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D, Parasail, Pascal, and Rust vs The Steelman

29/10/2017
Overview

From 1975 to 1978 the United States Department of Defense sought to establish a set of requirements for a single high level programming language that would also be appropriate for use in Defense embedded systems. After successively more refined versions of the requirements from Strawman through to Ironman, this effort culminated in Steelman. The Ada programming language, possibly the gold standard language for writing safe and secure software, was designed to comply with Steelman.

In 1996 David A. Wheeler wrote a paper that compared Ada, C, C++, and Java against the Steelman. This served to highlight the strengths and weaknesses of those languages, areas that could be improved, and a scant few requirement points that perhaps aren't even applicable anymore. Since then several more programming languages capable of systems work have been created, so it's time for an update. More datapoints! Hence, this article will conduct a similar comparison, instead using D, Parasail, Pascal, and Rust.

The Languages

D was created originally as a reworking of C++ in 2000-2001. It serves to represent a progression of the C language family, adding features including contracts, optional garbage collection, and a standard threading model.

Parasail is a research language created in 2009 by AdaCore, the main vendor of Ada compiler tooling today. The language is designed with implicit parallelism throughout, simplifying and adding static checking to eliminate as many sources of errors as possible. It represents a possible future direction for Ada derived languages.

Pascal, like C, predates the Steelman requirements and so they cannot have had any influence at all on the language. It was designed for formal specification and teaching algorithms. Later dialects were used to develop several high profile software projects, including Skype, Photoshop, and the original Mac OS. It is useful to consider as a precusor of Ada, sharing many points of functionality and style.

Rust is the newest language here, created in 2010. It is an odd mix of C and ML influence, placing more emphasis on the functional paradigm than other systems languages. Its main claim to fame is adding another method of heap memory safety via affine typing.

Logo for the D programming language
D
Logo for the Parasail programming language
Parasail
A picture of Blaise Pascal to stand in as a logo for the Pascal programming language
Pascal*
Logo for the Rust programming language
Rust

* Pascal does not have an official logo, so a picture of Blaise Pascal, in whose honour the language is named, will have to do.

Rules for Comparison

The rule used for this article is that a language provides a feature if:

  1. that feature is defined in the documents widely regarded as the language's defining document(s), or
  2. that feature is widely implemented by compilers typically used for that language with essentially the same semantics.

Note the bolded difference from the rules in Wheeler's paper. This is so later dialects of Pascal can be considered, rather than strictly adhering to the ISO standard. The other three languages are unaffected by this change. Aside from that, effort has been made to keep the evaluation as similar as practical to the previous work.

The defining documents used for each of these languages are as follows:

Results and Conclusions

The appendix lists the Steelman requirements and how well each language supports them. The following table shows a summary:

Language "No" "Partial" "Mostly" "Yes" Percentage with "Mostly" or "Yes"
D 7 15 25 66 81%
Parasail 11 6 11 85 85%
Pascal 19 16 11 67 69%
Rust 12 19 23 59 73%

Note that these raw numbers should not be taken at face value. They are a summary of how well the overall requirements are met, no more, no less. Attention should be directed towards specific requirements to determine the strengths and weaknesses of each language and the suitability for a particular purpose. Furthermore, some features are not covered by Steelman at all, such as support for functional programming or object oriented programming.

The following are high level comments on these programming languages and how they relate:

Appendix: Table Comparing the Languages to Steelman

As in Wheeler's paper, this table shows each Steelman requirement on the left and then how well each of the four languages considered meet that requirement on the right. Some explanatory notes are included for a few of the requirements.

Note that due to the standardisation issues each of these languages has, a (fortunately quite low) number of these turned out to be educated guesses. Fairness was the goal, but nonetheless reader discretion is advised.

Requirement D Parasail Pascal Rust
1A. Generality. The language shall provide generality only to the extent necessary to satisfy the needs of embedded computer applications. Such applications involve real time control, self diagnostics, input-output to nonstandard peripheral devices, parallel processing, numeric computation, and file processing. yes no? yes? yes?
Parasail does not specify ways to directly control hardware, nor any interfaces to other languages.
1B. Reliability. The language should aid the design and development of reliable programs. The language shall be designed to avoid error prone features and to maximize automatic detection of programming errors. The language shall require some redundant, but not duplicative, specifications in programs. Translators shall produce explanatory diagnostic and warning messages, but shall not attempt to correct programming errors. yes? yes partial? mostly?
1C. Maintainability. The language should promote ease of program maintenance. It should emphasize program readability (i.e., clarity, understandability, and modifiability of programs). The language should encourage user documentation of programs. It shall require explicit specification of programmer decisions and shall provide defaults only for instances where the default is stated in the language definition, is always meaningful, reflects the most frequent usage in programs, and may be explicitly overridden. partial? yes? mostly? partial?
Parasail was designed with readability in mind, although it suffers slightly from having several different ways to do something. Pascal was designed for teaching structured programming. D inherits a lot of the syntactical traps of C-family languages. Rust has exceedingly terse and difficult to read syntax.
1D. Efficiency. The language design should aid the production of efficient object programs. Constructs that have unexpectedly expensive implementations should be easily recognizable by translators and by users. Features should be chosen to have a simple and efficient implementation in many object machines, to avoid execution costs for available generality where it is not needed, to maximize the number of safe optimizations available to translators, and to ensure that unused and constant portions of programs will not add to execution costs. Execution time support packages of the language shall not be included in object code unless they are called. partial? yes? yes yes?
D incorporates garbage collection to some extent. Parasail is designed to allow as much implicit parallelism as possible.
1E. Simplicity. The language should not contain unnecessary complexity. It should have a consistent semantic structure that minimizes the number of underlying concepts. It should be as small as possible consistent with the needs of the intended applications. It should have few special cases and should be composed from features that are individually simple in their semantics. The language should have uniform syntactic conventions and should not provide several notations for the same concept. No arbitrary restriction should be imposed on a language feature. yes? mostly yes yes
Parasail allows several syntactical forms that are identical in meaning.
1F. Implementability. The language shall be composed from features that are understood and can be implemented. The semantics of each feature should be sufficiently well specified and understandable that it will be possible to predict its interaction with other features. To the extent that it does not interfere with other requirements, the language shall facilitate the production of translators that are easy to implement and are efficient during translation. There shall be no language restrictions that are not enforceable by translators. yes yes yes yes
All of these languages have been reasonably implemented.
1G. Machine Independence. The design of the language should strive for machine independence. It shall not dictate the characteristics of object machines or operating systems except to the extent that such characteristics are implied by the semantics of control structures and built-in operations. It shall attempt to avoid features whose semantics depend on characteristics of the object machine or of the object machine operating system. Nevertheless, there shall be a facility for defining those portions of programs that are dependent on the object machine configuration and for conditionally compiling programs depending on the actual configuration. yes yes yes yes
1H. Complete Definition. The language shall be completely and unambiguously defined. To the extent that a formal definition assists in achieving the above goals (i.e., all of section 1), the language shall be formally defined. yes yes mostly? mostly?
While Pascal is the only one of these languages with an ISO standard, most Pascal programming is done with more recent extended dialects. Rust reference material does not completely describe the language.
2A. Character Set. The full set of character graphics that may be used in source programs shall be given in the language definition. Every source program shall also have a representation that uses only the following 55 character subset of the ASCII graphics: %&'()*+,-./:;<=>? 0123456789 ABCDEFGHIJKLMNOPQRSTUVWXYZ_ Each additional graphic (i.e., one in the full set but not in the 55 character set) may be replaced by a sequence of (one or more) characters from the 55 character set without altering the semantics of the program. The replacement sequence shall be specified in the language definition. mostly mostly mostly mostly
2B. Grammar. The language should have a simple, uniform, and easily parsed grammar and lexical structure. The language shall have free form syntax and should use familiar notations where such use does not conflict with other goals. partial? yes yes partial?
Use of familiar notations is something that Parasail arguably takes too far, see 1E. D inherits some grammar issues from C/C++. Rust has less borrowing from that source but is still very ad hoc.
2C. Syntactic Extensions. The user shall not be able to modify the source language syntax. In particular the user shall not be able to introduce new precedence rules or to define new syntactic forms. yes yes yes no
Rust has macros, which can be used to add new syntax to the language.
2D. Other Syntactic Issues. Multiple occurrences of a language defined symbol appearing in the same context shall not have essentially different meanings. Lexical units (i.e., identifiers, reserved words, single and multicharacter symbols, numeric and string literals, and comments) may not cross line boundaries of a source program. All key word forms that contain declarations or statements shall be bracketed (i.e., shall have a closing as well as an opening key word). Programs may not contain unmatched brackets of any kind. mostly yes mostly mostly
D, Pascal, and Rust permit multi line comments. D and Rust use opening and closing braces rather than key words.
2E. Mnemonic Identifiers. Mnemonically significant identifiers shall be allowed. There shall be a break character for use within identifiers. The language and its translators shall not permit identifiers or reserved words to be abbreviated. (Note that this does not preclude reserved words that are abbreviations of natural language words.) yes yes yes yes
2F. Reserved Words. The only reserved words shall be those that introduce special syntactic forms (such as control structures and declarations) or that are otherwise used as delimiters. Words that may be replaced by identifiers, shall not be reserved (e.g., names of functions, types, constants, and variables shall not be reserved). All reserved words shall be listed in the language definition. yes yes yes yes
2G. Numeric Literals. There shall be built-in decimal literals. There shall be no implicit truncation or rounding of integer and fixed point literals. mostly yes mostly? mostly
Parasail Univ_Integer and Univ_Real types provide arbitrary precision. Rust provides configurable ways to treat integer overflow, with the default release mode being wrapping by two's complement. D allows implicit wrapping of integers. Pascal real types are implementation defined. Only Parasail supports fixed point types.
2H. String Literals. There shall be a built-in facility for fixed length string literals. String literals shall be interpreted as one-dimensional character arrays. yes yes yes yes
Univ_Strings in Parasail are vectors of Univ_Character, not arrays.
2I. Comments. The language shall permit comments that are introduced by a special (one or two character) symbol and terminated by the next line boundary of the source program. yes yes partial yes
Line comments were introduced in Turbo Pascal, but are not part of the ISO standard.
3A. Strong Typing. The language shall be strongly typed. The type of each variable, array and record component, expression, function, and parameter shall be determinable during translation. mostly yes yes yes
Some implicit conversion is allowed in D, such as booleans to integral types, one way conversion from enums to integers, and some automatic promotion of integer types.
3B. Type Conversions. The language shall distinguish the concepts of type (specifying data elements with common properties, including operations), subtype (i.e., a subset of the elements of a type, that is characterized by further constraints), and representations (i.e., implementation characteristics). There shall be no implicit conversions between types. Explicit conversion operations shall be automatically defined between types that are characterized by the same logical properties. partial? yes yes? partial?
D, Rust do not have subtypes, although class structures in D can be used with contract programming to provide the same functionality. D allows implicit promotion of integer types. D, Rust have primitive types that are tightly coupled to the typical implementation characteristics of computer hardware.
3C. Type Definitions. It shall be possible to define new data types in programs. A type may be defined as an enumeration, an array or record type, an indirect type, an existing type, or a subtype of an existing type. It shall be possible to process type definitions entirely during translation. An identifier may be associated with each type. No restriction shall be imposed on user defined types unless it is imposed on all types. mostly yes yes? mostly
D, Rust do not have subtypes. D however can emulate similar functionality with type invariant contracts for user defined classes.
3D. Subtype Constraints. The constraints that characterize subtypes shall include range, precision, scale, index ranges, and user defined constraints. The value of a subtype constraint for a variable may be specified when the variable is declared. The language should encourage such specifications. [Note that such specifications can aid the clarity, efficiency, maintainability, and provability of programs.] partial yes? mostly? no
D does not have subtypes but has similar functionality with class type invariants. Rust doesn't have subtypes at all.
3-1A. Numeric Values. The language shall provide distinct numeric types for exact and for approximate computation. Numeric operations and assignment that would cause the most significant digits of numeric values to be truncated (e.g., when overflow occurs) shall constitute an exception situation. partial? yes partial? mostly?
Numeric overflow does not cause an exception in D, but the language provides standard ways to check for the situation. Overflow error handling in Pascal is implementation defined.
3-1B. Numeric Operations. There shall be built-in operations (i.e., functions) for conversion between the numeric types. There shall be operations for addition, subtraction, multiplication, division, negation, absolute value, and exponentiation to integer powers for each numeric type. There shall be built-in equality (i.e., equal and unequal) and ordering operations (i.e., less than, greater than, less than or equal, and greater than or equal) between elements of each numeric type. Numeric values shall be equal if and only if they have exactly the same abstract value. mostly yes mostly mostly
D uses a library abs() function instead of a built-in operator. Pascal does not have built-in operators for absolute value or exponentiation. Rust uses library abs() and pow() functions instead of built-in absolute value and exponentiation operators.
3-1C. Numeric Variables. The range of each numeric variable must be specified in programs and shall be determined by the time of its allocation. Such specifications shall be interpreted as the minimum range to be implemented and as the maximum range needed by the application. Explicit conversion operations shall not be required between numeric ranges. yes yes yes yes
Counting built-in integer types as specifying a range, all these languages do so to some extent. Parasail is the only one that supports user defined custom ranges.
3-1D. Precision. The precision (of the mantissa) of each expression result and variable in approximate computations must be specified in programs, and shall be determinable during translation. Precision specifications shall be required for each such variable. Such specifications shall be interpreted as the minimum accuracy (not significance) to be implemented. Approximate results shall be implicitly rounded to the implemented precision. Explicit conversions shall not be required between precisions. mostly yes no mostly
D defines specific precisions for double and float, along with a minimum precision for real. Custom precisions can be defined for storage only, but all operations happen on doubles/floats/reals. In Standard Pascal precision of real number types is entirely implementation defined. Rust defines specific precisions for 32 and 64 bit floats, but no other control over precision.
3-1E. Approximate Arithmetic Implementation. Approximate arithmetic will be implemented using the actual precisions, radix, and exponent range available in the object machine. There shall be built-in operations to access the actual precision, radix, and exponent range of the implementation. yes yes? partial yes?
In Standard Pascal the values taken by real number types are entirely implementation defined. In practice, this usually means implementation using the actual precisions available in the object machine.
3-1F. Integer and Fixed Point Numbers. Integer and fixed point numbers shall be treated as exact numeric values. There shall be no implicit truncation or rounding in integer and fixed point computations. mostly yes partial mostly
D, Pascal, Rust don't support fixed point numbers, and permit implicit wrapping with integer calculations. Dealing with overflow, wrapping, and other error conditions in Pascal is implementation defined.
3-1G. Fixed Point Scale. The scale or step size (i.e., the minimal representable difference between values) of each fixed point variable must be specified in programs and be determinable during translation. Scales shall not be restricted to powers of two. no yes no no
Of these four languages, inly Parasail supports fixed point types.
3-1H. Integer and Fixed Point Operations. There shall be integer and fixed point operations for modulo and integer division and for conversion between values with different scales. All built-in and predefined operations for exact arithmetic shall apply between arbitrary scales. Additional operations between arbitrary scales shall be definable within programs. no yes no no
All support "modulo" operators; D, Pascal, Rust don't support fixed point numbers.
3-2A. Enumeration Type Definitions. There shall be types that are definable in programs by enumeration of their elements. The elements of an enumeration type may be identifiers or character literals. Each variable of an enumeration type may be restricted to a contiguous subsequence of the enumeration. mostly yes yes mostly
D does not permit character literals in an enumeration, nor restriction to a subsequence. Rust enums are more flexible in content, but still don't support restriction to a subsequence.
3-2B. Operations on Enumeration Types. Equality, inequality, and the ordering operations shall be automatically defined between elements of each enumeration type. Sufficient additional operations shall be automatically defined so that the successor, predecessor, the position of any element, and the first and last element of the type may be computed. yes yes? yes? mostly?
3-2C. Boolean Type. There shall be a predefined type for Boolean values. yes yes yes yes
D's boolean type is weakly typed.
3-2D. Character Types. Character sets shall be definable as enumeration types. Character types may contain both printable and control characters. The ASCII character set shall be predefined. yes yes yes yes
3-3A. Composite Type Definitions. It shall be possible to define types that are Cartesian products of other types. Composite types shall include arrays (i.e., composite data with indexable components of homogeneous types) and records (i.e., composite data with labeled components of heterogeneous type). yes yes yes yes
All have arrays and records.
3-3B. Component Specifications. For elements of composite types, the type of each component (i.e., field) must be explicitly specified in programs and determinable during translation. Components may be of any type (including array and record types). Range, precision, and scale specifications shall be required for each component of appropriate numeric type. yes yes yes yes
Range, precision, and scale specifications are included in numeric type definitions (with support varying, see 3-1).
3-3C. Operations on Composite Types. A value accessing operation shall be automatically defined for each component of composite data elements. Assignment shall be automatically defined for components that have alterable values. A constructor operation (i.e., an operation that constructs an element of a type from its constituent parts) shall be automatically defined for each composite type. An assignable component may be used anywhere in a program that a variable of the component's type is permitted. There shall be no automatically defined equivalence operations between values of elements of a composite type. yes yes yes yes
3-3D. Array Specifications. Arrays that differ in number of dimensions or in component type shall be of different types. The range of subscript values for each dimension must be specified in programs and may be determinable at the time of array allocation. The range of each subscript value must be restricted to a contiguous sequence of integers or to a contiguous sequence from an enumeration type. mostly yes yes mostly
D and Rust array indexes can only start at zero and cannot use enumerations.
3-3E. Operations on Subarrays. There shall be built-in operations for value access, assignment, and catenation of contiguous sections of one-dimensional arrays of the same component type. The results of such access and catenation operations may be used as actual input parameter. yes yes? no partial
Pascal has extremely limited array slicing and does not have a built-in array concatenation operator. Rust has array slicing facilities, but lacks a built-in array concatenation operator.
3-3F. Nonassignable Record Components. It shall be possible to declare constants and (unary) functions that may be thought of as record components and may be referenced using the same notation as for accessing record components. Assignment shall not be permitted to such components. yes yes no? no?
D classes can include constants and functions. Parasail type inferfaces can include constants and functions.
3-3G. Variants. It shall be possible to define types with alternative record structures (i.e., variants). The structure of each variant shall be determinable during translation. yes yes? yes yes
D classes can be used to simulate runtime variants. D also has untagged unions. Pascal has variant records. Rust has tagged unions called "sum types".
3-3H. Tag Fields. Each variant must have a nonassignable tag field (i.e., a component that can be used to discriminate among the variants during execution). It shall not be possible to alter a tag field without replacing the entire variant. yes yes? yes? yes
3-3I. Indirect Types. It shall be possible to define types whose elements are indirectly accessed. Elements of such types may have components of their own type, may have substructure that can be altered during execution, and may be distinct while having identical component values. Such types shall be distinguishable from other composite types in their definitions. An element of an indirect type shall remain allocated as long as it can be referenced by the program. [Note that indirect types require pointers and sometimes heap storage in their implementation.] yes yes yes yes
3-3J. Operations on Indirect Types. Each execution of the constructor operation for an indirect type shall create a distinct element of the type. An operation that distinguishes between different elements, an operation that replaces all of the component values of an element without altering the element's identity, and an operation that produces a new element having the same component values as its argument, shall be automatically defined for each indirect type. yes yes yes yes
3-4A. Bit Strings (i.e., Set Types). It shall be possible to define types whose elements are one-dimensional Boolean arrays represented in maximally packed form (i.e, whose elements are sets). yes yes yes partial
D provides bit arrays in the standard library in std.bitmanip. It is easy enough to construct bit strings in Rust using structs or integer types, but the language itself does not provide them as built in functionality.
3-4B. Bit String Operations. Set construction, membership (i.e., subscription), set equivalence and nonequivalence, and also complement, intersection, union, and symmetric difference (i.e., component-by-component negation, conjunction, inclusive disjunction, and exclusive disjunction respectively) operations shall be defined automatically for each set type. yes yes? yes partial
3-5A. Encapsulated Definitions. It shall be possible to encapsulate definitions. An encapsulation may contain declarations of anything (including the data elements and operations comprising a type) that is definable in programs. The language shall permit multiple explicit instantiations of an encapsulation. yes yes yes yes
All of these languages have modules as their unit of encapsulation.
3-5B. Effect of Encapsulation. An encapsulation may be used to inhibit external access to implementation properties of the definition. In particular, it shall be possible to prevent external reference to any declaration within the encapsulation including automatically defined operations such as type conversions and equality. Definitions that are made within an encapsulation and are externally accessible may be renamed before use outside the encapsulation. yes yes yes yes
3-5C. Own Variables. Variables declared within an encapsulation, but not within a function, procedure, or process of the encapsulation, shall remain allocated and retain their values throughout the scope in which the encapsulation is instantiated. yes yes yes yes
4A. Form of Expressions. The parsing of correct expressions shall not depend on the types of their operands or on whether the types of the operands are built into the language. yes yes yes yes
4B. Type of Expressions. It shall be possible to specify the type of any expression explicitly. The use of such specifications shall be required only where the type of the expression cannot be uniquely determined during translation from the context of its use (as might be the case with a literal). yes yes yes yes
4C. Side Effects. The language shall attempt to minimize side effects in expressions, but shall not prohibit all side effects. A side effect shall not be allowed if it would alter the value of a variable that can be accessed at the point of the expression. Side effects shall be limited to own variables of encapsulations. The language shall permit side effects that are necessary to instrument functions and to do storage management within functions. The order of side effects within an expression shall not be guaranteed. [Note that the latter implies that any program that depends on the order of side effects is erroneous.] mostly? yes? mostly? mostly?
Parasail does not permit global variables.
4D. Allowed Usage. Expressions of a given type shall be allowed wherever both constants and variables of the type are allowed. yes yes yes yes
4E. Translation Time Expressions. Expressions that can be evaluated during translation shall be permitted wherever literals of the type are permitted. Translation time expressions that include only literals and the use of translation time facilities (see 11C) shall be evaluated during translation. yes yes yes yes
4F. Operator Precedence Levels. The precedence levels (i.e., binding strengths) of all (prefix and infix) operators shall be specified in the language definition, shall not be alterable by the user, shall be few in number, and shall not depend on the types of the operands. mostly yes yes mostly
Pascal has 5 levels, Parasail has 7, Rust has 13 and D has 15-19 depending on how you count them. For comparison, Ada has 6.
4G. Effect of Parentheses. If present, explicit parentheses shall dictate the association of operands with operators. The language shall specify where explicit parentheses are required and shall attempt to minimize the psychological ambiguity in expressions. [Note that this might be accomplished by requiring explicit parentheses to resolve the operator-operand association whenever a nonassociative operator appears to the left of an operator of the same precedence at the least-binding precedence level of any subexpression.] yes yes yes yes
5A. Declarations of Constants. It shall be possible to declare constants of any type. Such constants shall include both those whose values-are determined during translation and those whose value cannot be determined until allocation. Programs may not assign to constants. yes yes yes yes
5B. Declarations of Variables. Each variable must be declared explicitly. Variables may be of any type. The type of each variable must be specified as part of its declaration and must be determinable during translation. [Note, "variable" throughout this document refers not only to simple variables but also to composite variables and to components of arrays and records.] mostly yes yes partial
D permits "void *" as a type, which is really a pointer to an unknown type and subverts the type system. Rust does not require the type of each variable to be explicitly specified and will infer types instead.
5C. Scope of Declarations. Everything (including operators) declared in a program shall have a scope (i.e., a portion of the program in which it can be referenced). Scopes shall be determinable during translation. Scopes may be nested (i.e., lexically embedded). A declaration may be made in any scope. Anything other than a variable shall be accessable within any nested scope of its definition. yes? yes yes yes?
5D. Restrictions on Values. Procedures, functions, types, labels, exception situations, and statements shall not be assignable to variables, be computable as values of expressions, or be usable as nongeneric parameters to procedures or functions. no no no? no
D and Pascal allow pointers to functions. Parasail allows lambda expressions. Rust has first class functions.
5E. Initial Values. There shall be no default initial-values for variables. partial partial partial? mostly
D defines initial values for all types. Parasail sets initial values of all 'optional' types to null. Rust does not assign default initial values, but instead requires the programmer to always provide an initial value. All of these instances are done to support reliability.
5F. Operations on Variables. Assignment and an implicit value access operation shall be automatically defined for each variable. yes yes yes yes
5G. Scope of Variables. The language shall distinguish between open scopes (i.e., those that are automatically included in the scope of more globally declared variables) and closed scopes (i.e., those in which nonlocal variables must be explicitly Imported). Bodies of functions, procedures, and processes shall be closed scopes. Bodies of classical control structures shall be open scopes. yes yes yes yes
6A. Basic Control Facility. The (built-in) control mechanisms should be of minimal number and complexity. Each shall provide a single capability and shall have a distinguishing syntax. Nesting of control structures shall be allowed. There shall be no control definition facility. Local scopes shall be allowed within the bodies of control statements. Control structures shall have only one entry point and shall exit to a single point unless exited via an explicit transfer of control (where permitted, see 6G), or the raising of an exception (see 10C). yes yes yes yes
6B. Sequential Control. There shall be a control mechanism for sequencing statements. The language shall not impose arbitrary restrictions on programming style, such as the choice between statement terminators and statement separators, unless the restriction makes programming errors less likely. yes yes yes yes
D and Rust use statement terminators. Pascal and Parasail use statement separators.
6C. Conditional Control. There shall be conditional control structures that permit selection among alternative control paths. The selected path may depend on the value of a Boolean expression, on a computed choice among labeled alternatives, or on the true condition in a set of conditions. The language shall define the control action for all values of the discriminating condition that are not specified by the program. The user may supply a single control path to be used when no other path is selected. Only the selected branch shall be compiled when the discriminating condition is a translation time expression. yes yes yes yes
6D. Short Circuit Evaluation. There shall be infix control operations for short circuit conjunction and disjunction of the controlling Boolean expression in conditional and iterative control structures. yes yes partial yes
Standard Pascal does not provide infix control operations, but both Extended Pascal and Turbo Pascal do.
6E. Iterative Control. There shall be an iterative control structure. The iterative control may be exited (without reentry) at an unrestricted number of places. A succession of values from an enumeration type or the integers may be associated with successive iterations and the value for the current iteration accessed as a constant throughout the loop body. mostly yes yes yes
In D, the loop control variable is not considered a constant.
6G. Explicit Control Transfer. There shall be a mechanism for control transfer (i.e., the go to). It shall not be possible to transfer out of closed scopes, into narrower scopes, or into control structures. It shall be possible to transfer out of classical control structures. There shall be no control transfer mechanisms in the form of switches, designational expressions, label variables, label parameters, or alter statements. yes? partial yes partial?
Neither Parasail nor Rust support goto. However both support break/continue statements that serve the same purpose in many cases.
7A. Function and Procedure Definitions. Functions (which return values to expressions) and procedures (which can be called as statements) shall be definable in programs. Functions or procedures that differ in the number or types of their parameters may be denoted by the same identifier or operator (i.e., overloading shall be permitted). [Note that redefinition, as opposed to overloading, of an existing function or procedure is often error prone.] yes yes yes no
Rust does not support ad-hoc polymorphism. It must be emulated using the trait system.
7B. Recursion. It shall be possible to call functions and procedures recursively. yes yes yes yes
 
7C. Scope Rules. A reference to an identifier that is not declared in the most local scope shall refer to a program element that is lexically global, rather than to one that is global through the dynamic calling structure. yes yes yes yes
7D. Function Declarations. The type of the result for each function must be specified in its declaration and shall be determinable during translation. The results of functions may be of any type. If a result is of a nonindirect array or record type then the number of its components must be determinable by the time of function call. mostly mostly mostly? mostly
7F. Formal Parameter Classes. There shall be three classes of formal data parameters: (a) input parameters, which act as constants that are initialized to the value of corresponding actual parameters at the time of call, (b) input-output parameters, which enable access and assignment to the corresponding actual parameters, either throughout execution or only upon call and prior to any exit, and (c) output parameters, whose values are transferred to the corresponding actual parameter only at the time of normal exit. In the latter two cases the corresponding actual parameter shall be determined at time of call and must be a variable or an assignable component of a composite type. partial yes partial? partial?
D, Pascal and Rust do not identify in, in-out and out parameters. D, Pascal and Rust can support in-only parameters.
7G. Parameter Specifications. The type of each formal parameter must be explicitly specified in programs and shall be determinable during translation. Parameters may be of any type. The language shall not require user specification of subtype constraints for formal parameters. If such constraints are permitted they shall be interpreted as assertions and not as additional overloading. Corresponding formal and actual parameters must be of the same type. yes? yes yes yes?
7H. Formal Array Parameters. The number of dimensions for formal array parameters must be specified in programs and shall be determinable during translation. Determination of the subscript range for formal array parameters may be delayed until invocation and may vary from call to call. Subscript ranges shall be accessible within function and procedure bodies without being passed as explicit parameters. no yes yes no
Subscript ranges are not accessible in D or Rust.
7I. Restrictions to Prevent Aliasing. The language shall attempt to prevent aliasing (l.e., multiple access paths to the same variable or record component) that is not intended, but shall not prohibit all aliasing. Aliasing shall not be permitted between output parameters nor between an input-output parameter and a nonlocal variable. Unintended aliasing shall not be permitted between input-output parameters. A restriction limiting actual input-output parameters to variables that are nowhere referenced as nonlocals within a function or routine, is not prohibited. All aliasing of components of elements of an indirect type shall be considered intentional. no yes yes? yes
8A. Low Level Input-Output. There shall be a few low level input-output operations that send and receive control information to and from physical channels and devices. The low level operations shall be chosen to insure that all user level input-output operations can be defined within the language. partial? no? partial? no?
D and some Pascal dialects permit access to memory mapped locations.
8B. User Level Input-Output. The language shall specify (i.e., give calling format and general semantics) a recommended set of user level input-output operations. These shall include operations to create, delete, open, close, read, write, position, and interrogate both sequential and random access files and to alter the association between logical files and physical devices. yes yes yes yes
8C. Input Restrictions. User level input shall be restricted to data whose record representations are known to the translator (i.e., data that is created and written entirely within the program or data whose representation is explicitly specified in the program). yes yes yes yes
8D. Operating System Independence. The language shall not require the presence of an operating system. [Note that on many machines it will be necessary to provide run-time procedures to implement some features of the language.] yes yes yes yes
8E. Resource Control. There shall be a few low level operations to interrogate and control physical resources (e.g., memory or processors) that are managed (e.g., allocated or scheduled) by built-in features of the language. mostly mostly partial no
D supports custom garbage collection and thread priorities. Standard Pascal does not define ways to control physical resources, but popular implementations such as Free Pascal provide both custom memory management and thread facilities.
8F. Formating. There shall be predefined operations to convert between the symbolic and internal representation of all types that have literal forms in the language (e.g., strings of digits to integers, or an enumeration element to its symbolic form). These conversion operations shall have the same semantics as those specified for literals in programs. yes yes yes partial
In Rust, operations to convert between enumerations and strings are not predefined.
9A. Parallel Processing. It shall be possible to define parallel processes. Processes (i.e., activation instances of such a definition) may be initiated at any point within the scope of the definition. Each process (activation) must have a name. It shall not be possible to exit the scope of a process name unless the process is terminated (or uninitiated). yes mostly no yes
D provides this functionality with the std.parallelism and core.thread libraries. Rust provides this with the std::thread library. Parasail is designed to be implicitly parallel by default, and thus the lightweight threads used do not have names. Pascal does not have built in thread or process facilities, and must rely on operating system specific libraries.
9B. Parallel Process Implementation. The parallel processing facility shall be designed to minimize execution time and space. Processes shall have consistent semantics whether implemented on multicomputers, multiprocessors, or with interleaved execution on a single processor. yes yes no yes
9C. Shared Variables and Mutual Exclusion. It shall be.possible to mark variables that are shared among parallel processes. An unmarked variable that is assigned on one path and used on another shall cause a warning. It shall be possible efficiently to perform mutual exclusion in programs. The language shall not require any use of mutual exclusion. partial? yes no yes
D supports shared variables and atomic operations, however the idiomatic way of threading is to rely on immutable data and message passing.
9D. Scheduling. The semantics of the built-in scheduling algorithm shall be first-in-first-out within priorities. A process may alter its own priority. If the language provides a default priority for new processes it shall be the priority of its initiating process. The built-in scheduling algorithm shall not require that simultaneously executed processes on different processors have the same priority. [Note that this rule gives maximum scheduling control to the user without loss of efficiency. Note also that priority specification does not impose a specific execution order among parallel paths and thus does not provide a means for mutual exclusion.] yes? yes? no partial?
9E. Real Time. It shall be possible to access a real time clock. There shall be translation time constants to convert between the implementation units and the program units for real time. On any control path, it shall be possible to delay until at least a specified time before continuing execution. A process may have an accessible clock giving the cumulative processing time (i.e., CPU time) for that process. yes yes yes? yes?
D provides this in core.time. Parasail provides this in its standard library.
9G. Asynchronous Termination. It shall be possible to terminate another process. The terminated process may designate the sequence of statements it will execute in response to the induced termination. yes no? no yes
D achieves this with std.parallelism and std.process. Pascal programs call an operating system dependent library to perform this. Parasail is structured around implicit pervasive parallelism so it's questionable how applicable this requirement is. Rust achieves this with std::process::Child.
9H. Passing Data. It shall be possible to pass data between processes that do not share variables. It shall be possible to delay such data transfers until both the sending and receiving processes have requested the transfer. yes yes no yes
D synchronized calls allow this. Rust achieves this with std::sync::mpsc.
9I. Signalling. It shall be possible to set a signal (without waiting), and to wait for a signal (without delay, if it is already set). Setting a signal, that is not already set, shall cause exactly one waiting path to continue. mostly? mostly? no mostly?
9J. Waiting. It shall be possible to wait for, determine, and act upon the first completed of several wait operations (including those used for data passing, signalling, and real time). mostly? mostly? no mostly?
10A. Exception Handling Facility. There shall be an exception handling mechanism for responding to unplanned error situations detected in declarations and statements during execution. The exception situations shall include errors detected by hardware, software errors detected during execution, error situations in built-in operations, and user defined exceptions. Exception identifiers shall have a scope. Exceptions should add to the execution time of programs only if they are raised. yes partial? partial? partial
Standard Pascal does not specify how to treat errors, whether with exceptions or otherwise. However later variations including FreePascal and Delphi support exceptions. Parasail attempts to check for all possible errors at compile time, however it is unclear from the reference manual how hardware problems are handled. Rust opts for using return value types to show errors rather than exceptions.
10B. Error Situations. The errors detectable during execution shall include exceeding the specified range of an array subscript, exceeding the specified range of a variable, exceeding the implemented range of a variable, attempting to access an uninitialized variable, attempting to access a field of a variant that is not present, requesting a resource (such as stack or heap storage) when an insufficient quantity remains, and failing to satisfy a program specified assertion. [Note that some are very expensive to detect unless aided by special hardware, and consequently their detection will often be suppressed (see 10G).] partial? mostly partial partial?
Parasail is constructed to detect all of these mentioned errors, except the out of memory error, at compile time.
10C. Raising Exceptions. There shall be an operation that raises an exception. Raising an exception shall cause transfer of control to the most local enclosing exception handler for that exception without completing execution of the current statement or declaration, but shall not of itself cause transfer out of a function, procedure, or process. Exceptions that are not handled within a function or procedure shall be raised again at the point of call in their callers. Exceptions that are not handled within a process shall terminate the process. Exceptions that can be raised by built-in operations shall be given in the language definition. yes partial partial partial
Standard Pascal does not specify how to handle errors, whether exceptions or otherwise, see 10A. It is unclear from the Parasail reference manual whether actual exceptions are used in the language, but similar functionality is achieved with compile time annotations. Rust opts for using return value types to show errors rather than exceptions. Various functions and macros are provided that more or less covers the same thing, but not in a way that satisfies this requirement.
10D. Exception Handling. There shall be a control structure for discriminating among the exceptions that can occur in a specified statement sequence. The user may supply a single control path for all exceptions not otherwise mentioned in such a discrimination. It shall be possible to raise the exception that selected the current handler when exiting the handler. yes no? partial no
Standard Pascal does not specify how to handle errors, whether exceptions or otherwise, see 10A.
10E. Order of Exceptions. The order in which exceptions in different parts of an expression are detected shall not be guaranteed by the language or by the translator. yes? yes yes yes
10F. Assertions. It shall be possible to include assertions in programs. If an assertion is false when encountered during execution, it shall raise an exception. It shall also be possible to include assertions, such as the expected frequency for selection of a conditional path, that cannot be verified. [Note that assertions can be used to aid optimization and maintenance.] mostly mostly no mostly
None? of these languages permit assertions of frequency.
10G. Suppressing Exceptions. It shall be possible during translation to suppress individually the execution time detection of exceptions within a given scope. The language shall not guarantee the integrity of the values produced when a suppressed exception occurs. [Note that suppression of an exception is not an assertion that the corresponding error will not occur.] partial no no no
It is possible to statically disallow code from throwing exceptions in D, but that doesn't fulfil the same function as this requirement.
11A. Data Representation. The language shall permit but not require programs to specify a single physical representation for the elements of a type. These specifications shall be separate from the logical descriptions. Physical representation shall include object representation of enumeration elements, order of fields, width of fields, presence of "don't care" fields, positions of word boundaries, and object machine addresses. In particular, the facility shall be sufficient to specify the physical representation of any record whose format is determined by considerations that are entirely external to the program, translator, and language. The language and its translators shall not guarantee any particular choice for those aspects of physical representation that are unspecified by the program. It shall be possible to specify the association of physical resources (e.g., interrupts) to program elements (e.g., exceptions or signals). partial? no? no partial
11C. Translation Time Facilities. To aid conditional compilation, it shall be possible to interrogate properties that are known during translation including characteristics of the object configuration, of function and procedure calling environments, and of actual parameters. For example, it shall be possible to determine whether the caller has suppressed a given exception, the callers optimization criteria, whether an actual parameter is a translation time expression, the type of actual generic parameters, and the values of constraints characterizing the subtype of actual parameters. partial partial? no partial
11D. Object System Configuration. The object system configuration must be explicitly specified in each separately translated unit. Such specifications must include the object machine model, the operating system if present, peripheral equipment, and the device configuration, and may include special hardware options and memory size. The translator will use such specifications when generating object code. [Note that programs that depend on the specific characteristics of the object machine, may be made more portable by enclosing those portions in branches of conditionals on the object machine configuration.] no? no? no no?
11E. Interface to Other Languages. There shall be a machine independent interface to other programming languages including assembly languages. Any program element that is referenced in both the source language program and foreign code must be identified in the interface. The source language of the foreign code must also be identified. yes no mostly yes
D provides interfaces to C, C++ and assembly. Many Pascal implementations provide interfaces to C and assembly. Parasail provides no interfaces to other languages. Rust provides an interface to C and assembly.
11F. Optimization. Programs may advise translators on the optimization criteria to be used in a scope. It shall be possible in programs to specify whether minimum translation costs or minimum execution costs are more important, and whether execution time or memory space is to be given preference. All such specifications shall be optional. Except for the amount of time and space required during execution, approximate values beyond the specified precision, the order in which exceptions are detected, and the occurrence of side effects within an expression, optimization shall not alter the semantics of correct programs, (e.g., the semantics of parameters will be unaffected by the choice between open and closed calls). partial? no? partial? partial?
12A. Library. There shall be an easily accessible library of generic definitions and separately translated units. All predefined definitions shall be in the library. Library entries may include those used as input-output packages, common pools of shared declarations, application oriented software packages, encapsulations, and machine configuration specifications. The library shall be structured to allow entries to be associated with particular applications, projects, and users. yes yes yes yes
12B. Separately Translated Units. Separately translated units may be assembled into operational systems. It shall be possible for a separately translated unit to reference exported definitions of other units. All language imposed restrictions shall be enforced across such interfaces. Separate translation shall not change the semantics of a correct program. mostly? yes? yes? yes?
12D. Generic Definitions. Functions, procedures, types, and encapsulations may have generic parameters. Generic parameters shall be instantiated during translation and shall be interpreted in the context of the instantiation. An actual generic parameter may be any defined identifier (including those for variables, functions, procedures, processes, and types) or the value of any expression. yes yes partial mostly
Rust only accepts types as generic parameters. Standard Pascal does not support generics, but later Pascal derivatives such as Free Pascal and Delphi both do.
13A. Defining Documents. The language shall have a complete and unambiguous defining document. It should be possible to predict the possible actions of any syntactically correct program from the language definition. The language documentation shall include the syntax, semantics, and appropriate examples of each built-in and predefined feature. A recommended set of translation diagnostic and warning messages shall be included in the language definition. mostly? mostly? yes partial
13B. Standards. There will be a standard definition of the language. Procedures will be established for standards control and for certification that translators meet the standard. no no yes no
13C. Completeness of Implementations. Translators shall implement the standard definition. Every translator shall be able to process any syntactically correct program. Every feature that is available to the user shall be defined in the standard, in an accessible library, or in the source program. mostly? yes yes mostly
13D. Translator Diagnostics. Translators shall be responsible for reporting errors that are detectable during translation and for optimizing object code. Translators shall be responsible for the integrity of object code in affected translation units when any separately translated unit is modified, and shall ensure that shared definitions have compatible representations in all translation units. Translators shall do full syntax and type checking, shall check that all language imposed restrictions are met, and should provide warnings where constructs will be dangerous or unusually expensive in execution and shall attempt to detect exceptions during translation. If the translator determines that a call on a routine will not terminate normally, the exception shall be reported as a translation error at the point of call. mostly? yes yes? yes?
13E. Translator Characteristics. Translators for the language will be written in the language and will be able to produce code for a variety of object machines. The machine independent parts of translators should be separate from code generators. Although it is desirable, translators need not be able to execute on every object machine. The internal characteristics of the translator (i.e., the translation method) shall not be specified by the language definition or standards. mostly mostly mostly mostly
Many, but not all, compilers are written in their own language. Parasail and Rust both have one compiler each, both written in their respective language.
13F. Restrictions on Translators. Translators shall fail to translate otherwise correct programs only when the program requires more resources during translation than are available on the host machine or when the program calls for resources that are unavailable in the specified object system configuration. Neither the language nor its translators shall impose arbitrary restrictions on language features. For example, they shall not impose restrictions on the number of array dimensions, on the number of identifiers, on the length of identifiers, or on the number of nested parentheses levels. yes yes yes? yes
13G. Software Tools and Application Packages. The language should be designed to work in conjunction with a variety of useful software tools and application support packages. These will be developed as early as possible and will include editors, interpreters, diagnostic aids, program analyzers, documentation aids, testing aids, software maintenance tools, optimizers, and application libraries. There will be a consistent user interface for these tools. Where practical software tools and aids will be written in the language. Support for the design, implementation, distribution, and maintenance of translators, software tools and aids, and application libraries will be provided independently of the individual projects that use them. mostly partial yes partial