[erlang-questions] Generating a variable in a macro

Richard Carlsson <>
Tue Jan 9 13:11:26 CET 2007


Joel Reymont wrote:
> Is there a way to "auto-generate" a variable in a macro ala (gensym)  
> in Lisp?
>  [...]
> Unless I pass Any1, Any2, etc. into the macro, erl will complain that  
> the use of Any is unsafe. Any workarounds apart from passing the  
> error variable as a macro argument?

The standard trick is to wrap the macro body in a fun-application,
i.e.:
	((fun () -> ...body... end)())

this keeps new variables in the body from being exported into the
surrounding environment. To avoid collisions between local variables
and existing variables in the environment, it is a good idea to
prefix all local variables with e.g., "__" or similar, as in:

-define(foo(Table, Nodes),
  ((fun () ->
      case mnesia:create_table(Table,[...]) of
        {atomic, ok} -> ok;
        __Any -> error_logger:error_report(...)
      end
    end)())).

This is relatively foolproof, but note that the macros don't nest.

     /Richard




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