I remember writing APL for a part-time job in college, quite some time ago. The language was powerful, and compact. However, when I went back to a piece of code that I hadn't looked at in two weeks, the compact and powerful nature of the language made it hard to read. It was often denser and harder to read than mathematical proofs. Sometimes it was faster to re-write the code from scratch than to read it, so it was, to some extent, a write-only language.