United States. Department of Defense

“ On the other hand, the programmer should not be forced to overspecify his program and thereby cloud their logic, unnecessarily eliminate opportunities for optimization, and misrepresent arbitrary choices as essential to the program logic. Defaults should be allowed, in fact encouraged, in don't care situations. ”

“ Every translator for the language should implement the entire language. There should be no subset implementations.
If individual compilers implement only a subset of the language, then there is no chance for software commonality. If a translator does not implement the entire language, it cannot give its users access to standard supported libraries or to application programs implemented on some other translator. Requiring that the full language be implemented will be expensive only if the language is large, complex, and nonuniform. ”

“ Machine independence, in the use of real numbers, can be achieved only if the user can place constraints on the translator and object machine without forcing a specific mechanization of the arithmetic. Precision specifications, as the maximum required by the program and the minimum to be implemented by the object code, provides all the power and guarantees needed by the programmer without unnecessarily [x] sing on the object machine. Precision specifications do not change the type of reals or the set of applicable operations. ”

“ Whether an extension library should contain source or object code is a question of implementation efficiency and should not be determined by the definition of the source language. It should be remembered, however, that interfaces cannot be validated at program assembly time without some equivalent of their source language interface specifications, that object modules are machine dependent and therefore not portable, that source code is often more compact than object code, and that compilers for simple languages can often compile faster than a loader can load from relocatable object programs. ”

“ Language commonality is an essential step in achieving software commonality, but the real benefits accrue when projects and contractors can draw on existing software with assurance that it will be supported, when systems can build from off the shelf components or at least with common goals, and when funds can be spent to expand existing capabilities rather than building from scratch. Support of common widely used tools and aids must be provided independent of progects and their individual funding if common software is to be widely used. ”

“ Whether a variable will be assigned a value is in general unsolvable at compile time, but in those cases in which it is not easily determined by the translator, it will not be easily determined by the programmer and those who must maintain the program and should, therefore, be considered an error. ”

“ Recursion is required in order to avoid unnecessarily opaque, complex and confusing programs when operating on recursive data structures. If recursive and nonrecursive procedures are marked in the source language, that specification represents just one more special case to be learned and dealt with by the user. The objections to recursion come from a feeling that recursion requires greater run-time costs in time and space than does nonrecursive procedures. In fact, recursion and iteration have the same costs in many cases, and stack allocation of procedure bodies can save space at run-time. ”

“ The semantics of the language should be defined unambiguously and clearly. To the extent a formal definition assists in attaining these objectives, the language's semantics should be specified formally. ”

“ A problem which arises in many programming languages is that loop control variables are global to the iterative control and thus will have a value after loop termination but that value is usually an accident of the implementation. Specifying the meaning of control variables after loop termination in the language definition resolves the ambiguity but must be an arbitrary decision which will not aid program clarity or correctness, and will interfere with the generation of efficient object code. ”

“ There should be a source language capability for handling asynchronous hardware interrupts in a recoverable manner.
One cannot write programs such as operating systems, executives and monitors which service hardware interrupts without access to the interrupt system. Minimally there must be an ability in the source language to specify the interrupt processing routine, to dynamically determine what interrupt has occurred, and to return to the interrupted program. These capabilities can be provided in a machine independent form, but the set of available interrupts must be machine dependent. ”

“ There should be a control agent to ensure that there is only one version of the source language and that implementations of the language conform to that standard. Without controls a hopefully common language will become another umbrella under which new languages will proliferate while retaining the same name. ”

“ A common language must have capability for growth. It should contain all the power necessary to satisfy all the applications and the ability to specialize that power to the particular application task. An extensible language will make it possible to add new application-oriented features and to add new programming techniques and mechanisms to the language using descriptions written entirely within the language. Extensions should have the appearance and costs of features which are built into the language while actually being only catalogued accessible application packages. ”

“ The use of variable number of argument operations need not and should not cause relaxation of any compile-time checks, require use of multiple entry procedures, allow the number of actual parameters to vary at run-time, nor require special calling mechanisms. If the parameters which can vary are limited to a program specified type treated as any other argument on the call side and as elements of an array within the procedure definition, full type checking can be done at compile time. ”

“ More simply, the translator cannot give up program reliability and correctness, regardless of the excuse. It should be noted that for most programming languages there are few known safe optimizations and many unsafe ones. The number of applicable safe optimizations can be increased by making more information available to the compiler and by choosing language constructs which allow safe optimizations. This requirement allows optimization by code motion providing that motion does not change the effect of the program. ”

“ The same capabilities, operations and notations available for real time should be available for virtual time.
Real time capability is essential to many DoD applications. The source language should provide a machine independent form of access and operation to real time clocks. This should be to avoid the cost when the capability is not needed, to keep the kernel language simple and uncluttered by features for particular applications, and to insure that, when necessary, the user can define his own specialized real time facility. ”

“ When an operation is an abstraction of an existing operation for a new type or is a generalization of an existing operation, it is inconvenient, confusing and misleading to use any but the existing operator symbol or name. ”

“ There is currently no common widely used computer in the DoD. There are at least 250 different models of commercial machines in use in DoD with many more home grown varieties. A common language must be applicable to a wide variety of models and sizes of machines. Translators should be written so that they can produce object code for several machines. This reduces the proliferation of translators and makes the full power of an existing translator available at the cost of producing an additional code generator. ”

“ Growth of a language through definition of new data and operations and the introduction of new words and symbols to identify them is desirable but there should be no provision for changing the structure of the language. ”

“ An extensible kernel language alone is not sufficient for a common language. Even though in theory such a language provides the necessary power and the capability for extension to special applications, the users of the language cannot be expected to become language designers or to divert project funds to develop the required extensions to make the language useful. ”

“ Translators should continue checking after one error has been found but should be careful not to generate erroneous messages because of translator confusion. Warnings should be generated when a source language construct is exceptionally expensive or impossible to implement on the specified object machine. ”

“ Some less obvious, but often more important, advantages in saving run-time costs is the ability to execute constant portions of routines at compile time and thereby eliminate time and space for those portions of the procedure body at run time. Open Routine capability is especially important for machine language insertion. ”

“ Key words should be few in number because each new key word introduces another case in the parsing rules and, thereby, adds to the complexity of the language, and because large numbers of key words inconvenience and complicate the programmers task of chasing informative identifiers. It is more important that key words be informative than that they be short but cryptic. ”

“ The language should be extensible so that the user of the language can extend the apparent set of data types and operations available to his programs by means of specifications made within his programs. The number of specialized capabilities needed for a common language is large and diverse. ”

“ The operational requirements dictating specific specialized language capabilities are volatile and future needs cannot always be foreseen. No language can make available all the features useful to the broad spectrum of military applications, anticipate future applications and requirements, or even provide a universally "best" capability in support of a single application area. ”

“ The alternative notion, that all declarations which are external to a program should have the same scope, is inconvenient and costly in creating large systems which are composed from many subsystems because it forces global access scopes and the attendant naming conflicts on subsystems not using the defined items. ”

“ The intended source language product from this effort is a small simple uniform kernel language with the specialized features, support packages and complex features relegated to library routines not requiring direct translator support. If simple low cost translators are not feasible for the selected language, then the language is too large and complex to be standardized and the goal of language commonality will not be achievable. ”

“ Free union adds no new power not provided by discriminated union but does require giving up the security of types in return for programmer freedom. Range or subset specifications on variables are useful documentation and debugging aids but should not be construed as types. Subsets do not introduce new properties or operations not available to the superset and often do not form a closed system under the superset operations. ”

“ They aid in understanding and determining the correctness of programs. They can also be used as additional information by the compiler in deciding what storage and allocation to use (e.g., half words may be more efficient for integers in the range 0 to 1000) ; Range specifications also offer the opportunity for the translator to automatically insert range tests for run-time or debug-time validation of the program logic. With variable ranges specified in the program, it becomes possible to perform many subscript bounds checks at compile-time. ”

“ This follows from having an operational environment which includes many small machines which are unable to support the design, documentation, test, and debugging aids necessary for the development of timely, reliable or efficient software. It also follows from the need to avoid penalizing large machine users for the restrictions of small machines when a common language is used. It is desirable that the translator be able to run on a variety of machines, but this should not be used as an excuse to eliminate needed source language capabilities. ”

“ By key words of the language are meant those symbols and symbol strings which have special meaning in the syntax of programs. They introduce special syntactic forms such as are used for control structures and declarations, or they are used as in-fix operators, or as some form of parenthesis. Key words should be reserved, that is unusable as identifiers, to avoid confusion and ambiguity. ”

“ Defaults should be provided for special capabilities affecting object representation and other properties which the average programmer does not know or care about. Such defaults should always mean that the programmer does not care which choice is made. ”

“ In addition, there should be a formal parameter class for specifying the control action when exception conditions occur, and a class for parameters processed entirely at compile time.
The two data parameter classes are often called call by value and call by reference, respectively. They are the only two widely used parameter passing mechanisms and the many alternatives (at least 9 have been suggested) add complexity and cost to a language without increasing the clarity or power. ”

“ In particular, inovation should be encouraged in the development of translators for a common language providing they implement exactly the source language as defined. Translators like all computer programs should be written in expectation of change ”