Monadic parser combinators are a domain specific language for writing parsers which can be embedded in general purpose programming languages like Haskell. Monadic parser combinators have several advantages over traditional parser generators. For example, they allow users to define their own derived combinators which can reduce duplication; the monadic structure allows for data dependent parsing, for example for length headers in (binary) data formats or for disambiguation such as layout parsing; and the embedding allows for direct semantic parsing which avoids the need for concrete syntax trees.

However, most monadic parser combinator libraries force their users to contort their grammars to avoid left-recursion and, if left-recursion is introduced accidentally, produce non-terminating programs without clear error messages. Secondly, parser combinators usually have little support for backtracking or degrade to exponential worst case complexity if backtracking is used pervasively. Finally, Ambiguities are often resolved by choosing the first successful branch which can be unpredictable.

In this talk, I show how we can distill a GLL parsing algorithm to its essence and how to embed it as a parser combinator library in Haskell. The resulting library allows for writing left recursive parsers while retaining most of the advantages of monadic parser combinators. Futhermore, I will discuss new challenges and new opportunities that this approach brings.