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 a virtual machine to 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 the SNOBOL Implementation Language, 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 by Douglas 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 as Fortran and ALGOL. SNOBOL4 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 records of the earlier COBOL and the later Pascal programming languages. All SNOBOL command lines are of the form label subject pattern = object : transfer Each of the five elements 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 transfer can be an absolute branch or a conditional branch dependent upon the success or failure of the subject evaluation, the pattern evaluation,