In computer science, a high-level programming language is a programming language with strong
abstraction from the details of the computer.
In comparison to low-level programming languages, it may use natural language elements, be easier
to use, or may automate significant areas of computing systems, making the process of
developing a program simpler and more understandable relative to a lower-level language.
The amount of abstraction provided defines how "high-level" a programming language is.
In the 1960s, high-level programming languages using a compiler were commonly called autocodes.
Examples of autocodes are COBOL and Fortran.
The first high-level programming language designed for computers was Plankalkül, created
by Konrad Zuse.
However, it was not implemented in his time, and his original contributions were largely
isolated from other developments, although it influenced Heinz Rutishauser's language
"Superplan".
The first really widespread high-level language was Fortran, a machine independent development
of IBM's earlier Autocode systems.
Algol, defined in 1958 and 1960, by committees of European and American computer scientist,
introduced recursion as well as nested functions under lexical scope.
It was also the first language with a clear distinction between value and name-parameters
and their corresponding semantics.
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 by a formal method, called BNF,
for Backus-Naur form.
During roughly the same period Cobol introduced records and Lisp introduced a fully general
lambda abstraction in a programming language for the first time.
Examples of popular high-level programming languages today may include Java, Python,
Visual Basic, Delphi, Perl, PHP, ECMA Script, Ruby and many others.
Features "High-level language" refers to the higher
level of abstraction from machine language.
Rather than dealing with registers, memory addresses and call stacks, high-level languages
deal with variables, arrays, objects, complex arithmetic or boolean expressions, subroutines
and functions, loops, threads, locks, and other abstract computer science concepts,
with a focus on usability over optimal program efficiency.
Unlike low-level assembly languages, high-level languages have few, if any, language elements
that translate directly into a machine's native opcodes.
Other features, such as string handling routines, object-oriented language features, and file
input/output, may also be present.
Abstraction penalty While high-level languages are intended to
make complex programming simpler, low-level languages often produce more efficient code.
Abstraction penalty is the border that prevents high-level programming techniques from being
applied in situations where computational resources are limited.
High-level programming exhibits features like more generic data structures, run-time interpretation,
and intermediate code files; which often result in slower execution speed, higher memory consumption,
and larger binary program size.
For this reason, code which needs to run particularly quickly and efficiently may require the use
of a lower-level language, even if a higher-level language would make the coding easier.
In many cases, critical portions of a program mostly in a high-level language can be hand-coded
in assembly language, leading to a much faster or more efficient optimised program.
However, with the growing complexity of modern microprocessor architectures, well-designed
compilers for high-level languages frequently produce code comparable in efficiency to what
most low-level programmers can produce by hand, and the higher abstraction may allow
for more powerful techniques providing better overall results than their low-level counterparts
in particular settings.
High-level languages are designed independent of structure of a specific computer.
This facilitates executing a program written in such a language on different computers.
Relative meaning The terms high-level and low-level are inherently
relative.
Some decades ago, the C language, and similar languages, were most often considered "high-level",
as it supported concepts such as expression evaluation, parameterised recursive functions,
and data types and structures, while assembly language was considered "low-level".
Today, many programmers might refer to C as low-level, as it lacks a large runtime-system,
basically supports only scalar operations, and provides direct memory addressing.
It, therefore, readily blends with assembly language and the machine level of CPUs and
microcontrollers.
Assembly language may itself be regarded as a higher level representation of machine code,
as it supports concepts such as constants and expressions, sometimes even variables,
procedures, and data structures.
Machine code, in its turn, is inherently at a slightly higher level than the microcode
or micro-operations used internally in many processors.
Execution models There are three general models of execution
for modern high-level languages: Interpreted
Interpreted languages are read and then executed directly, with no compilation stage.
A program called an interpreter reads each program statement following the program flow,
decides what to do, and does it.
A hybrid of an interpreter and a compiler will compile the statement into machine code
and execute that; the machine code is then discarded, to be interpreted anew if the line
is executed again.
Interpreters are commonly the simplest implementations, compared to the other two variants listed
here.
Compiled Compiled languages are transformed into an
executable form before running.
There are two types of compilation: Machine code generation
Some compilers compile source code directly into machine code.
This is the original mode of compilation, and languages that are directly and completely
transformed to machine-native code in this way may be called "truly compiled" languages.
See assembly language.
Intermediate representations When a language is compiled to an intermediate
representation, that representation can be optimized or saved for later execution without
the need to re-read the source file.
When the intermediate representation is saved, it is often represented as byte code.
The intermediate representation must then be interpreted or further compiled to execute
it.
Virtual machines that execute byte code directly or transform it further into machine code
have blurred the once clear distinction between intermediate representations and truly compiled
languages.
Translated or Trans-compiled A language may be translated into a lower-level
programming language for which native code compilers are already widely available.
The C programming language is a common target for such translators.
See Chicken Scheme and the Eiffel as examples.
Specifically, the generated C and C++ code can be seen in the EIFGENs directory of any
compiled Eiffel project.
In Eiffel, the "Translated" process is referred to as Trans-compiling or Trans-compiled, and
the Eiffel compiler as a Transcompiler.
Note that languages are not strictly "interpreted" languages or "compiled" languages.
Rather, language implementations use interpretation or compilation.
For example, Algol 60 and Fortran have both been interpreted.
Similarly, Java shows the difficulty of trying to apply these labels to languages, rather
than to implementations; Java is compiled to bytecode and the bytecode is subsequently
executed by either interpretation or compilation.
Moreover, compilation, trans-compiling, and interpretation are not strictly limited just
a description of the compiler artifact.
The Eiffel programming language uses both forms of compilation in a single development
system.
While "Finalized" Eiffel code is trans-compiled to binary, "code-in-development" exists partially
as either fully compiled machine code or as intermediate byte code, being interpreted
by an Eiffel run-time.
Thus, for purposes of efficient development, the majority of an Eiffel project is compiled
to binary, with small code changes "melted" out of the binary into IL "byte-code" in an
Eiffel run-time virtual machine.
As development moves along, the programmer will periodically "freeze" the small "melted"
portions of code back into the "workbench" binary.
See also
Abstraction Generational list of programming languages
Low-level programming languages Very high-level programming languages
Categorical list of programming languages
References
External links http:c2.comwiki?HighLevelLanguage - The WikiWikiWeb's
article on high-level programming languages