Principles of Programming Languages


Subprograms and Blocks

  • Topics
    * Introduction
    * Fundamentals of Subprograms
    * Design Issues for Subprograms
    * Local Referencing Environments
    * Parameter-Passing Methods
    * Parameters That Are Subprogram Names
    * Overloaded Subprograms
    * Generic Subprograms
    * Design Issues for Functions
    * User-Defined Overloaded Operators
    * Coroutines
    * Two fundamental abstraction facilities
    > Process abstraction
    * Emphasized from early days
    > Data abstraction
    * Emphasized in the1980s
    Fundamentals of Subprograms
    * Each subprogram has a single entry point
    * The calling program is suspended during execution of the called subprogram
    * Control always returns to the caller when the called subprogram‘s execution terminates
    Basic Definitions
    * A subprogram definition describes the interface to and the actions of the subprogram abstraction
    * A subprogram call is an explicit request that the subprogram be executed
    * A subprogram header is the first part of the definition, including the name, the kind of subprogram, and the formal parameters
    * The parameter profile (aka signature) of a subprogram is the number, order, and types of its parameters
    * The protocol is a subprogram‘s parameter profile and, if it is a function, its return type
    * Function declarations in C and C++ are often called prototypes
    * A subprogram declaration provides the protocol, but not the body, of the subprogram
    * A formal parameter is a dummy variable listed in the subprogram header and used in the subprogram
    * An actual parameter represents a value or address used in the subprogram call statement
    Actual/Formal Parameter Correspondence
    * Positional
    > The binding of actual parameters to formal parameters is by position: the first actual parameter is bound to the first formal parameter and so forth
    > Safe and effective
    * Keyword
    > The name of the formal parameter to which an actual parameter is to be bound is specified with the actual parameter
    > Parameters can appear in any order
    Formal Parameter Default Values
    * In certain languages (e.g., C++, Ada), formal parameters can have default values (if not actual parameter is passed)
    > In C++, default parameters must appear last because parameters are positionally associated
    * C# methods can accept a variable number of parameters as long as they are of the same type
    Procedures and Functions
    * There are two categories of subprograms
    > Procedures are collection of statements that define parameterized computations
    > Functions structurally resemble procedures but are semantically modeled on mathematical functions
    * They are expected to produce no side effects
    * In practice, program functions have side effects
    Design Issues for Subprograms
    * What parameter passing methods are provided?
    * Are parameter types checked?
    * Are local variables static or dynamic?
    * Can subprogram definitions appear in other subprogram definitions?
    * Can subprograms be overloaded?
    * Can subprogram be generic?
    Local Referencing Environments
    * Local variables can be stack-dynamic (bound to storage)
    > Advantages
    * Support for recursion
    * Storage for locals is shared among some subprograms
    > Disadvantages
    * Allocation/de-allocation, initialization time
    * Indirect addressing
    * Subprograms cannot be history sensitive
    * Local variables can be static
    > More efficient (no indirection)
    > No run-time overhead
    > Cannot support recursion
    Parameter Passing Methods
    * Ways in which parameters are transmitted to and/or from called subprograms
    > Pass-by-value
    > Pass-by-result
    > Pass-by-value-result
    > Pass-by-reference
    > Pass-by-name
    Models of Parameter Passing

    Pass-by-Value (In Mode)
    * The value of the actual parameter is used to initialize the corresponding formal parameter
    > Normally implemented by copying
    > Can be implemented by transmitting an access path but not recommended (enforcing write protection is not easy)
    > When copies are used, additional storage is required
    > Storage and copy operations can be costly
    Pass-by-Result (Out Mode)
    * When a parameter is passed by result, no value is transmitted to the subprogram; the corresponding formal parameter acts as a local variable; its value is transmitted to caller‘s actual parameter when control is returned to the caller
    > Require extra storage location and copy operation
    * Potential problem: sub(p1, p1); whichever formal parameter is copied back will represent the current value of p1
    Pass-by-Value-Result (inout Mode)
    * A combination of pass-by-value and pass-by-result
    * Sometimes called pass-by-copy
    * Formal parameters have local storage
    * Disadvantages:
    > Those of pass-by-result
    > Those of pass-by-value
    Pass-by-Reference (Inout Mode)
    * Pass an access path
    * Also called pass-by-sharing
    * Passing process is efficient (no copying and no duplicated storage)
    * Disadvantages
    > Slower accesses (compared to pass-by-value) to formal parameters
    > Potentials for un-wanted side effects
    > Un-wanted aliases (access broadened)
    Pass-by-Name (Inout Mode)
    * By textual substitution
    * Formals are bound to an access method at the time of the call, but actual binding to a value or address takes place at the time of a reference or assignment
    * Allows flexibility in late binding
    Implementing Parameter-Passing Methods
    * In most language parameter communication takes place thru the run-time stack
    * Pass-by-reference are the simplest to implement; only an address is placed in the stack
    * A subtle but fatal error can occur with pass-by-reference and pass-by-value-result: a formal parameter corresponding to a constant can mistakenly be changed
    Parameter Passing Methods of Major Languages
    * Fortran
    > Always used the inout semantics model
    > Before Fortran 77: pass-by-reference
    > Fortran 77 and later: scalar variables are often passed by value-result
    * C
    > Pass-by-value
    > Pass-by-reference is achieved by using pointers as parameters
    * C++
    > A special pointer type called reference type for pass-by-reference
    * Java
    > All parameters are passed are passed by value
    > Object parameters are passed by reference
    * Ada
    > Three semantics modes of parameter transmission: in, out, in out; in is the default mode
    > Formal parameters declared out can be assigned but not referenced; those declared in can be referenced but not assigned; in out parameters can be referenced and assigned
    * C#
    > Default method: pass-by-value
    > Pass-by-reference is specified by preceding both a formal parameter and its actual parameter with ref
    * PHP: very similar to C#
    * Perl: all actual parameters are implicitly placed in a predefined array named @_
    Type Checking Parameters
    * Considered very important for reliability
    * FORTRAN 77 and original C: none
    * Pascal, FORTRAN 90, Java, and Ada: it is always required
    * ANSI C and C++: choice is made by the user
    > Prototypes
    * Relatively new languages Perl, JavaScript, and PHP do not require type checking
    Multidimensional Arrays as Parameters
    * If a multidimensional array is passed to a subprogram and the subprogram is separately compiled, the compiler needs to know the declared size of that array to build the storage mapping function
    Multidimensional Arrays as Parameters: C and C++
    * Programmer is required to include the declared sizes of all but the first subscript in the actual parameter
    * Disallows writing flexible subprograms
    * Solution: pass a pointer to the array and the sizes of the dimensions as other parameters; the user must include the storage mapping function in terms of the size parameters
    Multidimensional Arrays as Parameters: Pascal and Ada
    * Pascal
    > Not a problem; declared size is part of the array‘s type
    * Ada
    > Constrained arrays - like Pascal
    > Unconstrained arrays - declared size is part of the object declaration
    Multidimensional Arrays as Parameters: Fortran
    * Formal parameter that are arrays have a declaration after the header
    > For single-dimension arrays, the subscript is irrelevant
    > For multi-dimensional arrays, the subscripts allow the storage-mapping function
    Multidimensional Arrays as Parameters: Java and C#
    * Similar to Ada
    * Arrays are objects; they are all single-dimensioned, but the elements can be arrays
    * Each array inherits a named constant (length in Java, Length in C#) that is set to the length of the array when the array object is created
    Design Considerations for Parameter Passing
    * Two important considerations
    > Efficiency
    > One-way or two-way data transfer
    * But the above considerations are in conflict
    > Good programming suggest limited access to variables, which means one-way whenever possible
    > But pass-by-reference is more efficient to pass structures of significant size
    Parameters that are Subprogram Names
    * It is sometimes convenient to pass subprogram names as parameters
    * Issues:
    * Are parameter types checked?
    * What is the correct referencing environment for a subprogram that was sent as a parameter?
    Parameters that are Subprogram Names: Parameter Type Checking
    * C and C++: functions cannot be passed as parameters but pointers to functions can be passed; parameters can be type checked
    * FORTRAN 95 type checks
    * Later versions of Pascal and
    * Ada does not allow subprogram parameters; a similar alternative is provided via Ada‘s generic facility
    Parameters that are Subprogram Names: Referencing Environment
    * Shallow binding: The environment of the call statement that enacts the passed subprogram
    * Deep binding: The environment of the definition of the passed subprogram
    * Ad hoc binding: The environment of the call statement that passed the subprogram
    Overloaded Subprograms
    * An overloaded subprogram is one that has the same name as another subprogram in the same referencing environment
    > Every version of an overloaded subprogram has a unique protocol
    * C++, Java, C#, and Ada include predefined overloaded subprograms
    * In Ada, the return type of an overloaded function can be used to disambiguate calls (thus two overloaded functions can have the same parameters)
    * Ada, Java, C++, and C# allow users to write multiple versions of subprograms with the same name
    Generic Subprograms
    * A generic or polymorphic subprogram takes parameters of different types on different activations
    * Overloaded subprograms provide ad hoc polymorphism
    * A subprogram that takes a generic parameter that is used in a type expression that describes the type of the parameters of the subprogram provides parametric polymorphism
    Examples of parametric polymorphism: C++
    template <class Type>
    Type max(Type first, Type second) {
    return first > second ? first : second;
    * The above template can be instantiated for any type for which operator > is defined
    int max (int first, int second) {
    return first > second? first : second;
    Design Issues for Functions
    * Are side effects allowed?
    > Parameters should always be in-mode to reduce side effect (like Ada)
    * What types of return values are allowed?
    > Most imperative languages restrict the return types
    > C allows any type except arrays and functions
    > C++ is like C but also allows user-defined types
    > Ada allows any type
    > Java and C# do not have functions but methods can have any type
    User-Defined Overloaded Operators
    * Operators can be overloaded in Ada and C++
    * An Ada example
    Function ―*‖(A,B: in Vec_Type): return Integer is
    Sum: Integer := 0;
    for Index in A‘range loop
    Sum := Sum + A(Index) * B(Index)
    end loop
    return sum;
    end ―*‖;

    c = a * b; -- a, b, and c are of type Vec_Type
    * A coroutine is a subprogram that has multiple entries and controls them itself
    * Also called symmetric control: caller and called coroutines are on a more equal basis
    * A coroutine call is named a resume
    * The first resume of a coroutine is to its beginning, but subsequent calls enter at the point just after the last executed statement in the coroutine
    * Coroutines repeatedly resume each other, possibly forever
    * Coroutines provide quasi-concurrent execution of program units (the coroutines); their execution is interleaved, but not overlapped Coroutines Illustrated: Possible Execution Controls
    Coroutines Illustrated: Possible Execution Controls
    Coroutines Illustrated: Possible Execution Controls with Loops

    * A subprogram definition describes the actions represented by the subprogram
    * Subprograms can be either functions or procedures
    * Local variables in subprograms can be stack-dynamic or static
    * Three models of parameter passing: in mode, out mode, and inout mode
    * Some languages allow operator overloading
    * Subprograms can be generic
    * A coroutine is a special subprogram with multiple entries
    1) Explain the general subprogram characteristics.
    2) What is the general problem with static scoping?
    3) What is block? How scope of a variable is dependent on block.
    4) Explain extent with the help of an example.
    5) Discuss the design issues of sub programs.
    6) What are the advantages and disadvantages of dynamic local variables?
    7) Define static, stack-dynamic, explicitly heap dynamic and implicit heap dynamic
    variables. What are the advantages and disadvantages of these.
    8) Explain how multi dimensional arrays are passed as parameters.
    9) Discuss about actual parameters, positional parameters and keyword parameters.
    10) Explain type-checking technique in parameter passing.
    11) Explain with examples pass-by-value and pass by reference parameter passing techniques.
    12) Explain how subprogram is overloaded. Give examples.
    13) Explain how generic functions are implemented in C++?
    14) What is parametric polymorphism?
    15) Explain how sub program names are passed as parameters?
    16) In what ways coroutines are different from conventional sub programs?

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