"undefined reference to" error. can't compile with custom modules

[Adam D. Ruppe]

I think this is the wrong newsgroup. Normally you'd use 
digitalmars.D.learn.

But when compiling you should put all your custom modules on the 
command line.

dmd tests.d datastructures.d

And that will solve it. Alternatively you can compile them 
separately then put all the .o files together to link, but the 
easiest way is to just list them at once.