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

Chapter 7K. Louden, Programming Languages2Introduction "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.

Chapter 7K. Louden, Programming Languages3 Expressions 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.

Chapter 7K. Louden, Programming Languages4 Side 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.

Chapter 7K. Louden, Programming Languages5 Prefix notation Ordinary prefix Function 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 known Example: *+ab/cd Named because the Polish mathematician Lukasiewiez invented the notation. Difficult to read Works for any number of operands (unlike infix notation) Easy to decode mathematically.

Chapter 7K. Louden, Programming Languages6 Postfix Notation (suffix or reverse Polish) notation Not used in programming languages, but frequently for execution time representation Easily evaluated using a stack - Easy code generation Infix Notation Suitable only for binary operations Common use in mathematics and programming languages

Chapter 7K. Louden, Programming Languages7 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 operators In 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 operator Hard to read

Chapter 7K. Louden, Programming Languages8 Functional 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

Chapter 7K. Louden, Programming Languages9 Functional Side Effects 2. Write the language definition to demand that operand evaluation order be fixed –Disadvantage: limits some compiler optimizations

Chapter 7K. Louden, Programming Languages10 Overloaded 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

Chapter 7K. Louden, Programming Languages11 Type 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

Chapter 7K. Louden, Programming Languages12 Type Conversions 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

Chapter 7K. Louden, Programming Languages13 Type Conversions Explicit Type Conversions Often called casts

Chapter 7K. Louden, Programming Languages14 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

Chapter 7K. Louden, Programming Languages15 Short 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

Chapter 7K. Louden, Programming Languages16 Short 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)

Chapter 7K. Louden, Programming Languages17 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.

Chapter 7K. Louden, Programming Languages18 Strictness 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.

Chapter 7K. Louden, Programming Languages19 Function 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?).

Chapter 7K. Louden, Programming Languages20 Examples 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.

Chapter 7K. Louden, Programming Languages21 Sequencing 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.

Chapter 7K. Louden, Programming Languages22Statements 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.

Chapter 7K. Louden, Programming Languages23 Summary 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.