Chapter 9: Subprograms

Local Variables

Static

Require no run-time overhead for allocation and deallocation

Stack Dynamic

Bound to storage at start of execution(of subprogram) and unbound when execution terminates

Nested Subprograms

created to be able to create a hierarchy of both logic and scopes

Semantics

In mode

Receive data from the corresponding actual parameter

Out mode

Transmit data to actual parameter

Inout mode

Receive and transmit data from and to the parameter

Models

Pass-by-Value

Value of actual parameter is used to initialize the corresponding former parameter

Pass-by-Result

(For out-mode parameters) No value is transmitted to the subprogram

Pass-by-Value-Result

(For inout-mode parameters) Actual values are copied

Combination between pass-by-value and pass-by-result

Pass-by-Reference

transmits an access path (an address) to the called subprogram

Pass-by-Name

The actual parameter is textually substituted for the corresponding formal parameter in all its occurrences in the subprogram

Type Checking Parameters

Types of actual parameters checked for consistency w/ the types of corresponding formal parameters

Design Considerations

Efficiency

Complications

Type checking the parameters of the activations of the subprograms that were passed as parameters

Which referencing environment should be used

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 as an actual parameter

Subprogram has same name as another but has a least one difference in:

Number of parameters

Order of parameters

Types of parameters

Return type

Can be found in

C++

Java

C#

Polymorphic

Can take parameters of different types on seperate calls

Subtype polymorphism

The type of an object can be derived from T

Parametric polymorphism

Able to use different parameters per call instance

Generic in C++

Template function

Class Identifier

Typename Identifier

Generic in Java 5.0

Uses

Collection<?>

is a wildcard type

Generic in C#

No support for Wildcard type

actual type can be omitted if compiler can infer

Generic in F#

Automatic generalization

If no type info included, assume generic

Which type of Subprogram

Overloaded Subprograms

Same name for
multiple subprograms

Generic Subprograms

Subprogram which can be
used with any input type

Closure

Nested Subprogram

Characteristics

Single Entry Point

Calling program suspended until end

Returns to calling program at end

Definitions

Subprogram Definitions

Describes interface

Subprogram call

Request subprogram execution

Considered active during call

Subprogram Header

Chooses call name

Names parameters

Specifies subprogram variety

Specifies return value

Parameters

Formal parameters

Ones in function call,
exist only while the
subprogram is called

Actual Parameters

Parameters initialized within subprogram

Positional Parameters

Actual parameters which take
the values of their corresponding
formal parameter in order

Keyword Parameter

Formal Parameter attached
to Actual Parameter in Subprogram call

Procedures and Functions

Procedures

Can change outside parameters

Functions

Mathematic equation

Two fundamental abstractions

Process abstraction

First programmable computer used
via reusable cards

Modern versions are written as subprograms

Not the same as methods.

Data abstraction

Functional Side Effects

In-mode only

No side effects from parameters or aliasing of parameters/globals

Pass-by-value/pass-by-reference

Functions can possibly cause side effects and aliasing

Types of Returned Values

No arrays/functions

C

Any type

Ada

Python

Ruby

Subprograms as objects

Ruby

Python

Number of Returned values

1 Return

Most languages

Multiple Returns

Ruby

Tuples

ML

Python

F#