Happy is a parser generator system for Haskell, similar to the tool `yacc' for C. Like `yacc', it takes a file containing an annotated BNF specification of a grammar and produces a Haskell module containing a parser for the grammar.

Happy is flexible: you can have several Happy parsers in the same program, and several entry points to a single grammar. Happy can work in conjunction with a lexical analyser supplied by the user (either hand-written or generated by another program), or it can parse a stream of characters directly (but this isn't practical in most cases).