Ada is a structured, statically typed, imperative, and object-oriented high-level computer programming language based on Pascal. It was originally designed by a team led by Jean Ichbiah of CII Honeywell Bull under contract to the United States Department of Defense during 1977–1983 to supersede the hundreds of programming languages then used by the US Department of Defense (DoD). Ada is strongly typed and compilers are validated for reliability in mission-critical applications, such as avionics software. Ada is an international standard; the current version (known as Ada 2005) is defined by joint ISO/ANSI standard (ISO-8652:1995), combined with major Amendment ISO/IEC 8652:1995/Amd 1:2007
Ada was named after Ada Lovelace (1815–1852), who is often credited as being the first computer programmer.
Notable features of Ada include: strong typing, modularity mechanisms (packages), run-time checking, parallel processing (tasks), exception handling, and generics. Ada 95 added support for object-oriented programming, including dynamic dispatch.
Ada supports run-time checks in order to protect against access to unallocated memory, buffer overflow errors, off-by-one errors, array access errors, and other avoidable bugs. These checks can be disabled in the interest of runtime efficiency, but can often be compiled efficiently. It also includes facilities to help program verification. For these reasons, Ada is widely used in critical systems, where any anomaly might lead to very serious consequences, i.e., accidental death or injury. Examples of systems where Ada is used include avionics, weapon systems (including thermonuclear weapons), and spacecraft.
Ada also supports a large number of compile-time checks to help avoid bugs that would not be detectable until run-time in some other languages or would require explicit checks to be added to the source code.
Ada's dynamic memory management is high-level and type-explicit, requiring explicit instantiation of the Unchecked_Deallocation package to explicitly free allocated memory. The specification does not require any particular implementation. Though the semantics of the language allow automatic garbage collection of inaccessible objects, most implementations do not support it. Ada does support a limited form of region-based storage management. Invalid accesses can always be detected at run time (unless of course the check is turned off) and sometimes at compile time.
The syntax of Ada is simple, consistent and readable. It minimizes choices of ways to perform basic operations, and prefers English keywords (eg "OR") to symbols (eg. "||"). Ada uses the basic mathematical symbols (i.e.: "+", "-", "*" and "/") for basic mathematical operations but avoids using other symbols. Code blocks are delimited by words such as "declare", "begin" and "end". Conditional statements are closed with "end if", avoiding dangling else. Code for complex systems is typically maintained for many years, by programmers other than the original author. It can be argued that these language design principles apply to most software projects, and most phases of software development, but when applied to complex, safety critical projects, benefits in correctness, reliability, and maintainability take precedence over (arguable) costs in initial development.
Unlike most ISO standards, the Ada language definition (known as the Ada Reference Manual or ARM, or sometimes the Language Reference Manual or LRM) is free content. Thus, it is a common reference for Ada programmers, not just programmers implementing Ada compilers. Apart from the reference manual, there is also an extensive rationale document which explains the language design and the use of various language constructs. This document is also widely used by programmers. When the language was revised, a new rationale document was written.
Requests for proposals for a new programming language were issued and four contractors were hired to develop their proposals under the names of Red (Intermetrics led by Benjamin Brosgol), Green (CII Honeywell Bull, led by Jean Ichbiah), Blue (SofTech, led by John Goodenough), and Yellow (SRI International, led by Jay Spitzen). In April 1978, after public scrutiny, the Red and Green proposals passed to the next phase. In May 1979, the Green proposal, designed by Jean Ichbiah at CII Honeywell Bull, was chosen and given the name Ada—after Augusta Ada, Countess of Lovelace. This proposal was influenced by the programming language LIS that Ichbiah and his group had developed in the 1970s. The preliminary Ada reference manual was published in ACM SIGPLAN Notices in June 1979. The Military Standard reference manual was approved on December 10, 1980 (Ada Lovelace's birthday), and given the number MIL-STD-1815 in honor of Ada Lovelace's birth year. In 1981, C. A. R. Hoare took advantage of his Turing Award speech to criticize Ada for being overly complex and hence unreliable.
In 1987, the US Department of Defense began to require the use of Ada (the Ada mandate) for every software project where new code was more than 30% of result, though exceptions to this rule were often granted. This requirement was effectively removed in 1997, as the DoD began to embrace COTS (commercial off-the-shelf) technology. Similar requirements existed in other NATO countries.
Because Ada is a strongly typed language, it has been used outside the military in commercial aviation projects, where a software bug can cause fatalities. The fly-by-wire system software in the Boeing 777 was written in Ada. The Canadian Automated Air Traffic System (completed in year 2000 by Raytheon Canada) was written in 1 million lines of Ada (SLOC count). It featured advanced (for the time) distributed processing, a distributed Ada database, and object-oriented design.
Ada 95, the joint ISO/ANSI standard (ISO-8652:1995) is the latest standard for Ada. It was published in February 1995, making Ada 95 the first ISO standard object-oriented programming language. To help with the standard revision and future acceptance, the US Air Force funded the development of the GNAT Compiler. Presently, the GNAT Compiler is part of the GNU Compiler Collection.
Work has continued on improving and updating the technical content of the Ada programming language. A Technical Corrigendum to Ada 95 was published in October 2001, and a major Amendment, ISO/IEC 8652:1995/Amd 1:2007, was published on March 9, 2007. Other related standards include ISO 8651-3:1988 Information processing systems -- Computer graphics -- Graphical Kernel System (GKS) language bindings -- Part 3: Ada
Ada also provides alternative constructions that are more streamlined.
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