SNOBOL

This article is about the programming language. For the cleaning product, seeSnoBol (cleaner).

SNOBOL

Paradigm	Imperative,procedural,unstructured
Designed by	David J. Farber,Ralph GriswoldandIvan P. Polonsky
Developer	David J. Farber, Ralph E. Griswold, Ivan P. Polonsky, andBell Labs
First appeared	1962(1962)
Stable release	SNOBOL4 / 1967(1967)
Website	https://www.regressive.org/snobol4/
Majorimplementations
SNOBOL,SPITBOL
Influenced by
COMIT
Influenced
AWK,SL5,Icon,bs,Lua

SNOBOL( "StriNg Oriented and symBOlic Language" ) is a series ofprogramming languagesdeveloped between 1962 and 1967 atAT&TBell LaboratoriesbyDavid J. Farber,Ralph Griswoldand Ivan P. Polonsky, culminating in SNOBOL4. It was one of a number oftext-string-oriented languages developed during the 1950s and 1960s; others includedCOMITandTRAC.

SNOBOL4 stands apart from most programming languages of its era by having patterns as afirst-class data type(i.e.a data type whose values can be manipulated in all ways permitted to any other data type in the programming language) and by providing operators for patternconcatenationandalternation.SNOBOL4 patterns are a type of object and admit various manipulations, much like laterobject-oriented languagessuch asJavaScriptwhose patterns are known asregular expressions.In addition SNOBOL4 strings generated during execution can be treated as programs and either interpreted or compiled and executed (as in theevalfunction of other languages).

SNOBOL4 was quite widely taught in larger U.S. universities in the late 1960s and early 1970s and was widely used in the 1970s and 1980s as a text manipulation language in thehumanities.

In the 1980s and 1990s, its use faded as newer languages such asAWKandPerlmade string manipulation by means ofregular expressionsfashionable. SNOBOL4 patterns subsumeBNFgrammars, which are equivalent tocontext-free grammarsand more powerful than regular expressions. The "regular expressions" in current versions of AWK and Perl are in fact extensions of regular expressions in thetraditional sense,but regular expressions, unlike SNOBOL4 patterns, are not recursive, which gives a distinct computational advantage to SNOBOL4 patterns.(Recursive expressions did appear inPerl 5.10,though, released in December 2007.)

The later SL5 (1977)andIcon(1978) languages were designed by Griswold to combine the backtracking of SNOBOL4 pattern matching with more standardALGOL-like structuring.

Development

SNOBOL1

The initial SNOBOL language was created as a tool to be used by its authors to work with the symbolic manipulation of polynomials. It was written in assembly language for theIBM 7090.It had a simple syntax, only one datatype, the string, no functions, and no declarations and very little error control. However, despite its simplicity and its "personal" nature its use began to spread to other groups. As a result, the authors decided to extend it and tidy it up.

SNOBOL2

SNOBOL2 did exist but it was a short-lived intermediate development version without user-defined functions and was never released.

SNOBOL3

SNOBOL was rewritten to add functions, both standard and user-defined, and the result was released as SNOBOL3. SNOBOL3 became quite popular and was rewritten for other computers than the IBM 7090 by other programmers. As a result, several incompatible dialects arose.

SNOBOL4

As SNOBOL3 became more popular, the authors received more and more requests for extensions to the language. They also began to receive complaints about incompatibility and bugs in versions that they hadn't written. To address this and to take advantage of the new computers being introduced in the late 1960s, the decision was taken to develop SNOBOL4 with many extra datatypes and features but based on avirtual machineto allow improved portability across computers.The SNOBOL4 language translator was still written in assembly language. However the macro features of the assembler were used to define the virtual machine instructions of theSNOBOLImplementationLanguage, the SIL. This very much improved the portability of the language by making it relatively easy to port the virtual machine which hosted the translator by recreating its virtual instructions on any machine which included a macro assembler or indeed a high level language.

The machine-independent language SIL arose as a generalization of string manipulation macros byDouglas McIlroy,which were used extensively in the initial SNOBOL implementation. In 1969, McIlroy influenced the language again by insisting on addition of the table type to SNOBOL4.

SNOBOL4 features

SNOBOL is distinctive in format and programming style, which are radically different from contemporary procedural languages such asFortranandALGOL.

SNOBOL4 supports a number of built-indata types,such asintegersand limited precisionreal numbers,strings,patterns,arrays,andtables(associative arrays), and also allows the programmer to define additional data types and newfunctions.SNOBOL4's programmer-defined data type facility was advanced at the time—it is similar to the records of the earlierCOBOLand the laterPascalprogramming languages.

All SNOBOL command lines are of the form

label subject pattern=object:transfer

Each of the five elements is optional. In general, thesubjectis matched against thepattern.If theobjectis present, any matched portion is replaced by theobjectvia rules for replacement. Thetransfercan be an absolute branch or a conditional branch dependent upon the success or failure of the subject evaluation, the pattern evaluation, the pattern match, the object evaluation or the final assignment. It can also be a transfer to code created and compiled by the program itself during a run.

A SNOBOL pattern can be very simple or extremely complex. A simple pattern is just a text string (e.g. "ABCD" ), but a complex pattern may be a large structure describing, for example, the complete grammar of a computer language. It is possible to implement a language interpreter in SNOBOL almost directly from aBackus–Naur formexpression of it, with few changes. Creating a macro assembler and an interpreter for a completely theoretical piece of hardware could take as little as a few hundred lines, with a new instruction being added with a single line.

Complex SNOBOL patterns can do things that would be impractical or impossible using the more primitive regular expressions used in most other pattern-matching languages. Some of this power derives from the so-called "SPITBOL extensions" (which have since been incorporated in basically all modern implementations of the original SNOBOL 4 language too), although it is possible to achieve the same power without them. Part of this power comes from the side effects that it is possible to produce during the pattern matching operation, including saving numerous intermediate/tentative matching results and the ability to invoke user-written functions during the pattern match which can perform nearly any desired processing, and then influence the ongoing direction the interrupted pattern match takes, or even to indeed change the pattern itself during the matching operation. Patterns can be saved like any other first-class data item, and can be concatenated, used within other patterns, and used to create very complex and sophisticated pattern expressions. It is possible to write, for example, a SNOBOL4 pattern which matches "a complete name and international postal mailing address", which is well beyond anything that is practical to even attempt using regular expressions.

SNOBOL4 pattern-matching uses a backtracking algorithm similar to that used in thelogic programminglanguageProlog,which provides pattern-like constructs viaDCGs.This algorithm makes it easier to use SNOBOL as a logic programming language than is the case for most languages.

SNOBOL stores variables, strings and data structures in a singlegarbage-collectedheap.

Example programs

The "Hello, World!" program might be as follows...

OUTPUT="Hello, World!"
END

A simple program to ask for a user's name and then use it in an output sentence...

OUTPUT="What is your name?"
Username=INPUT
OUTPUT="Thank you,"Username
END

To choose between three possible outputs...

OUTPUT="What is your name?"
Username=INPUT
Username"J":S(LOVE)
Username"K":S(HATE)
MEHOUTPUT="Hi,"Username:(END)
LOVEOUTPUT="How nice to meet you,"Username:(END)
HATEOUTPUT="Oh. It's you,"Username
END

To continue requesting input until no more is forthcoming...

OUTPUT="This program will ask you for personal names"
OUTPUT="until you press return without giving it one"
NameCount=0:(GETINPUT)
AGAINNameCount=NameCount+1
OUTPUT="Name"NameCount":"PersonalName
GETINPUTOUTPUT="Please give me name"NameCount+1
PersonalName=INPUT
PersonalNameLEN(1):S(AGAIN)
OUTPUT="Finished."NameCount"names requested."
END

Implementations

The classic implementation was on thePDP-10;it has been used to studycompilers,formal grammars,andartificial intelligence,especiallymachine translationand machine comprehension ofnatural languages.The original implementation was on an IBM 7090 at Bell Labs, Holmdel, N.J. SNOBOL4 was specifically designed for portability; the first implementation was started on an IBM 7094 in 1966 but completed on an IBM 360 in 1967. It was rapidly ported to many other platforms.

It is normally implemented as aninterpreterbecause of the difficulty in implementing some of its very high-level features, but there is acompiler,theSPITBOL compiler,which provides nearly all the facilities that the interpreter provides.

The classic implementation on thePDP-10was quite slow, and in 1972 James Gimpel of Bell Labs, Holmdel, N.J. designed a native implementation of SNOBOL4 for thePDP-10that he named SITBOL. He used the design as the basis of a graduate class in string processing that he taught that year atStevens Institute of Technology(which is why it was named SITBOL). Students were given sections to implement (in PDP-10 assembler) and the entire semester was focused on implementing SITBOL. It was over 80% complete by the end of the semester and was subsequently completed by Professor Gimpel and several students over the summer. SITBOL was a full-featured, high-performance SNOBOL4 interpreter.

TheGnatAdaCompiler comes with a package (GNAT.Spitbol) that implements all of the Spitbol string manipulation semantics. This can be called from within an Ada program.

The file editor for theMichigan Terminal System(MTS) provided pattern matching based on SNOBOL4 patterns.

Several implementations are currently available. Macro SNOBOL4 in C written by Phil Budne is a free, open source implementation, capable of running on almost any platform.Catspaw, Inc provided a commercial implementation of the SNOBOL4 language for many different computer platforms, including DOS, Macintosh, Sun, RS/6000, and others, and these implementations are now available free from Catspaw. Minnesota SNOBOL4, by Viktors Berstis, the closest PC implementation to the original IBM mainframe version (even including Fortran-like FORMAT statement support) is also free.

Although SNOBOL itself has nostructured programmingfeatures, a SNOBOL preprocessor calledSnostormwas designed and implemented during the 1970s by Fred G. Swartz for use under theMichigan Terminal System(MTS) at theUniversity of Michigan.Snostorm was used at the eight to fifteen sites that ran MTS. It was also available atUniversity College London(UCL) between 1982 and 1984.

Snocone byAndrew Koenigadds block-structured constructs to the SNOBOL4 language. Snocone is a self-contained programming language, rather than a proper superset of SNOBOL4.

The SPITBOL implementation also introduced a number of features which, while not using traditional structured programming keywords, nevertheless can be used to provide many of the equivalent capabilities normally thought of as "structured programming", most notably nested if/then/else type constructs. These features have since been added to most recent SNOBOL4 implementations. After many years as a commercial product, in April 2009 SPITBOL was released as free software under theGNU General Public License.

Naming

According to Dave Farber,he, Griswold and Polonsky "finally arrived at the name Symbolic EXpression Interpreter SEXI."

All went well until one day I was submitting a batch job to assemble the system and as normal on my JOBcard— the first card in the deck, I, in BTL standards, punched my job and my name — SEXI Farber.

One of the Comp Center girls looked at it and said, "That's what you think" in a humorous way.

That made it clear that we needed another name!! We sat and talked and drank coffee and shot rubber bands and after much too much time someone said — most likely Ralph — "We don't have aSnowball's chance in hellof finding a name ". All of us yelled at once," WE GOT IT — SNOBOL "in the spirit of all theBOL languages.We then stretched our mind to find what it stood for.

Commonbackronymsof "SNOBOL" are 'String Oriented Symbolic Language'or (as aquasi-initialism) 'StriNg Oriented symBOlic Language'.

See also

• Icon (programming language)
• Snowball (programming language)
• Snostorm
• SPITBOL
• Unicon (programming language)

Further reading

• Emmer, Mark B. (1985).SNOBOL4+: The SNOBOL4 Language for the Personal Computer User.Prentice Hall.ISBN0-13-815119-9.
• Gimpel, James F. (1976).Algorithms in SNOBOL4.Wiley.ISBN0-471-30213-9.republished Salida, CO: Catspaw, 1986 (ISBN0-939793-00-8).
• Griswold, Ralph E. (1972).The Macro Implementation of SNOBOL4.W.H. Freeman.ISBN0-7167-0447-1.
• Griswold, Ralph E.; Poage, J.F.; Polonsky, I.P. (1968).The SNOBOL4 Programming Language.Prentice Hall.ISBN0-13-815373-6.
• Griswold, Ralph E. (1975).String and List Processing in SNOBOL4: Techniques and Applications.Prentice Hall.ISBN0-13-853010-6.
• Hockey, Susan M.(1985).Snobol Programming for the Humanities.Clarendon Press.ISBN0-19-824676-5.

External links

• CSNOBOL4is a free and open source BSD-licensed port of the original Bell Labs SNOBOL4 to systems with a C compiler, and includes SPITBOL and Blocks enhancements.
• Catspaw, Inc. offers implementations of and commercial support for SNOBOL4
• SNOBOLatCurlie
• Griswold, Ralph E. (25 July 1990)."Oral history interview with Ralph E. Griswold — discusses development of SNOBOL".Minneapolis:Charles Babbage Institute,University of Minnesota.].
• "Charles Hall Collection on the SNOBOL Programming Language".Minneapolis: Charles Babbage Institute, University of Minnesota.
• For a small brief taste of what SNOBOL4 is about try this online compiler
• Try It Online (Snobol4/CSNOBOL)Online compiler