3 ms·

Side effects are no issue if every local variable name in a defmacro is generated with (gensym).

nyir 9y ago
Exactly, you have to specify what you want. Evaluating an expression argument multiple times is probably a mistake, as is not having a new gensym for local variables ... until it is not, e.g. for anaphoric macros (binding some value to "it", https://en.wikipedia.org/wiki/Anaphoric_macro https://en.wikipedia.org/wiki/Anaphoric_macro, you can still argue that that's bad style of course).
TeMPOraL 9y ago
And if you're tired of doing that by hand, you can... write a macro that'll do that for you. In fact, those were already written, and are available in many libraries as well as featured in many books. See e.g. Alexandria[0] for with-gensyms (automatically binding your variables to gensyms) and once-only (similar, but ensures the value will be evaluated only once). Also note that lack of hygiene is actually a useful feature - it allows macros to capture variables from code passed to it as well as the environment they expand it. Some cool stuff can be done that way (some uncool foot-shooting too, though). -- [0] - https://common-lisp.net/project/alexandria/draft/alexandria.html#Macro-Writing https://common-lisp.net/project/alexandria/draft/alexandria....
- sedachv 9y ago
  > Also note that lack of hygiene is actually a useful feature - it allows macros to capture variables from code passed to it as well as the environment they expand it. Some cool stuff can be done that way (some uncool foot-shooting too, though). Doug Hoyte's excellent book _Let Over Lambda_ has a lot of examples.
  - TeMPOraL 9y ago
    Oh yes. This book is very fun, in the "what on Earth did he just do here" kind of way.
- Johnny_Brahms 9y ago
  Syntax-case allows you to introduce bindings into the lexical context (breaking hygiene in a very controlled manner). It does not work with lists, but with syntax objects.

nyir 9y ago

Exactly, you have to specify what you want. Evaluating an expression argument multiple times is probably a mistake, as is not having a new gensym for local variables ... until it is not, e.g. for anaphoric macros (binding some value to "it", https://en.wikipedia.org/wiki/Anaphoric_macro https://en.wikipedia.org/wiki/Anaphoric_macro, you can still argue that that's bad style of course).

TeMPOraL 9y ago

And if you're tired of doing that by hand, you can... write a macro that'll do that for you. In fact, those were already written, and are available in many libraries as well as featured in many books. See e.g. Alexandria[0] for with-gensyms (automatically binding your variables to gensyms) and once-only (similar, but ensures the value will be evaluated only once). Also note that lack of hygiene is actually a useful feature - it allows macros to capture variables from code passed to it as well as the environment they expand it. Some cool stuff can be done that way (some uncool foot-shooting too, though). -- [0] - https://common-lisp.net/project/alexandria/draft/alexandria.html#Macro-Writing https://common-lisp.net/project/alexandria/draft/alexandria....

sedachv 9y ago
> Also note that lack of hygiene is actually a useful feature - it allows macros to capture variables from code passed to it as well as the environment they expand it. Some cool stuff can be done that way (some uncool foot-shooting too, though). Doug Hoyte's excellent book _Let Over Lambda_ has a lot of examples.
- TeMPOraL 9y ago
  Oh yes. This book is very fun, in the "what on Earth did he just do here" kind of way.
Johnny_Brahms 9y ago
Syntax-case allows you to introduce bindings into the lexical context (breaking hygiene in a very controlled manner). It does not work with lists, but with syntax objects.