You have the compiler use a special reader which records the source locations of each form in e.g. a hash table. When compiling a specific source, look it up in

You have the compiler use a special reader which records the source locations of each form in e.g. a hash table. When compiling a specific source, look it up in the hash table to get a source location mapping. There's a couple of downsides to this: - This only works when the reader returns unique objects. It's possible to track the location of a cons cell, but it's not possible to do it for any specific use of a symbol or a integer. - CL macros can do arbitrary transformations on the data. If those transformations involve loss of object identity, the tracking breaks down. (E.g. code-walking macros will often end up doing spurious copies, and cause unnecessary loss of source location mappings).