What effect does a foreign patent application have on a domestic application?

It depends upon several factors. The first factor that you have to determine is whether what you CLAIM, or are intending to claim, in your US application, is DISCLOSED within the foreign application. In other words, does that foreign patent application show, teach, or clearly demonstrate what you want to claim as your invention. If what you claim, or eventually claim, is not disclosed within the foreign application, then you should hopefully be able to obtain a US patent. The second factor to consider is time and publication - when was the foreign application filed and when was it published. If it has not yet been published, it is not considered to be prior art with respect to your US application if you file your US application prior to the publication of the foreign patent application. This of course also presumes that the foreign invention was not disclosed to the public by some other means other than the foreign-filed patent application. If there was a public disclosure, then that is effectively the same as publication of the foreign patent application.