Andy Wingo <wi...@pobox.com> writes: > I replaced the text with this: > > We do not recommend #,() reader extensions, however, and for three > reasons. > > First of all, this SRFI is not modular: the tag is matched by name, > not as an identifier within a scope. Defining a reader extension in one > part of a program can thus affect unrelated parts of a program because > the tag is not scoped. > > Secondly, reader extensions can be hard to manage from a time > perspective: when does the reader extension take effect? *Note Eval > When::, for more discussion. > > Finally, reader extensions can easily produce objects that can’t be > reified to an object file by the compiler. For example if you define a > reader extension that makes a hash table (*note Hash Tables::), then it > will work fine when run with the interpreter, and you think you have a > neat hack. But then if you try to compile your program, after wrangling > with the ‘eval-when’ concerns mentioned above, the compiler will carp > that it doesn’t know how to serialize a hash table to disk. > > In the specific case of hash tables, it would be possible for Guile > to know how to pack hash tables into compiled files, but this doesn’t > work in general. What if the object you produce is an instance of a > record type? Guile would then have to serialize the record type to disk > too, and then what happens if the program independently loads the code > that defines the record type? Does it define the same type or a > different type? Guile’s record types are nominal, not structural, so > the answer is not clear at all. > > For all of these reasons we recommend macros over reader extensions. > Macros fulfill many of the same needs while preserving modular > composition, and their interaction with ‘eval-when’ is well-known. If > you need brevity, instead use ‘read-hash-extend’ and make your reader > extension expand to a macro invocation. In that way we preserve scoping > as much as possible. *Note Reader Extensions::.
I find this documentation helpful. Thank you. -- Mathieu Lirzin
