A pattern matcher for miniKanren or How to get into trouble with CPS macros

CPS macros written using Scheme’s syntax-rules macro system allow for guaranteed composition of macros and control over the order of macro expansion. We identify a limitation of CPS macros when used to generate bindings from a non-unique list of user-specified identifiers. Implementing a pattern matcher for the miniKanren relational programming language revealed this limitation. Identifiers come from the pattern, and repetition indicates that the same variable binding should be used. Using a CPS macro, binding is delayed until after the comparisons are performed. This may cause free identifiers that are symbolically equal to be conflated, even when they are introduced by different parts of the source program. After expansion, this leaves some identifiers unbound that should be bound. In our first solution, we use syntax-case with bound-identifier=? to correctly compare the delayed bindings. Our second solution uses eager binding with syntax-rules. This requires abandoning the CPS approach when discovering new identifiers.