CS 334 Lecture 13

1. Call by Reference (FORTRAN, Pascal):
2. Call by Copy (Algol 60, Pascal, C, etc.):
3. Call by Name (Algol-60)
4. Procedures and functions as parameters and return values
1. Two major problems which arise with subprograms:
   1. Side-effects:
   2. Aliasing:
2. Correspondence Principle
3. Problems with writing large programs:
   1. Characteristics of solution:
4. Abstract Data Types
   1. Specification:
   2. Implementation (Representation):
   3. Simula 67
      1. Intermezzo: What are problems with defining a new type in terms of an old?
   4. Ada (~1979)

1. Call by Reference (FORTRAN, Pascal):

2. Call by Copy (Algol 60, Pascal, C, etc.):

3. Call by Name (Algol-60)

Ex.

        Procedure  swap(a, b : integer);
            var temp : integer;
            begin
                temp := a;
                a := b;
                b := temp
            end;

swap(i, a[i]) with i = 1, a[1] = 3, a[3] = 17.

No way to define a correct swap in Algol-60!

Expressive power - Jensen's device:

n

To compute x = Sum for i=1 to n of V_i

    real procedure SUM (k, lower, upper, ak);
        value lower, upper;     
        integer k, lower, upper;
        real ak;
        begin
            real s;
            s := 0;
            for k := lower step 1 until upper do
                s := s + ak;
            sum := s
        end;

What about sum(i, 1, m, sum(j, 1, n, B[i,j]))?

If evaluating parameters has side-effects (e.g., read), then must know how and how many times parameter is evaluated to predict what will happen.

Therefore try to avoid call-by-name with expressions with side-effects.

Lazy evaluation is efficient implementation of call-by-name where only evaluate parameter once. Requires that there be no side-effects, since owise get diff. results.

Implement call-by-name using thunks - procedures which evaluate expressions - difficult and slow. Must pass around code for evaluating expression (including environment defined in). Can use the same THUNK's as show up in environment based interpreter.

Note different from call-by-text (which would allow capture of free vbles).

Can classify parameter passing by copying (value, result, or value-result) or definitional.

Definitional have constant, variable, procedural, and functional.

Constant parameters are treated as values, not variables - different from call-by-value.
Default for Ada in parameters.

Can think of call-by-name as definitional with expression parameter.

Note that difference in parameter passing depends on what is bound (value or address) and when it is bound.

Procedures and functions as parameters and return values

When pass function (or procedure) parameters in stack-based languages, must also pass the equivalent of a closure. In particular must pass the environment in which the function was defined. This is accomplished by passing the appropriate static pointer with the function so that can find non-local variables. Usually pass the pair (ep,ip) of environment pointer and instruction pointer as the "closure" of a procedure, when it is passed as a parameter.

Returning functions from functions is harder since defining environment may go away:

program ret;

function a(): function (integer): integer;
	var m: integer;
	
	function addm (n: integer): integer;
		begin
			return (n + m)
		end;

	begin (* a *)
		m := 5;
		return addm
	end; (* a *)

procedure b (g: function(integer): integer);
	begin (* b *)
		writeln(g(2))
	end (* b *)

begin (* main *)
	b(a())		(* note that a() returns a function, which is 					
	               then passed to b *)
end.

Two major problems which arise with subprograms:

aliasing

Side-effects:

Often happens with global vbles

Also call by reference parameters, very dangerous in call-by-name.

Very disturbing in functions since can make it hard to figure out values of expressions. Example:

        A[f(j)] := j * f(j) + j 

Aliasing:

Most common ways of arising: global and parameter, two parameters, pointers

Example:

        Procedure swap( var x, y: integer);
        begin   
            x := x + y;
            y := x - y;
            x := x - y
        end;

Tricky way of completing swap of x and y w/out extra space.

Doesn't always work - swap (a,a) (but does work with value-result! )

Can get similar probs with A, A[i] as parameters and pointers

Another problem: Overlap btn global vble and by-reference parameter.

Causes problems with correctness since any two vbles may refer to the same object.

Also makes it difficult to optimize if can't predict when a vble might be changed.

If no aliasing, can't detect difference btn call-by-reference and call-by-value-result.

• But not semantically equivalent if aliasing is possible.

(Illegal program if it makes a difference - but not detectable!)

Unfortunately Ada doesn't enforce no aliasing.

Therefore possible problems with in out parameters.

Euclid (variant of Pascal) designed to write verifiable programs.

• Attempted to eliminate aliasing.

• Unfortunately some can only be caught at run-time, e.g. p(A[i], A[j]). Legality assertions generated to check run-time problems.

• Global vbles had to be explicitly imported to avoid problems

  i.e. treated as implicit parameters

Correspondence Principle

• constant parameter: constant def - value bound to identifier.

• variable parameter: variable renaming definition - new name given to old variable

• value parameter: new variable declaration w/ initialization

• procedure parameter: procedure declaration

E.g., constant, variable (def. & declaration), procedure & function, type(?)

Problems with writing large programs:

1. Side effects - hidden access

2. Indiscriminant access - can't prevent access - may be difficult to make changes later

3. Screening - may lose access via new declaration of vble

4. Aliasing - control shared access to prevent more than one name for reference variables.

Characteristics of solution:

1. No implicit inheritance of variables

2. Right to access by mutual consent

3. Access to structure not imply access to substructure

4. Provide different types of access (e.g. read-only)

5. Decouple declaration, name access, and allocation of space. (e.g. scope indep of where declared, similarly w/allocation of space - like Pascal new)

Abstract Data Types

Package data structure and its operations in same module - Encapsulation

Data type consists of set of objects plus set of operations on the objects of the type (constructors, inspectors, destructors).

Want mechanism to build new data types (extensible types).

Should be treated same way as built-in types.

Representation should be hidden from users (abstract).

Users only have access via operations provided by the ADT.

Distinguish between specification and implementation.

Specification:

Method for defining data type and the operations on that type (all in same place). The definitions should not depend on any implementation details. The definitions of the operations should include a specification of their semantics.

Provides user-interface with ADT.

Typically includes

1. Data structures: constants, types, & variables accessible to user (although details may be hidden)

2. Declarations of functions and procedures accessible to user (bodies not provided here).

Ex: pop(push(S,x)) = S,

if not empty(S) then push(pop(S), top(S)) = S

Data + Operations (+ possibly equations) = Algebra

Implementation (Representation):

Method for collecting the implementation details of the type and its operations (in one place), and of restricting access to these details by programs that use the data type.

Usually not accessible to user.

Provides details on all data structures (including some hidden to users) and bodies of all operations.

Note that ADT methodology is orthogonal to top-down design

• Partition first into modules corresponding to ADT's.

• Use top-down within ADT's and in larger programs using ADT's

How to represent ADT's in programming languages?

Three predominant concerns in language design:

• Simplicity of design

• Application of formal techniques to specification & verification

• Keep down lifetime costs

Reusable modules to represent ADT's quite important.

• Separate (but not independent) compilation.

• Want to maintain type checking

• Control over export and import of names (scope)

Examine implementation in Simula 67, Ada, Modula 2, Clu, and ML..

Simula 67

Derived from Algol 60. Simulation language.

Provided notion of class.

• Each kind of object being simulated belongs to a class.
• Objects called class instances.
• Class similar to type but includes procedures, functions, and variables.

class vehicle(weight,maxload);
    real weight, maxload;
begin
    integer licenseno;      (* attributes of class instance *)
    real load;
    Boolean procedure tooheavy;
        tooheavy := weight + load > maxload;
    load := 0;      (* initialization code *)
end

    ref(vehicle) rv, pickup;
    rv1:- new vehicle(2000,2500);
    pickup:- rv1;       (* special assignment via sharing *)
    pickup.licenseno := 3747;
    pickup.load := pickup.load +150;
    if pickup.tooheavy then ...

Representation not hidden.

Come back to discuss subclasses later when discussing object-oriented languages.

Intermezzo: What are problems with defining a new type in terms of an old?

1. Representation might have several values that do not correspond to any values of the desired type (e.g., (3,0)).

2. Representation might have multiple values corresponding to the same abstract value (e.g., (1,2), (2,4), etc.)

3. Values of the new type can be confused with values of the representation type.

Abstract data type is one that is defined by group of operations (including constants) and (possibly) a set of equations. Set of values only defined indirectly as those values which can be generated by ops, starting from constructors or constants.

E.g., Stack defined by EmptyStack, push, pop, top, and empty operations and equations.

   pop(push(fst,rest)) = rest, 
   top(push(fst,rest)) = fst, 
   empty(EmptyStack) = true, 
   empty(push(fst,rest)) = false, 
   etc.

Ada (~1979)

Packages used to define abstract data types.

Package together type, operations (& state) and hide rep.

Provides support for parameterized packages (polymorphism)

package <package-name> is
    --  declarations of visible types, variables, constants, and subprograms
private
    --  complete definitions of private types and constants
end <package-name>;

package body <package-name> is
    -- definitions of local variables, types, and subprograms, and complete bodies for 
    -- subprograms declared in the specification part above.  Code for initialization
    -- and exception handlers
    end <package-name>;

package VECT_PACKAGE is  -- declarations only
    type REAL_VECT is array (INTEGER range <>) of float;
    function SUM(V: in REAL_VECT) return FLOAT;
    procedure VECT_PRODUCT(V1,V2 : in REAL_VECT) return                                 FLOAT;
    function MAX(V: in REAL_VECT) return FLOAT;
end VECT_PACKAGE ;

package body VECT_PACKAGE is  -- details of implementation
    function SUM(V: in REAL_VECT) return FLOAT is
        TEMP : FLOAT := 0.0;
    begin
        for I in V'FIRST..V'LAST loop
            TEMP:= TEMP + V(I);
        end loop;
        return TEMP;
    end;
        -- definitions of VECT_PRODUCT and MAX subprograms would appear here
end VECT_PACKAGE ;

with VECT_PACKAGE, TEXT_IO; -- used to make separately compiled package visible
procedure MAIN is
    use VECT_PACKAGE, TEXT_IO;  -- eliminates need for qualifiers
    package INT_IO is new INTEGER_IO(INTEGER); --instantiation of generic packages
    package REAL_IO is new FLOAT_IO(FLOAT);
    use INT_IO, REAL_IO;
    K: INTEGER range 0..99;
begin
    loop
        GET(K);
        exit when K<1;
        declare         -- start of block
            A : REAL_VECT(1..K);  -- provides subscript bounds
        begin
            for J in 1..K loop
                GET(A(J));
                PUT(A(J));
            end loop;
            PUT("SUM = ");
            PUT(SUM(A));      -- uses package function
        end; -- of block
    end loop;
end MAIN ;

Stack represented internally in package (closer to object-oriented than get w/ Modula 2)

generic
    length : Natural := 100;      -- generic parameters
    type element is private;
    -- only assignment and tests for = may be done on objects of "private" type 
    -- "limited private" is also available.
package stack is
    procedure push (X : in element);
    procedure pop (X: out element);
    function empty return boolean;
    stack_error : exception;
end stack;

package body stack is
    space   : array (1..length) of element;
    top : integer range 0..length := 0;

    procedure push (X : in element) is
    begin
        if full()  then 
            raise stack_error;
        else
            top := top + 1;
            space(top) := X;
        end if;
    end push;

    procedure pop (X: out element) is
    begin
        if empty() then 
            raise stack_error;
        else
            X := space(top);
            top := top - 1;
        end if;
    end pop;

    function empty return boolean is
    begin
        return (top = 0);
    end;

    function full return boolean is
    begin
        return (top = length);
    end;

end stack;

Notice: Data structure of stack is entirely hidden from user -

there is no object of type stack available to user.

How to use:

    package stack1 is new stack(20,integer);
    package stack2 is new stack(100, character);
        -- Note that this initializes length in both cases to 0
    use stack2;
    stack1.push(5)
    if not stack1.empty() then 
        stack1.pop(Z);
    endif;
    push('z');

        stack =  package
                         push : procedure (X : in element);
                         pop : procedure (X: out element);
                         empty : function return boolean;
                         stack_error : exception;
                     end package;

One of two key ideas behind object-oriented programming.