66.7 CCL

CCL (Code Conversion Language) is a simple structured programming language designed for character coding conversions. A CCL program is compiled to CCL code (represented by a vector of integers) and executed by the CCL interpreter embedded in SXEmacs. The CCL interpreter implements a virtual machine with 8 registers called r0, ..., r7, a number of control structures, and some I/O operators. Take care when using registers r0 (used in implicit set statements) and especially r7 (used internally by several statements and operations, especially for multiple return values and I/O operations).

CCL is used for code conversion during process I/O and file I/O for non-ISO2022 coding systems. (It is the only way for a user to specify a code conversion function.) It is also used for calculating the code point of an X11 font from a character code. However, since CCL is designed as a powerful programming language, it can be used for more generic calculation where efficiency is demanded. A combination of three or more arithmetic operations can be calculated faster by CCL than by SXEmacs Lisp.

Warning: The code in src/mule-ccl.c and $packages/lisp/mule-base/mule-ccl.el is the definitive description of CCL’s semantics. The previous version of this section contained several typos and obsolete names left from earlier versions of MULE, and many may remain. (I am not an experienced CCL programmer; the few who know CCL well find writing English painful.)

A CCL program transforms an input data stream into an output data stream. The input stream, held in a buffer of constant bytes, is left unchanged. The buffer may be filled by an external input operation, taken from a SXEmacs buffer, or taken from a Lisp string. The output buffer is a dynamic array of bytes, which can be written by an external output operation, inserted into a SXEmacs buffer, or returned as a Lisp string.

A CCL program is a (Lisp) list containing two or three members. The first member is the buffer magnification, which indicates the required minimum size of the output buffer as a multiple of the input buffer. It is followed by the main block which executes while there is input remaining, and an optional EOF block which is executed when the input is exhausted. Both the main block and the EOF block are CCL blocks.

A CCL block is either a CCL statement or list of CCL statements. A CCL statement is either a set statement (either an integer or an assignment, which is a list of a register to receive the assignment, an assignment operator, and an expression) or a control statement (a list starting with a keyword, whose allowable syntax depends on the keyword).