I tried to use nom for one of my projects, but I found that it had two major problems. First, the macro syntax was hard to use, hard to learn, and hard to debug. Sometimes you had to put an expression in parentheses, even if it seems that it should be unnecessary, so that the macros would be able to parse your code. I also found the macros hard to compose, which is ironic for a parser combinator library.

My other problem was with error handling. I wanted to have my own `Error` enum to represent errors in my program. Nom supports a custom error type, but I now had to write out type annotations in many places with the fish syntax (e.g. foo::<&[u8], Ast, Error>()) and they are quite long annotations. In addition, some of the macros would not work—or at least, I couldn't get them to work—with my custom error type, and I had to revert to using functions in a way that was absolutely not compositional.

In the end, since the format I was parsing was simple and regular (Erlang's External Term Format), I wrote a parser by hand and removed the dependency on nom. The result is as just as fast and produces meaningful error messages. It's actually less tedious, because I am using simple functions that have the syntax and semantics that I already know; maybe if I had a more complex format to parse I would sing a different tune, but for the moment I prefer to avoid using nom.