SNOBOL - Features

Features

SNOBOL4 is the fourth and latest incarnation of a series of special purpose programming languages for character string manipulation.

The SNOBOL4 variant of the language supports a number of built-in data types, such as integers and limited precision real numbers, strings, patterns, arrays, and tables (associative arrays), and also allows the programmer to define additional data types and new functions. SNOBOL4's programmer-defined data type facility was advanced at the time—it is similar to the earlier Cobol's and the later Pascal's records.

All SNOBOL command lines are of the form Label1 Subject Pattern = Object :(Goto Label2) Each of the five sections is optional. In general, the subject is matched against the pattern. If the object is present, any matched portion is replaced by the object via rules for replacement. The goto can be an absolute branch, or conditionally based on success or failure of the subject evaluation, the pattern evaluation, the pattern match, the object evaluation or the final assignment.

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 a Backus-Naur form expression 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 the logic programming language Prolog, which provides pattern-like constructs via DCGs. 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 single garbage-collected heap.

SNOBOL rivals APL for its distinctiveness in format and programming style, both being radically unlike more "standard" procedural languages such as BASIC, Fortran, or C.