Ada (programming language)
Ada is a structured, statically typed, imperative, and object-oriented high-level programming language, inspired by Pascal and other languages. It has built-in language support for design by contract, extremely strong typing, explicit concurrency, tasks, synchronous message passing, protected objects, and non-determinism. Ada improves code safety and maintainability by using the compiler to find errors in favor of runtime errors. Ada is an international technical standard, jointly defined by the International Organization for Standardization, and the International Electrotechnical Commission., the standard, ISO/IEC 8652:2023, is called Ada 2022 informally.
Ada was originally designed by a team led by French computer scientist Jean Ichbiah of Honeywell under contract to the United States Department of Defense from 1977 to 1983 to supersede over 450 programming languages then used by the DoD. Ada was named after Ada Lovelace, who has been credited as the first computer programmer.
Features
Ada was originally designed for embedded and real-time systems. The Ada 95 revision, designed by S. Tucker Taft of Intermetrics between 1992 and 1995, improved support for systems, numerical, financial, and object-oriented programming.Features of Ada include strong typing, modular programming mechanisms, run-time checking, parallel processing, exception handling, and generics. Ada 95 added support for object-oriented programming, including dynamic dispatch.
The syntax of Ada minimizes choices of ways to perform basic operations, and prefers English keywords to symbols. Ada uses the basic arithmetical operators,,, and, but avoids using other symbols. Code blocks are delimited by words such as 'declare', 'begin', and 'end', where the 'end' is followed by the keyword of the block that it closes. In the case of conditional blocks this avoids a dangling else that could pair with the wrong nested 'if'-expression in other languages such as C or Java.
Ada is designed for developing very large software systems. Ada packages can be compiled separately. Ada package specifications can also be compiled separately without the implementation to check for consistency. This makes it possible to detect problems early during the design phase, before implementation starts.
A large number of compile-time checks are supported 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. For example, the syntax requires explicitly named closing of blocks to prevent errors due to mismatched end tokens. The adherence to strong typing allows detecting many common software errors either during compile time, or otherwise during run time. As concurrency is part of the language specification, the compiler can in some cases detect potential deadlocks. Compilers also commonly check for misspelled identifiers, visibility of packages, redundant declarations, etc. and can provide warnings and useful suggestions on how to fix the error.
Ada also supports run-time checks to protect against access to unallocated memory, buffer overflow errors, range violations, off-by-one errors, array access errors, and other detectable 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 sometimes used in critical systems, where any anomaly might lead to very serious consequences, e.g., accidental death, injury or severe financial loss. Examples of systems where Ada is used include avionics, air traffic control, railways, banking, military and space technology.
Ada's dynamic memory management is high-level and type-safe. Ada has no generic or untyped pointers, nor does it implicitly declare any pointer type. Instead, all dynamic memory allocation and deallocation must occur via explicitly declared access types. Each access type has an associated storage pool that handles the low-level details of memory management; the programmer can either use the default storage pool or define new ones. It is even possible to declare several different access types that all designate the same type but use different storage pools. Also, the language provides for accessibility checks, both at compile time and at run time, that ensures that an access value cannot outlive the type of the object it points to.
Though the semantics of the language allow automatic garbage collection of inaccessible objects, most implementations do not support it by default, as it would cause unpredictable behaviour in real-time systems. Ada supports a limited form of region-based memory management, and in Ada, destroying a storage pool also destroys all the objects in the pool.
A double dash, resembling an em dash, denotes comment text. Comments stop at end of line; there is intentionally no way to make a comment span multiple lines, to prevent unclosed comments from accidentally voiding whole sections of source code. Disabling a whole block of code therefore requires the prefixing of each line individually with. While this clearly denotes disabled code by creating a column of repeated '--' down the page, it also renders the experimental dis/re-enablement of large blocks a more drawn-out process in editors without block commenting support.
The semicolon is a statement terminator, and the null or no-operation statement is. A single without a statement to terminate is not allowed.
Unlike most ISO standards, the Ada language definition is free content. Thus, it is a common reference for Ada programmers, not only 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.
One notable free software tool that is used by many Ada programmers to aid them in writing Ada source code is the GNAT Programming Studio, and GNAT which is part of the GNU Compiler Collection.
Alire is a package and toolchain management tool for Ada.
History
In the 1970s the US Department of Defense became concerned by the number of different programming languages being used for its embedded computer system projects, many of which were obsolete or hardware-dependent, and none of which supported safe modular programming. In 1975, a working group, the High Order Language Working Group, was formed with the intent to reduce this number by finding or creating a programming language generally suitable for the department's and the UK Ministry of Defence's requirements. After many iterations beginning with an original straw-man proposal the eventual programming language was named Ada. The total number of high-level programming languages in use for such projects fell from over 450 in 1983 to 37 by 1996.HOLWG crafted the Steelman language requirements, a series of documents stating the requirements they felt a programming language should satisfy. Many existing languages were formally reviewed, but the team concluded in 1977 that no existing language met the specifications. The requirements were created by the United States Department of Defense in The Department of Defense Common High Order Language program in 1978. The predecessors of this document were called, in order, "Strawman", "Woodenman", "Tinman" and "Ironman". The requirements focused on the needs of embedded computer applications, and emphasised reliability, maintainability, and efficiency. Notably, they included exception handling facilities, run-time checking, and parallel computing.
It was concluded that no existing language met these criteria to a sufficient extent, so a contest was called to create a language that would be closer to fulfilling them. The design that won this contest became the Ada programming language. The resulting language followed the Steelman requirements closely, though not exactly.
Requests for proposals for a new programming language were issued and four contractors were hired to develop their proposals under the names of Red, Green, Blue and Yellow. 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 Honeywell, was chosen and given the name Ada—after Augusta Ada King, Countess of Lovelace, usually known as Ada Lovelace. This proposal was influenced by the 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, and given the number MIL-STD-1815 in honor of Ada Lovelace's birth year. In 1981, Tony Hoare took advantage of his Turing Award speech to criticize Ada for being overly complex and hence unreliable, but subsequently seemed to recant in the foreword he wrote for an Ada textbook.
Image:ACT Ada validation certificate and company award.jpg|thumb|left|upright=0.7|Passing the ACVC validation tests was an arduous task for Ada compiler vendors; here the certificate awarded by the Ada Joint Program Office and an award given by vendor management to its compiler staff
Ada attracted much attention from the programming community as a whole during its early days. Its backers and others predicted that it might become a dominant language for general purpose programming and not only defense-related work. Ichbiah publicly stated that within ten years, only two programming languages would remain: Ada and Lisp. Early Ada compilers struggled to implement the large, complex language, and both compile-time and run-time performance tended to be slow and tools primitive. Compiler vendors expended most of their efforts in passing the massive, language-conformance-testing, government-required Ada Compiler Validation Capability validation suite that was required in another novel feature of the Ada language effort.
The first validated Ada implementation was the NYU Ada/Ed translator, certified on April 11, 1983. NYU Ada/Ed is implemented in the high-level set language SETL. Several commercial companies began offering Ada compilers and associated development tools, including Alsys, TeleSoft, DDC-I, Advanced Computer Techniques, Tartan Laboratories, Irvine Compiler, TLD Systems, and Verdix. Computer manufacturers who had a significant business in the defense, aerospace, or related industries, also offered Ada compilers and tools on their platforms; these included Concurrent Computer Corporation, Cray Research, Inc., Digital Equipment Corporation, Harris Computer Systems, and Siemens Nixdorf Informationssysteme AG.
In 1991, the US Department of Defense began to require the use of Ada for all software, though exceptions to this rule were often granted. The Department of Defense Ada mandate was effectively removed in 1997, as the DoD began to embrace commercial off-the-shelf technology. Similar requirements existed in other NATO countries: Ada was required for NATO systems involving command and control and other functions, and Ada was the mandated or preferred language for defense-related applications in countries such as Sweden, Germany, and Canada.
By the late 1980s and early 1990s, Ada compilers had improved in performance, but there were still barriers to fully exploiting Ada's abilities, including a tasking model that was different from what most real-time programmers were used to.
Because of Ada's safety-critical support features, it is now used not only for military applications, but also in commercial projects where a software bug can have severe consequences, e.g., avionics and air traffic control, commercial rockets such as the Ariane 4 and 5, satellites and other space systems, railway transport and banking.
For example, the Primary Flight Control System, the fly-by-wire system software in the Boeing 777, was written in Ada, as were the fly-by-wire systems for the aerodynamically unstable Eurofighter Typhoon, Saab Gripen, Lockheed Martin F-22 Raptor and the DFCS replacement flight control system for the Grumman F-14 Tomcat. The Canadian Automated Air Traffic System was written in 1 million lines of Ada. It featured advanced distributed processing, a distributed Ada database, and object-oriented design. Ada is also used in other air traffic systems, e.g., the UK's next-generation Interim Future Area Control Tools Support air traffic control system is designed and implemented using SPARK Ada.
It is also used in the French TVM in-cab signalling system on the TGV high-speed rail system, and the metro suburban trains in Paris, London, Hong Kong and New York City.
The Ada 95 revision of the language went beyond the Steelman requirements, targeting general-purpose systems in addition to embedded ones, and adding features supporting object-oriented programming.