>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?

Take a look at Boomerang: http://www.seas.upenn.edu/~harmony/

>From the overview:

Boomerang is a programming language for writing lenses—well-behaved bidirectional transformations—that operate on ad-hoc, textual data formats. Every lens program, when read from left to right, describes a function that maps an input to an output; when read from right to left, the very same program describes a "backwards" function that maps a modified output, together with the original input, back to a modified input.

Lenses have been used to solve problems across a wide range of areas in computing including: [...] in parsers and pretty printers