[erlang-questions] Reason for "format string not a textual constant"

[Eric Merritt]

Guys,

  I have a little build system that I put together. By default it
turns on all of the available warnings/error messages. For the most
part this is a good thing. However, in a couple of places where pass a
non-constant string to io:format I get the warning "format string not
a textual constant". Whats the reasoning behind this warning?

Thanks,
 Eric