The C preprocessor is a thing of beauty

Converting programs from one language to another can be easy or difficult, depending on the languages. More specifically, the ease of conversion depends on the commonality of language features.

For example: converting a program from FORTRAN IV to C. Converting a FORTRAN program to C is easy, converting a C program to FORTRAN IV is hard. The FORTRAN constructs of subroutines and functions are easily handled in C, as are the data types of integer, real, and character. C has constructs that are unavailable in FORTRAN: pointers, dynamic memory, and recursion. The FORTRAN language elements of IF/THEN and DO are available in C, but the C elements of 'while' and 'longjmp' are not. (We have to shoe-horn the non-FORTRAN aspects of C into FORTRAN.)

When converting a program from one language to another, the difficulties of conversion are the language-specific features of the 'origin' language -- the things that the 'destination' language cannot handle.

In the past, I have considered the C preprocessor to be a hideous thing, an orc-like programming language that has no manners. But I have been wrong.

The C preprocessor (which is the same as the C++ preprocessor), has some very interesting constructs. It is a language with concepts found only in advanced programming languages. It has access to the caller's scope, something that exists in the lambdas of LISP. It can use a parameter as a substitution token, or it can substitute the text value of the token name into the program ("stringification").

These features of the C preprocessor make conversion to other languages difficult. We think of the C preprocessor as an ugly beast not because it is ugly, but because we cannot easily convert its code to another language. The preprocessor language is beautiful -- simple, elegant, and powerful. We hate it because we cannot think of it in the terms of languages that we know and love.

It may be some time before mainstream languages have the features of the C preprocessor. (I'm considering LISP outside of the mainstream, at least for now.) Until such a time, the advanced language of the preprocessor will pose conversion challenges to programmers.