Define parser and printer consistently

[Dawid Toton <d0@wp.pl>]

I'm going to define a parser and a printer for a simple grammar.
Is there a way to define both of them in a single construct using some 
existing OCaml tool?

For example, I have a keyword "function". The usual parser would contain 
a mapping like:
"function" -> `Function
and the straightforward printer would do:
`Function -> "function"

What is the best way to combine these definitions, so that duplication 
would be minimized?
To be precise, avoiding duplication is not exactly what I need. I'm 
looking for something that would prevent making inconsistent changes to 
the parser and the printer.

Dawid