfalls in language design cont d1
Pitfalls in Language Design (cont’d)
  • The same applies to functions:

procedure f;

...

end;

procedure A;

procedure B;

begin

f; {intent is to call }

{outer f. }

end; {error: f used before}

{it's declared }

...

procedure f;

...

end;

end;

pitfalls in language design cont d2
Pitfalls in Language Design (cont’d)
  • Some Pascal compilers (and most Pascal successors) specify that the scope of a declaration is limited to the remainder of (not the entire) block.
  • Confusingly, Pascal allows use of pointer types before they are declared:

type

aptr = ^a;

a = record

data: ...;

next: aptr;

end;

pitfalls in language design cont d3
Pitfalls in Language Design (cont’d)
  • Modula-3 changed this: scope of name is still the entire block, but names need not be declared before they are used.
  • C, C++, Java have a mixed strategy: variables must be declared before they are used, but not type (class) names. Class names are visible over the entire containing module (package).
  • Separate compilation (read in book).
scope rules1

Scope Rules

Programming Language Principles

Lecture 16

Prepared by

Manuel E. Bermúdez, Ph.D.

Associate Professor

University of Florida

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