Louden, 20031 Chapter 7 - Control I: Expressions and Statements Programming Languages: Principles and Practice, 2nd Ed. Kenneth C. Louden.

Louden, 2003 1

Chapter 7 - Control I: Chapter 7 - Control I: Expressions and StatementsExpressions and Statements

Programming Languages:

Principles and Practice, 2nd Ed.

Kenneth C. Louden

Chapter 7 K. Louden, Programming Languages 2

IntroductionIntroduction "Control" is the general study of the

semantics of execution paths through code: what gets executed, when, and in what order.

Most important control issue in modern languages: procedure/function/method call and return, studied in Chapter 8.

Here we study more localized control issues in expressions and statements, and:

Exception handling, which involves non-local control but has a local component too.


ExpressionsExpressions In their purest form, expressions do not involve

control issues: subexpressions can be evaluated in arbitrary order, and the order does not affect the result. Functional programming tries to achieve this goal for whole programs.

Of course, there must always be a few expressions that can modify the execution/evaluation process: if-then-else expressions, short-circuit boolean operators, case/switch expressions.

If these could have arbitrary evaluation order, programs would become non-deterministic: any of a number of different outcomes might be possible. Usually this is not desirable, but see later.


Side EffectsSide Effects A side effect is any observable change to

memory, input or output. A program without any side effect is useless. Side effects expose evaluation order:

class Order{ static int x = 1; public static int getX() { return x++; } public static void main(String[] args) { System.out.println( x+getX() ); }}

This prints 2, but the corresponding C program will usually print 3!

Referential transparency limits side effects, so this can't happen for r. t. expressions.


Prefix notation Prefix notation

Ordinary prefixFunction name precedes its arguments f(a,b,c)Example:(a+b)*(c/d) becomes *(+(a,b),/(c,d))

A variant (Cambridge Polish or fully parenthesized) moved the left paren before the operand and deletes the commas Example: (a+b)*(c/d) becomes (*(+a b) (/c d)) - LISP

Polish, allows parens to be dropped Parens are unnecessary if number of args is fixed and knownExample: *+ab/cd Named because the Polish mathematician Lukasiewiez invented the notation.Difficult to readWorks for any number of operands (unlike infix notation)Easy to decode mathematically.


Postfix Notation (suffix or reverse Polish) notation Not used in programming languages, but frequently for execution time representationEasily evaluated using a stack - Easy code generation

Infix Notation Suitable only for binary operationsCommon use in mathematics and programming languages


Problems with infix Since only works for binary operations, others must use prefix

(or postfix) making translation worse ambiguity: parens, precedence

Comparison of notations Infix is a natural representation,

but requires complex implicit rules and doesn't work for non-binary operatorsIn the absence of implicit rules, large number of parens are required

prefix and Cambridge Polish require large number of parens Polish requires no parens, but requires you know the arity of

each operatorHard to read


Functional Side EffectsFunctional Side Effects

Two Possible Solutions to the Problem:1. Write the language definition to disallow

functional side effects– No two-way parameters in functions– No non-local references in functions– Advantage: it works!– Disadvantage: Programmers want the

flexibility of two-way parameters (what about C?) and non-local references


Functional Side EffectsFunctional Side Effects

2. Write the language definition to demand that operand evaluation order be fixed

– Disadvantage: limits some compiler optimizations


Overloaded OperatorsOverloaded Operators

C++ and Ada allow user-defined overloaded operators

Potential problems: – Users can define nonsense operations– Readability may suffer, even when the

operators make sense


Type ConversionsType Conversions

Def: A narrowing conversion is one that converts an object to a type that cannot include all of the values of the original type e.g., float to int

Def: A widening conversion is one in which an object is converted to a type that can include at least approximations to all of the values of the original type e.g., int to float



Def: A mixed-mode expression is one that has operands of different types

Def: A coercion is an implicit type conversion The disadvantage of coercions:

– They decrease in the type error detection ability of the compiler

In most languages, all numeric types are coerced in expressions, using widening conversions

In Ada, there are virtually no coercions in expressions



Explicit Type Conversions Often called casts


Eager evaluation For each operation node in the expression tree, first evaluate (or

generate code to do so) each operand, then apply the operation. Sounds good - but complications: Z+(y==0?x:x/y)If we evaluate the operands of condition first, we do what the condition was set up to avoid.

Sometimes optimizations make use of the fact that association can be changed. Sometimes, reordering causes problems:

adding large and small values together - lose small ones due to number of significant digits which can be stored

can be confusing to reader - exception thrown or side effects seen to be out of order

Evaluation Order Bottom-up evaluation of syntax tree For function calls: all arguments evaluated before call. translator may rearrange the order of computation so it is more

efficient. If order of evaluation is unspecified, not portable


Short Circuit EvaluationShort Circuit Evaluation

Suppose Java did not use short-circuit evaluation

Problem: table look-up

index = 1;

while (index <= length) && (LIST[index] != value)

index++;Problem: divide by zero


Short Circuit EvaluationShort Circuit Evaluation

C, C++, and Java: use short-circuit evaluation for the usual Boolean operators (&& and ||), but also provide bitwise Boolean operators that are not short circuit (& and |)

Ada: programmer can specify either (short-circuit is specified with and then and or else)

Short-circuit evaluation exposes the potential problem of side effects in expressions e.g. (a > b) || (b++ / 3)


Delayed order evaluation used in functional languages. Don't evaluate

until actually needed.Example:

function sq(x:integer):integer;

begin sq = x*x;

end;

sq(i++)

Becomes sq(i++) = (i++)*(i++) is evaluated twice.


StrictnessStrictness An evaluation order for expressions is strict if all

subexpressions of an expression are evaluated, whether or not they are needed to determine the value of the result, non-strict otherwise.

Arithmetic is almost always strict. Every language has at least a few non-strict

expressions (?:, &&, || in Java). Some languages use a form of non-strictness

called normal-order evaluation: no expression is ever evaluated until it is needed (Haskell). Also called delayed evaluation.

A form of strict evaluation called applicative-order is more common: "bottom-up" or "inside-out".

Still leaves open whether left-to-right or not.


Function callsFunction calls Obey evaluation rules like other

expressions. Applicative order: evaluate all arguments

(left to right?), then call the procedure. Normal order: pass in unevaluated

representations of the arguments. Only evaluate when needed.

With side effects, order makes a difference.

Representation of argument value also makes a difference (value or reference?).


ExamplesExamples C and Scheme: no explicit order required for

subexpressions or arguments to calls. Java always says left to right, but warns against

using that knowledge. Case/switch/cond expressions imply a top-down

order:(define (f) (cond (#t 1) (#t 2)))

Theoretically, this could return either 1 or 2 (non-determinism—the "guarded if" of text).

Java and C outlaw this: no duplicate cases.


Sequencing and StatementsSequencing and Statements A sequence of expressions makes no sense

without side effects. Thus, a referentially transparent program should not need sequences.

Both ML and Scheme have sequences:(e1;e2;…) [ML] and (begin e1 e2 …) [Scheme].

What about a let expression? Is there an implied sequence? (let val x = e1 in e2 end;)

Applicative order would say yes: e1 is an argument to a call: (fn x => e2) e1.

Normal order would say no: only evaluate e1 if the value of x is actually needed in e2.

Statements by definition imply sequencing, since there is no computed value.


StatementsStatements Can be viewed as expressions with no

value. Java, C have very few: if, while, do, for,

switch, plus "expression statements." Scheme: valueless expressions also exist:

define, set! (some versions give these values).

ML: "valueless" expressions have value (). What about val and fun? Declarations may be neither expressions

nor statements.


SummarySummary Every language has three major program

components: expressions, statements, and declarations.

Expressions are executed for their values (but may have side effects), and may or may not be sequenced.

Statements are executed solely for their side effects, and they must be sequenced.

Declarations define names; they can also give values to those names. They may or may not be viewed by a language as expressions or statements.

Louden, 20031 Chapter 7 - Control I: Expressions and Statements Programming Languages: Principles and Practice, 2nd Ed. Kenneth C. Louden.

Documents

programming languages4

functional programming

caseswitch expressions

reverse polish notation

infix notation easy

arbitrary order

non local control

number of args