$172 STATIC TYPING:WHY AND HOW 615 17.2 STATIC TYPING:WHY AND HOW Although the advantages of static typing seem obvious,it is necessary to review the terms of the debate. The benefits The reasons for using a statically typed form of object technology were listed at the very beginning of this chapter:reliability,readability and efficiency. The reliability value comes from the use of static typing to detect errors that would otherwise manifest themselves only at run time,and only in certain runs.The rule that forces you to declare entities and functions-the first of our three type rules above- introduces redundancy into the software text;this enables the compiler,through the other two rules,to detect inconsistencies between the purpose and actual use of an entity,feature or expression. Catching errors early is essential,as correction cost grows quickly with the detection delay.This property,intuitively clear to all software professionals,is confirmed quantitatively,for specification errors,by Boehm's well-known studies,plotting the cost of correcting an error against the time at which it is found(base 1 if found at requirements time),for both a set of large industrial projects and a controlled small project experiment: Relative cost of Correction cost correcting 1000 errors After [Boehm 1981]. Reproduced with permission 500 LARGE PROJECTS 20 SMALL PROJECT 1 十 HTime Require- Design Code Develop- Accep- Opera- error ments ment test tance test tion found The readability benefit is also appreciable.As the examples appearing throughout this book should show convincingly,declaring every entity and function with a certain type is a powerful way of conveying to the software reader some information about its intended uses.This is particularly precious for maintainers of the software If readability were not part of the goal we might be able to obtain some of the other benefits of typing without explicit declarations.It is possible indeed,under certain conditions,to use an implicit form of typing in which the compiler,instead of requiring software authors to declare entity types,attempts to determine the type of each entity automatically from its uses.This is known as type inference.But from a software engineering perspective explicit declarations are a help,not a penalty;types should be clear not just to the compiler but to the human reader.§17.2 STATIC TYPING: WHY AND HOW 615 17.2 STATIC TYPING: WHY AND HOW Although the advantages of static typing seem obvious, it is necessary to review the terms of the debate. The benefits The reasons for using a statically typed form of object technology were listed at the very beginning of this chapter: reliability, readability and efficiency. The reliability value comes from the use of static typing to detect errors that would otherwise manifest themselves only at run time, and only in certain runs. The rule that forces you to declare entities and functions — the first of our three type rules above — introduces redundancy into the software text; this enables the compiler, through the other two rules, to detect inconsistencies between the purpose and actual use of an entity, feature or expression. Catching errors early is essential, as correction cost grows quickly with the detection delay. This property, intuitively clear to all software professionals, is confirmed quantitatively, for specification errors, by Boehm’s well-known studies, plotting the cost of correcting an error against the time at which it is found (base 1 if found at requirements time), for both a set of large industrial projects and a controlled small project experiment: The readability benefit is also appreciable. As the examples appearing throughout this book should show convincingly, declaring every entity and function with a certain type is a powerful way of conveying to the software reader some information about its intended uses. This is particularly precious for maintainers of the software. If readability were not part of the goal we might be able to obtain some of the other benefits of typing without explicit declarations. It is possible indeed, under certain conditions, to use an implicit form of typing in which the compiler, instead of requiring software authors to declare entity types, attempts to determine the type of each entity automatically from its uses. This is known as type inference. But from a software engineering perspective explicit declarations are a help, not a penalty; types should be clear not just to the compiler but to the human reader. Relative cost of correcting errors After [Boehm 1981]. Reproduced with permission. 1000 500 20 Require￾ments Design Code Accep￾tance test Opera￾tion Develop￾ment test LARGE PROJECTS SMALL PROJECT 1 Time error found Correction cost