High-level programming language

A high-level programming language is a programming language with strong abstraction from the details of the computer. In contrast to low-level programming languages, it may use natural language elements, be easier to use, or may automate (or even hide entirely) significant areas of computing systems (e.g. memory management), making the process of developing a program simpler and more understandable than when using a lower-level language. The amount of abstraction provided defines how "high-level" a programming language is.^[1]

High-level refers to a level of abstraction from the hardware details of a processor inherent in machine and assembly code. Rather than dealing with registers, memory addresses, and call stacks, high-level languages deal with variables, arrays, objects, arithmetic and Boolean expressions, functions, loops, threads, locks, and other computer science abstractions, intended to facilitate correctness and maintainability. Unlike low-level assembly languages, high-level languages have few, if any, language elements that translate directly to a machine's native opcodes. Other features, such as string handling, Object-oriented programming features, and file input/output, may also be provided. A high-level language allows for source code that is detached and separated from the machine details. That is, unlike low-level languages like assembly and machine code, high-level language code may result in data movements without the programmer's knowledge. Some control of what instructions to execute is handed to the compiler.

History

In the 1960s, a high-level programming language using a compiler was commonly called an autocode.^[2] Examples of autocodes are COBOL and Fortran.^[3]

The first high-level programming language designed for computers was Plankalkül, created by Konrad Zuse.^[4] However, it was not implemented in his time, and his original contributions were largely isolated from other developments due to World War II, aside from the language's influence on the "Superplan" language by Heinz Rutishauser and also to some degree ALGOL. The first significantly widespread high-level language was Fortran, a machine-independent development of IBM's earlier Autocode systems. The ALGOL family, with ALGOL 58 defined in 1958 and ALGOL 60 defined in 1960 by committees of European and American computer scientists, introduced recursion as well as nested functions under lexical scope. ALGOL 60 was also the first language with a clear distinction between value and name-parameters and their corresponding semantics.^[5] ALGOL also introduced several structured programming concepts, such as the while-do and if-then-else constructs and its syntax was the first to be described in formal notation – Backus–Naur form (BNF). During roughly the same period, COBOL introduced records (also called structs) and Lisp introduced a fully general lambda abstraction in a programming language for the first time.

Abstraction penalty

A high-level language provides features that standardize common tasks, permit rich debugging, and maintain architectural agnosticism. On the other hand, a low-level language requires the coder to work at a lower-level of abstraction which is generally more challenging, but does allow for optimizations that are not possible with a high-level language. This abstraction penalty for using a high-level language instead of a low-level language is real, but in practice, low-level optimizations rarely improve performance at the user experience level.^[6][7][8] None the less, code that needs to run quickly and efficiently may require the use of a lower-level language, even if a higher-level language would make the coding easier to write and maintain. In many cases, critical portions of a program mostly in a high-level language are coded in assembly in order to meet tight timing or memory constraints. A well-designed compiler for a high-level language can produce code comparable in efficiency to what could be coded by hand in assembly, and the higher-level abstractions sometimes allow for optimizations that beat the performance of hand-coded assembly.^[9] Since a high-level language is designed independent of a specific computing system architecture, a program written in such a language can run on any computing context with a compatible compiler or interpreter.

Unlike a low-level language that is inherently tied to processor hardware, a high-level language can be improved, and new high-level languages can evolve from others with the goal of aggregating the most popular constructs with improved features. For example, Scala maintains backward compatibility with Java. Code written in Java continue to be usable even if a developer switches to Scala. This makes the transition easier and extends the lifespan of a codebase. In contrast, low-level programs rarely survive beyond the system architecture which they were written for.

Relative meaning

The terms high-level and low-level are inherently relative, and languages can be compared as higher or lower level to each other. Sometimes the C language is considered as either high-level or low-level depending on one's perspective. Regardless, most agree that C is higher level than assembly and lower level than most other languages.

C supports constructs such as expression evaluation, parameterized and recursive functions, data types and structures which are generally not supported in assembly or directly by a processor but C does provide lower-level features such as auto-increment and pointer math. But C lacks many higher-level abstracts common in other languages such as garbage collection and a built-in string type. In the introduction of The C Programming Language (second edition) by Brian Kernighan and Dennis Ritchie, C is described as "not a very high level" language.^[10]

Assembly language is higher-level than machine code, but still highly tied to the processor hardware. But, assembly may provide some higher-level features such as macros, relatively limited expressions, constants, variables, procedures, and data structures.

Machine code is at a slightly higher level abstraction than the microcode or micro-operations used internally in many processors.^[11]

Execution modes

The source code of a high-level language may be processed in various ways, including

Compiled

A compiler transforms source code into other code. Sometimes, and traditionally, a compiler generates native machine code that is interpreted by the processor. But, today many execution models involve generating an intermediate representation (i.e. bytecode) that is later interpreted in software or converted to native code at runtime (via JIT compilation).

Transpiled

Code may be translated into source code of another language (typically lower-level) for which a compiler or interpreter is available. JavaScript and the C are common targets for such translators. For example, C and C++ code can be seen as generated from Eiffel code when using the EiffelStudio IDE. In Eiffel, the translated process is referred to as transcompiling or transcompiled, and the Eiffel compiler as a transcompiler or source-to-source compiler.

Software interpreted

A software interpreter performs the actions encoded in source code without generating native machine code.

Hardware interpreted

Although uncommon, a processor with a high-level language computer architecture can process a high-level language without a compilation step. For example, the Burroughs large systems were target machines for ALGOL 60.^[12]

Note that a language is not strictly interpreted or compiled. Rather, an execution model involves a compiler or an interpreter and the same language might be used with different execution models. For example, ALGOL 60 and Fortran have both been interpreted even though they were more typically compiled. Similarly, Java shows the difficulty of trying to apply these labels to languages, rather than to implementations. Java is compiled to bytecode which is then executed by either interpreting in a Java virtual machine (JVM) or JIT compiled.

See also

• Computer programming portal

• Generational list of programming languages
• Categorical list of programming languages
• Very high-level programming languages
• Low-level programming languages
• High-level assembler
• Abstraction (computer science)