B.1 The Template File - UNFUG Expression Evaluation

UNFUG template source files are regular source files of the respective target programming language with special UNFUG expressions inserted in angle brackets <>. Angle brackets were chosen since they resemble the notations for ``placeholders'', but they may conflict with the genuine usage of angle brackets in a programming language syntax (mostly as relational operators). To overcome this situation, some precautions are taken to avoid syntactical conflicts.

Two forms of an UNFUG expression are possible: A simple LISP variable name, e.g. <variable_name>, or a full LISP expression, e.g. <(strcat "int " variable_name)>. In both forms, no spaces are allowed immediately after the opening or before the closing angle bracket of the expression to be properly recognized by UNFUG. In the first case, the variable name may just contain upper- or lowercase letters, numbers or an underscore character ``_'', thus avoiding any conflicts with typical conditional constructs in most programming languages, where 1<variable_name would be an invalid expression syntax. In the second case, the same holds true if the variable is substituted with a parenthesized expression (with, of course, matching parentheses). Thus it is guaranteed that no UNFUG expression may conflict with a syntactically correct programming language construct of common programming languages like C, FORTRAN, or PASCAL.

When running UNFUG on a template file, UNFUG appends all lines which do not contain a valid UNFUG expression unmodified to the output file. When encountering a valid UNFUG expression in a template source file, the UNFUG expression evaluator now pursues the following strategy:

• If the expression is a LISP variable name which is not bound to a string value, the empty string ``'' is appended to the output file in place of the UNFUG expression.

• If the expression is a LISP variable name which is bound to a string value, the string's value is appended to the output file in place of the UNFUG expression.

• If the expression is a LISP variable name which is bound to a LISP function, and if the corresponding function evaluates to a string value, this value is appended to the output file in place of the UNFUG expression, otherwise the empty string ``'' is appended.

The LISP expression is taken ``as is'' and is just evaluated by the LISP interpreter. Any syntactical or semantical errors are therefore not detected by UNFUG, but only by XLISP upon evaluation. This may be considered a drawback, but it offers the utmost flexibility in code generation, however complex this process may be. A few LISP functions are predefined by UNFUG, which perform the actual tasks UNFUG was developed for. These are described in the following section.