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102-478: ‹The template Manual is being considered for merging .›   Rexx ( Restructured Extended Executor ) is a programming language that can be interpreted or compiled . It was developed at IBM by Mike Cowlishaw . It is a structured, high-level programming language designed for ease of learning and reading. Proprietary and open source Rexx interpreters exist for a wide range of computing platforms; compilers exist for IBM mainframe computers . Rexx

204-484: A garbage collector and debugger . Programs written in a high-level language are either directly executed by some kind of interpreter or converted into machine code by a compiler (and assembler and linker ) for the CPU to execute. While compilers (and assemblers) generally produce machine code directly executable by computer hardware, they can often (optionally) produce an intermediate form called object code . This

306-449: A scripting programming language to replace the languages EXEC and EXEC 2 . It was designed to be a macro or scripting language for any system. As such, Rexx is considered a precursor to Tcl and Python . Rexx was also intended by its creator to be a simplified and easier to learn version of the PL/I programming language. However, some differences from PL/I may trip up the unwary. It

408-468: A variable-length code requiring 3, 6, 10, or 18 bits, and address operands include a "bit offset". Many BASIC interpreters can store and read back their own tokenized internal representation. An interpreter might well use the same lexical analyzer and parser as the compiler and then interpret the resulting abstract syntax tree . Example data type definitions for the latter, and a toy interpreter for syntax trees obtained from C expressions are shown in

510-466: A virtual machine , which is implemented not in hardware, but in the bytecode interpreter. Such compiling interpreters are sometimes also called compreters . In a bytecode interpreter each instruction starts with a byte, and therefore bytecode interpreters have up to 256 instructions, although not all may be used. Some bytecodes may take multiple bytes, and may be arbitrarily complicated. Control tables - that do not necessarily ever need to pass through

612-458: A Rexx interpreter for the Windows command line is supplied in most Resource Kits for various versions of Windows and works under all of them as well as DOS. Originally the language was called Rex ( Reformed Executor ); the extra "X" was added to avoid collisions with other products' names. REX was originally all uppercase because the mainframe code was uppercase oriented. The style in those days

714-517: A Rexx script or command is sometimes referred to as an EXEC in a nod to the CMS file type used for EXEC , EXEC 2 and REXX scripts on CP/CMS and VM/370 through z/VM . Rexx has the following characteristics and features: Rexx has just twenty-three, largely self-evident, instructions (such as call , parse , and select ) with minimal punctuation and formatting requirements. It is essentially an almost free-form language with only one data-type,

816-453: A built-in dynamic object system, TclOO, in 2012. It includes features such as: Tcl did not have object oriented (OO) syntax until 2012, so various extension packages emerged to enable object-oriented programming. They are widespread in existing Tcl source code. Popular extensions include: TclOO was not only added to build a strong object oriented system, but also to enable extension packages to build object oriented abstractions using it as

918-441: A bytecode interpreter, because of nodes related to syntax performing no useful work, of a less sequential representation (requiring traversal of more pointers) and of overhead visiting the tree. Further blurring the distinction between interpreters, bytecode interpreters and compilation is just-in-time (JIT) compilation, a technique in which the intermediate representation is compiled to native machine code at runtime. This confers

1020-413: A compiler works. However, a compiled program still runs much faster, under most circumstances, in part because compilers are designed to optimize code, and may be given ample time for this. This is especially true for simpler high-level languages without (many) dynamic data structures, checks, or type checking . In traditional compilation, the executable output of the linkers (.exe files or .dll files or

1122-439: A compiling phase - dictate appropriate algorithmic control flow via customized interpreters in similar fashion to bytecode interpreters. Threaded code interpreters are similar to bytecode interpreters but instead of bytes they use pointers. Each "instruction" is a word that points to a function or an instruction sequence, possibly followed by a parameter. The threaded code interpreter either loops fetching instructions and calling

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1224-413: A computer language is usually done in relation to an abstract machine (so-called operational semantics ) or as a mathematical function ( denotational semantics ). A language may also be defined by an interpreter in which the semantics of the host language is given. The definition of a language by a self-interpreter is not well-founded (it cannot define a language), but a self-interpreter tells a reader about

1326-476: A dual interpreter model with the untrusted interpreter running code in an untrusted script. It was designed by Nathaniel Borenstein and Marshall Rose to include active messages in e-mail. Safe-Tcl can be included in e-mail when the application/safe-tcl and multipart/enabled-mail are supported. The functionality of Safe-Tcl has since been incorporated as part of the standard Tcl/Tk releases. The syntax and semantics of Tcl are covered by twelve rules known as

1428-412: A foundation. After the release of TclOO, incr Tcl was updated to use TclOO as its foundation. Tcl Web Server is a pure-Tcl implementation of an HTTP protocol server. It runs as a script on top of a vanilla Tcl interpreter. Apache Rivet is an open source programming system for Apache HTTP Server that allows developers to use Tcl as a scripting language for creating dynamic web applications. Rivet

1530-418: A library, see picture) is typically relocatable when run under a general operating system, much like the object code modules are but with the difference that this relocation is done dynamically at run time, i.e. when the program is loaded for execution. On the other hand, compiled and linked programs for small embedded systems are typically statically allocated, often hard coded in a NOR flash memory, as there

1632-714: A list of these commands in the order a programmer wishes to execute them. Each command (also known as an Instruction ) contains the data the programmer wants to mutate, and information on how to mutate the data. For example, an interpreter might read ADD Books, 5 and interpret it as a request to add five to the Books variable . Interpreters have a wide variety of instructions which are specialized to perform different tasks, but you will commonly find interpreter instructions for basic mathematical operations , branching , and memory management , making most interpreters Turing complete . Many interpreters are also closely integrated with

1734-422: A new value instead. The most important commands that refer to program execution and data operations are: The usual execution control commands are: Those above looping commands can be additionally controlled by the following commands: uplevel allows a command script to be executed in a scope other than the current innermost scope on the stack. Because the command script may itself call procedures that use

1836-412: A numerical index. Instead it provides compound variables . A compound variable consists of a stem followed by a tail. A . (dot) is used to join the stem to the tail. If the tails used are numeric, it is easy to produce the same effect as an array. Afterwards the following variables with the following values exist: stem.1 = 9, stem.2 = 8, stem.3 = 7 ... Unlike arrays, the index for a stem variable

1938-439: A parse tree, and both may generate immediate instructions (for a stack machine , quadruple code , or by other means). The basic difference is that a compiler system, including a (built in or separate) linker, generates a stand-alone machine code program, while an interpreter system instead performs the actions described by the high-level program. A compiler can thus make almost all the conversions from source code semantics to

2040-405: A similar effect to obfuscation, but bytecode could be decoded with a decompiler or disassembler . The main disadvantage of interpreters is that an interpreted program typically runs more slowly than if it had been compiled . The difference in speeds could be tiny or great; often an order of magnitude and sometimes more. It generally takes longer to run a program under an interpreter than to run

2142-622: A single word. From Tcl 8.5 onwards, any word may be prefixed by {*} , which causes the word to be split apart into its constituent sub-words for the purposes of building the command invocation (similar to the ,@ sequence of Lisp 's quasiquote feature). As a consequence of these rules, the result of any command may be used as an argument to any other command. Note that, unlike in Unix command shells , Tcl does not reparse any string unless explicitly directed to do so, which makes interactive use more cumbersome, but scripted use more predictable (e.g.,

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2244-494: A suitable interpreter. If the interpreter needs to be supplied along with the source, the overall installation process is more complex than delivery of a monolithic executable, since the interpreter itself is part of what needs to be installed. The fact that interpreted code can easily be read and copied by humans can be of concern from the point of view of copyright . However, various systems of encryption and obfuscation exist. Delivery of intermediate code, such as bytecode, has

2346-472: A template interpreter. Rather than implement the execution of code by virtue of a large switch statement containing every possible bytecode, while operating on a software stack or a tree walk, a template interpreter maintains a large array of bytecode (or any efficient intermediate representation) mapped directly to corresponding native machine instructions that can be executed on the host hardware as key value pairs (or in more efficient designs, direct addresses to

2448-614: A visual development system from Watcom VX-REXX . Another dialect was VisPro REXX from Hockware. Portable Rexx by Kilowatt and Personal Rexx by Quercus are two Rexx interpreters designed for DOS and can be run under Windows as well using a command prompt. Since the mid-1990s, two newer variants of Rexx have appeared: In 1996 American National Standards Institute (ANSI) published a standard for Rexx: ANSI X3.274–1996 "Information Technology – Programming Language REXX". More than two dozen books on Rexx have been published since 1985. Rexx marked its 25th anniversary on 20 March 2004, which

2550-415: A wide range of computational tasks, including binary emulation and internet applications. Interpreter performance is still a worry despite their adaptability, particularly on systems with limited hardware resources. Advanced instrumentation and tracing approaches provide insights into interpreter implementations and processor resource utilization during execution through evaluations of interpreters tailored for

2652-472: A word that begins with a double-quote character ( " ) extends to the next double-quote character. Such a word can thus contain whitespace and semicolons without those characters being interpreted as having any special meaning (i.e., they are treated as normal text characters). A word that begins with an opening curly-brace character ( { ) extends to the next closing curly-brace character ( } ). Inside curly braces all forms of substitution are suppressed except

2754-564: Is Rexx's CASE structure , derived from the SELECT; form of the PL/I SELECT statement. Like some implementations of CASE constructs in other dynamic languages, Rexx's WHEN clauses specify full conditions, which need not be related to each other. In that, they are more like cascaded sets of IF-THEN-ELSEIF-THEN-...-ELSE code than they are like the C or Java switch statement. The NOP instruction performs "no operation", and

2856-415: Is a high-level , general-purpose , interpreted , dynamic programming language . It was designed with the goal of being very simple but powerful. Tcl casts everything into the mold of a command , even programming constructs like variable assignment and procedure definition. Tcl supports multiple programming paradigms , including object-oriented , imperative , functional , and procedural styles. It

2958-432: Is a few decades old, appearing in languages such as Smalltalk in the 1980s. Just-in-time compilation has gained mainstream attention amongst language implementers in recent years, with Java , the .NET Framework , most modern JavaScript implementations, and Matlab now including JIT compilers. Making the distinction between compilers and interpreters yet again even more vague is a special interpreter design known as

3060-509: Is a full language that can be used as a scripting , macro language, and application development language. It is often used for processing data and text and generating reports; this means that Rexx works well in Common Gateway Interface (CGI) programming and is used for this purpose, like later languages such as Perl . Rexx is the primary scripting language in some operating systems, e.g. OS/2 , MVS , VM , AmigaOS , and

3162-688: Is a layer of hardware-level instructions that implement higher-level machine code instructions or internal state machine sequencing in many digital processing elements. Microcode is used in general-purpose central processing units , as well as in more specialized processors such as microcontrollers , digital signal processors , channel controllers , disk controllers , network interface controllers , network processors , graphics processing units , and in other hardware. Microcode typically resides in special high-speed memory and translates machine instructions, state machine data or other input into sequences of detailed circuit-level operations. It separates

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3264-465: Is a package of file and directory functions, windowed I/O, and functions to access system services such as WAIT and POST. Rexx/Tk, a toolkit for graphics to be used in Rexx programmes in the same fashion as Tcl/Tk is widely available. A Rexx IDE, RxxxEd, has been developed for Windows. RxSock for network communication as well as other add-ons to and implementations of Regina Rexx have been developed, and

3366-536: Is a relatively simple way to achieve software compatibility between different products in a processor family. Even a non microcoding computer processor itself can be considered to be a parsing immediate execution interpreter that is written in a general purpose hardware description language such as VHDL to create a system that parses the machine code instructions and immediately executes them. Interpreters, such as those written in Java, Perl, and Tcl, are now necessary for

3468-502: Is also used as an internal macro language in some other software, such as SPF/PC , KEDIT , THE and the ZOC terminal emulator. Additionally, the Rexx language can be used for scripting and macros in any program that uses Windows Scripting Host ActiveX scripting engines languages (e.g. VBScript and JScript) if one of the Rexx engines is installed. Rexx is supplied with VM/SP Release 3 on up, TSO/E Version 2 on up, OS/2 (1.3 and later, where it

3570-583: Is basically the same machine specific code but augmented with a symbol table with names and tags to make executable blocks (or modules) identifiable and relocatable. Compiled programs will typically use building blocks (functions) kept in a library of such object code modules. A linker is used to combine (pre-made) library files with the object file(s) of the application to form a single executable file. The object files that are used to generate an executable file are thus often produced at different times, and sometimes even by different languages (capable of generating

3672-425: Is commonly used embedded into C applications, for rapid prototyping , scripted applications, GUIs, and testing. Tcl interpreters are available for many operating systems , allowing Tcl code to run on a wide variety of systems. Because Tcl is a very compact language, it is used on embedded systems platforms, both in its full form and in several other small-footprint versions. The popular combination of Tcl with

3774-439: Is compiled into "F code" (a bytecode), which is then interpreted by a virtual machine . In the spectrum between interpreting and compiling, another approach is to transform the source code into an optimized abstract syntax tree (AST), then execute the program following this tree structure, or use it to generate native code just-in-time . In this approach, each sentence needs to be parsed just once. As an advantage over bytecode,

3876-529: Is conventionally written Tcl rather than TCL . Tcl conferences and workshops are held in both the United States and Europe. Several corporations, including FlightAware use Tcl as part of their products. Tcl's features include Safe-Tcl is a subset of Tcl that has restricted features so that Tcl scripts cannot harm their hosting machine or application. File system access is limited and arbitrary system commands are prevented from execution. It uses

3978-432: Is executed and then perform the desired action, whereas the compiled code just performs the action within a fixed context determined by the compilation. This run-time analysis is known as "interpretive overhead". Access to variables is also slower in an interpreter because the mapping of identifiers to storage locations must be done repeatedly at run-time rather than at compile time . There are various compromises between

4080-430: Is implemented using closures in the interpreter language or implemented "manually" with a data structure explicitly storing the environment. The more features implemented by the same feature in the host language, the less control the programmer of the interpreter has; for example, a different behavior for dealing with number overflows cannot be realized if the arithmetic operations are delegated to corresponding operations in

4182-565: Is more difficult to maintain due to the interpreter having to support translation to multiple different architectures instead of a platform independent virtual machine/stack. To date, the only template interpreter implementations of widely known languages to exist are the interpreter within Java's official reference implementation, the Sun HotSpot Java Virtual Machine, and the Ignition Interpreter in

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4284-457: Is not required to have an integer value. For example, the following code is valid: In Rexx it is also possible to set a default value for a stem. After these assignments the term stem.3 would produce 'Unknown' . Interpreter (computing) Early versions of Lisp programming language and minicomputer and microcomputer BASIC dialects would be examples of the first type. Perl , Raku , Python , MATLAB , and Ruby are examples of

4386-441: Is officially named Procedures Language/2 ), AmigaOS Version 2 on up, PC DOS ( 7.0 or 2000 ), ArcaOS , and Windows NT 4.0 (Resource Kit: Regina). REXX scripts for CMS share the filetype EXEC with EXEC and EXEC2, and the first line of the script specifies the interpreter to be used. REXX scripts for MVS may be recognized by the low level qualifier EXEC or may be recognized by context and the first line. REXX scripts for OS/2 share

4488-449: Is often no secondary storage and no operating system in this sense. Historically, most interpreter systems have had a self-contained editor built in. This is becoming more common also for compilers (then often called an IDE ), although some programmers prefer to use an editor of their choice and run the compiler, linker and other tools manually. Historically, compilers predate interpreters because hardware at that time could not support both

4590-423: Is reached: The increment may be omitted and defaults to 1. The limit can also be omitted, which makes the loop continue forever. Rexx permits counted loops, where an expression is computed at the start of the loop and the instructions within the loop are executed that many times: Rexx can even loop until the program is terminated: A program can break out of the current loop with the leave instruction, which

4692-502: Is similar to PHP , ASP , and JSP . Rivet was primarily developed by Damon Courtney, David Welton, Massimo Manghi, Harald Oehlmann and Karl Lehenbauer . Rivet can use any of the thousands of publicly available Tcl packages that offer countless features such as database interaction (Oracle, PostgreSQL, MySQL, SQLite, etc.), or interfaces to popular applications such as the GD Graphics Library . Tcl interfaces natively with

4794-490: Is such a language, because XSLT programs are written in XML. A sub-domain of metaprogramming is the writing of domain-specific languages (DSLs). Clive Gifford introduced a measure quality of self-interpreter (the eigenratio), the limit of the ratio between computer time spent running a stack of N self-interpreters and time spent to run a stack of N − 1 self-interpreters as N goes to infinity. This value does not depend on

4896-419: Is the normal way to exit a do forever loop, or can short-circuit it with the iterate instruction. Like PL/I , Rexx allows both conditional and repetitive elements to be combined in the same loop: Testing conditions with IF : The ELSE clause is optional. For single instructions, DO and END can also be omitted: Indentation is optional, but it helps improve the readability. SELECT

4998-551: Is used when the programmer wishes to do nothing in a place where one or more instructions would be required. The OTHERWISE clause is optional. If omitted and no WHEN conditions are met, then the SYNTAX condition is raised. Variables in Rexx are typeless, and initially are evaluated as their names, in upper case. Thus a variable's type can vary with its use in the program: Unlike many other programming languages, classic Rexx has no direct support for arrays of variables addressed by

5100-403: The C language. This is because it was originally written to be a framework for providing a syntactic front-end to commands written in C, and all commands in the language (including things that might otherwise be keywords , such as if or while ) are implemented this way. Each command implementation function is passed an array of values that describe the (already substituted) arguments to

5202-522: The TIOBE index as one of the fifty languages in its top 100 not belonging to the top 50. In 2019, the 30th Rexx Language Association Symposium marked the 40th anniversary of Rexx. The symposium was held in Hursley, England, where Rexx was first designed and implemented. The RexxUtil toolkit is a package of functions that is available for most Rexx implementations and most host operating systems. RexxUtil

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5304-610: The Tk extension is referred to as Tcl/Tk (pronounced "tickle teak" or as an initialism ) and enables building a graphical user interface (GUI) natively in Tcl. Tcl/Tk is included in the standard Python installation in the form of Tkinter . The Tcl programming language was created in the spring of 1988 by John Ousterhout while he was working at the University of California, Berkeley . Originally "born out of frustration", according to

5406-427: The development speed when using an interpreter and the execution speed when using a compiler. Some systems (such as some Lisps ) allow interpreted and compiled code to call each other and to share variables. This means that once a routine has been tested and debugged under the interpreter it can be compiled and thus benefit from faster execution while other routines are being developed. Many interpreters do not execute

5508-493: The puts (short for "put string") command to display a string of text on the host console: This sends the string "Hello, World!" to the standard output device along with an appended newline character. Variables and the results of other commands can be substituted into strings, such as in this example which uses the set and expr commands to store the result of a calculation in a variable (note that Tcl does not use = as an assignment operator), and then uses puts to print

5610-450: The uplevel command, this has the net effect of transforming the call stack into a call tree. It was originally implemented to permit Tcl procedures to reimplement built-in commands (like for , if or while ) and still have the ability to manipulate local variables . For example, the following Tcl script is a reimplementation of the for command (omitting exception handling ): upvar arranges for one or more local variables in

5712-471: The AST keeps the global program structure and relations between statements (which is lost in a bytecode representation), and when compressed provides a more compact representation. Thus, using AST has been proposed as a better intermediate format for just-in-time compilers than bytecode. Also, it allows the system to perform better analysis during runtime. However, for interpreters, an AST causes more overhead than

5814-475: The Dodekalogue. A Tcl script consists of a series of command invocations. A command invocation is a list of words separated by whitespace and terminated by a newline or semicolon. The first word is the name of a command, which may be built into the language, found in an available library , or defined in the script itself. The subsequent words serve as arguments to the command: The following example uses

5916-508: The Google V8 javascript execution engine. A self-interpreter is a programming language interpreter written in a programming language which can interpret itself; an example is a BASIC interpreter written in BASIC. Self-interpreters are related to self-hosting compilers . If no compiler exists for the language to be interpreted, creating a self-interpreter requires the implementation of

6018-676: The Lisp eval function could be implemented in machine code. The result was a working Lisp interpreter which could be used to run Lisp programs, or more properly, "evaluate Lisp expressions". The development of editing interpreters was influenced by the need for interactive computing. In the 1960s, the introduction of time-sharing systems allowed multiple users to access a computer simultaneously, and editing interpreters became essential for managing and modifying code in real-time. The first editing interpreters were likely developed for mainframe computers, where they were used to create and modify programs on

6120-499: The MIPS instruction set and programming languages such as Tcl, Perl, and Java. Performance characteristics are influenced by interpreter complexity, as demonstrated by comparisons with compiled code. It is clear that interpreter performance is more dependent on the nuances and resource needs of the interpreter than it is on the particular application that is being interpreted. Tcl Tcl (pronounced " tickle " or as an initialism )

6222-415: The amount of analysis performed before the program is executed. For example, Emacs Lisp is compiled to bytecode , which is a highly compressed and optimized representation of the Lisp source, but is not machine code (and therefore not tied to any particular hardware). This "compiled" code is then interpreted by a bytecode interpreter (itself written in C ). The compiled code in this case is machine code for

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6324-537: The author, with programmers devising their own languages for extending electronic design automation (EDA) software and, more specifically, the VLSI design tool Magic , which was a professional focus for John at the time. Later Tcl gained acceptance on its own. Ousterhout was awarded the ACM Software System Award in 1997 for Tcl/Tk. The name originally comes from T ool C ommand L anguage, but

6426-464: The box. Interpretation cannot be used as the sole method of execution: even though an interpreter can itself be interpreted and so on, a directly executed program is needed somewhere at the bottom of the stack because the code being interpreted is not, by definition, the same as the machine code that the CPU can execute. There is a spectrum of possibilities between interpreting and compiling, depending on

6528-600: The character string; this philosophy means that all data are visible (symbolic) and debugging and tracing are simplified. Rexx's syntax looks similar to PL/I , but has fewer notations; this makes it harder to parse (by program) but easier to use, except for cases where PL/I habits may lead to surprises. One of the REXX design goals was the principle of least astonishment . Rexx was designed and first implemented, in assembly language , as an 'own-time' project between 20 March 1979 and mid-1982 by Mike Cowlishaw of IBM, originally as

6630-421: The command, and is free to interpret those values as it sees fit. Digital logic simulators often include a Tcl scripting interface for simulating Verilog , VHDL and SystemVerilog hardware languages . Tools exist (e.g. SWIG , Ffidl ) to automatically generate the necessary code to connect arbitrary C functions and the Tcl runtime, and Critcl does the reverse, allowing embedding of arbitrary C code inside

6732-418: The compiled code but it can take less time to interpret it than the total time required to compile and run it. This is especially important when prototyping and testing code when an edit-interpret-debug cycle can often be much shorter than an edit-compile-run-debug cycle. Interpreting code is slower than running the compiled code because the interpreter must analyze each statement in the program each time it

6834-408: The current procedure to refer to variables in an enclosing procedure call or to global variables . The upvar command simplifies the implementation of call-by-name procedure calling and also makes it easier to build new control constructs as Tcl procedures. A decr command that works like the built-in incr command except it subtracts the value from the variable instead of adding it: Tcl 8.6 added

6936-436: The efficiency of running native code, at the cost of startup time and increased memory use when the bytecode or AST is first compiled. The earliest published JIT compiler is generally attributed to work on LISP by John McCarthy in 1960. Adaptive optimization is a complementary technique in which the interpreter profiles the running program and compiles its most frequently executed parts into native code. The latter technique

7038-414: The expressiveness and elegance of a language. It also enables the interpreter to interpret its source code, the first step towards reflective interpreting. An important design dimension in the implementation of a self-interpreter is whether a feature of the interpreted language is implemented with the same feature in the interpreter's host language. An example is whether a closure in a Lisp -like language

7140-401: The filename extension .cmd with other scripting languages, and the first line of the script specifies the interpreter to be used. REXX macros for REXX-aware applications use extensions determined by the application. In the late 1980s, Rexx became the common scripting language for IBM Systems Application Architecture , where it was renamed "SAA Procedure Language REXX". In mainframe programming,

7242-411: The first public release of Open Object Rexx (ooRexx) was announced. This product contains a WSH scripting engine which allows for programming of the Windows operating system and applications with Rexx in the same fashion in which Visual Basic and JScript are implemented by the default WSH installation and Perl , Tcl , Python third-party scripting engines. As of January 2017 REXX was listed in

7344-587: The fly. One of the earliest examples of an editing interpreter is the EDT (Editor and Debugger for the TECO) system, which was developed in the late 1960s for the PDP-1 computer. EDT allowed users to edit and debug programs using a combination of commands and macros, paving the way for modern text editors and interactive development environments. An interpreter usually consists of a set of known commands it can execute , and

7446-504: The forming of the REXX Language Association. Symposia are held annually. Several freeware versions of Rexx are available. In 1992, the two most widely used open-source ports appeared: Ian Collier's REXX/imc for Unix and Anders Christensen's Regina (later adopted by Mark Hessling) for Windows and Unix. BREXX is well known for WinCE and Pocket PC platforms, and has been "back-ported" to VM/370 and MVS . OS/2 has

7548-515: The functions they point to, or fetches the first instruction and jumps to it, and every instruction sequence ends with a fetch and jump to the next instruction. Unlike bytecode there is no effective limit on the number of different instructions other than available memory and address space. The classic example of threaded code is the Forth code used in Open Firmware systems: the source language

7650-479: The host language. Some languages such as Lisp and Prolog have elegant self-interpreters. Much research on self-interpreters (particularly reflective interpreters) has been conducted in the Scheme programming language , a dialect of Lisp. In general, however, any Turing-complete language allows writing of its own interpreter. Lisp is such a language, because Lisp programs are lists of symbols and other lists. XSLT

7752-414: The interpreter and interpreted code and the typical batch environment of the time limited the advantages of interpretation. During the software development cycle , programmers make frequent changes to source code. When using a compiler, each time a change is made to the source code, they must wait for the compiler to translate the altered source files and link all of the binary code files together before

7854-403: The keyword LOOP instead of DO for looping, while ooRexx treats LOOP and DO as equivalent when looping. Rexx supports a variety of traditional structured-programming loops while testing a condition either before ( do while ) or after ( do until ) the list of instructions are executed: Like most languages, Rexx can loop while incrementing an index variable and stop when a limit

7956-453: The language in a host language (which may be another programming language or assembler ). By having a first interpreter such as this, the system is bootstrapped and new versions of the interpreter can be developed in the language itself. It was in this way that Donald Knuth developed the TANGLE interpreter for the language WEB of the de-facto standard TeX typesetting system . Defining

8058-405: The language into native calls one opcode at a time rather than creating optimized sequences of CPU executable instructions from the entire code segment. Due to the interpreter's simple design of simply passing calls directly to the hardware rather than implementing them directly, it is much faster than every other type, even bytecode interpreters, and to an extent less prone to bugs, but as a tradeoff

8160-633: The limitations of computers at the time (e.g. a shortage of program storage space, or no native support for floating point numbers). Interpreters were also used to translate between low-level machine languages, allowing code to be written for machines that were still under construction and tested on computers that already existed. The first interpreted high-level language was Lisp . Lisp was first implemented by Steve Russell on an IBM 704 computer. Russell had read John McCarthy 's paper, "Recursive Functions of Symbolic Expressions and Their Computation by Machine, Part I", and realized (to McCarthy's surprise) that

8262-572: The machine instructions from the underlying electronics so that instructions can be designed and altered more freely. It also facilitates the building of complex multi-step instructions, while reducing the complexity of computer circuits. Writing microcode is often called microprogramming and the microcode in a particular processor implementation is sometimes called a microprogram . More extensive microcoding allows small and simple microarchitectures to emulate more powerful architectures with wider word length , more execution units and so on, which

8364-421: The machine level once and for all (i.e. until the program has to be changed) while an interpreter has to do some of this conversion work every time a statement or function is executed. However, in an efficient interpreter, much of the translation work (including analysis of types, and similar) is factored out and done only the first time a program, module, function, or even statement, is run, thus quite akin to how

8466-430: The native instructions), known as a "Template". When the particular code segment is executed the interpreter simply loads or jumps to the opcode mapping in the template and directly runs it on the hardware. Due to its design, the template interpreter very strongly resembles a just-in-time compiler rather than a traditional interpreter, however it is technically not a JIT due to the fact that it merely translates code from

8568-452: The parsing of any commands or arguments. If the final character on a line (i.e., immediately before a newline) is a backslash, then the backslash-newline combination (and any spaces or tabs immediately following the newline) are replaced by a single space. This provides a line continuation mechanism, whereby long lines in the source code can be wrapped to the next line for the convenience of readers. Continuing with normal argument processing,

8670-479: The presence of spaces in filenames does not cause difficulties). The single equality sign ( = ) serves no special role in the language at all. The double equality sign ( == ) is the test for equality which is used in expression contexts such as the expr command and in the first argument to if . (Both commands are part of the standard library; they have no special place in the library and can be replaced if desired.) The majority of Tcl commands, especially in

8772-401: The previously mentioned backslash-newline elimination. Words not enclosed in either construct are known as bare words. In bare and double-quoted words, three types of substitution may occur: Substitution proceeds left-to-right in a single scan through each word. Any substituted text will not be scanned again for possible further substitutions. However, any number of substitutions can appear in

8874-410: The program being run. The book Structure and Interpretation of Computer Programs presents examples of meta-circular interpretation for Scheme and its dialects. Other examples of languages with a self-interpreter are Forth and Pascal . Microcode is a very commonly used technique "that imposes an interpreter between the hardware and the architectural level of a computer". As such, the microcode

8976-411: The program can be executed. The larger the program, the longer the wait. By contrast, a programmer using an interpreter does a lot less waiting, as the interpreter usually just needs to translate the code being worked on to an intermediate representation (or not translate it at all), thus requiring much less time before the changes can be tested. Effects are evident upon saving the source code and reloading

9078-458: The program. Compiled code is generally less readily debugged as editing, compiling, and linking are sequential processes that have to be conducted in the proper sequence with a proper set of commands. For this reason, many compilers also have an executive aid, known as a Makefile and program. The Makefile lists compiler and linker command lines and program source code files, but might take a simple command line menu input (e.g. "Make 3") which selects

9180-398: The result together with some explanatory text: The # character introduces a comment . Comments can appear anywhere the interpreter is expecting a command name. As seen in these examples, there is one basic construct in the language: the command. Quoting mechanisms and substitution rules determine how the arguments to each command are processed. One special substitution occurs before

9282-619: The same object format). A simple interpreter written in a low-level language (e.g. assembly ) may have similar machine code blocks implementing functions of the high-level language stored, and executed when a function's entry in a look up table points to that code. However, an interpreter written in a high-level language typically uses another approach, such as generating and then walking a parse tree , or by generating and executing intermediate software-defined instructions, or both. Thus, both compilers and interpreters generally turn source code (text files) into tokens, both may (or may not) generate

9384-586: The second, while UCSD Pascal is an example of the third type. Source programs are compiled ahead of time and stored as machine independent code, which is then linked at run-time and executed by an interpreter and/or compiler (for JIT systems). Some systems, such as Smalltalk and contemporary versions of BASIC and Java , may also combine two and three types. Interpreters of various types have also been constructed for many languages traditionally associated with compilation, such as Algol , Fortran , Cobol , C and C++ . While interpretation and compilation are

9486-537: The source code as it stands but convert it into some more compact internal form. Many BASIC interpreters replace keywords with single byte tokens which can be used to find the instruction in a jump table . A few interpreters, such as the PBASIC interpreter, achieve even higher levels of program compaction by using a bit-oriented rather than a byte-oriented program memory structure, where commands tokens occupy perhaps 5 bits, nominally "16-bit" constants are stored in

9588-573: The standard library, are variadic , and the proc (the constructor for scripted command procedures) allows one to define default values for unspecified arguments and a catch-all argument to allow the code to process arbitrary numbers of arguments. Tcl is not statically typed: each variable may contain integers, floats, strings, lists, command names, dictionaries, or any other value; values are reinterpreted (subject to syntactic constraints) as other types on demand. However, values are immutable and operations that appear to change them actually just return

9690-506: The third group (set) of instructions then issues the commands to the compiler, and linker feeding the specified source code files. A compiler converts source code into binary instruction for a specific processor's architecture, thus making it less portable . This conversion is made just once, on the developer's environment, and after that the same binary can be distributed to the user's machines where it can be executed without further translation. A cross compiler can generate binary code for

9792-515: The two main means by which programming languages are implemented, they are not mutually exclusive, as most interpreting systems also perform some translation work, just like compilers. The terms " interpreted language " or " compiled language " signify that the canonical implementation of that language is an interpreter or a compiler, respectively. A high-level language is ideally an abstraction independent of particular implementations. Interpreters were used as early as 1952 to ease programming within

9894-410: The user machine even if it has a different processor than the machine where the code is compiled. An interpreted program can be distributed as source code. It needs to be translated in each final machine, which takes more time but makes the program distribution independent of the machine's architecture. However, the portability of interpreted source code is dependent on the target machine actually having

9996-606: Was celebrated at the REXX Language Association's 15th International REXX Symposium in Böblingen, Germany, in May 2004. On October 12, 2004, IBM announced their plan to release their Object REXX implementation's sources under the Common Public License . Recent releases of Object REXX contain an ActiveX Windows Scripting Host (WSH) scripting engine implementing this version of the Rexx language. On February 22, 2005,

10098-572: Was first described in public at the SHARE 56 conference in Houston, Texas, in 1981, where customer reaction, championed by Ted Johnston of SLAC , led to it being shipped as an IBM product in 1982. Over the years IBM included Rexx in almost all of its operating systems ( VM/CMS , MVS TSO/E , IBM OS/400 , VSE/ESA , AIX , PC DOS , and OS/2 ), and has made versions available for Novell NetWare , Windows , Java , and Linux . The first non-IBM version

10200-422: Was included with AmigaOS 2 onwards and was popular for scripting as well as application control. Many Amiga applications have an "ARexx port" built into them which allows control of the application from Rexx. One single Rexx script could even switch between different Rexx ports in order to control several running applications. In 1990, Cathie Dager of SLAC organized the first independent Rexx symposium, which led to

10302-466: Was to have all-caps names, partly because almost all code was still all-caps then. For the product it became REXX, and both editions of Mike Cowlishaw's book use all-caps. The expansion to REstructured eXtended eXecutor was used for the system product in 1984. ‹The template Manual is being considered for merging .›   The loop control structure in Rexx begins with a DO and ends with an END but comes in several varieties. NetRexx uses

10404-617: Was written for PC DOS by Charles Daney in 1984/5 and marketed by the Mansfield Software Group (founded by Kevin J. Kearney in 1986). The first compiler version appeared in 1987, written for CMS by Lundin and Woodruff. Other versions have also been developed for Atari , AmigaOS , Unix (many variants), Solaris , DEC , Windows , Windows CE , Pocket PC , DOS , Palm OS , QNX , OS/2 , Linux , BeOS , EPOC32 / Symbian , AtheOS , OpenVMS , Apple Macintosh , and Mac OS X . The Amiga version of Rexx, called ARexx ,

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