In the previous post we wrote a simple but correct JSON parser in Haskell. The parser was written very naively: if it failed, it returned nothing. You couldn’t tell what the failure was or where it happened. That’s OK for a toy parser but error reporting is an absolute must requirement for all good parsers. So in this post, we’ll add simple but useful error reporting capability to our JSON parser.

JSON is probably the most used standard file format for storing and transmitting data on the internet in recent times. Though it was historically derived from JavaScript, it is a programming language independent format and is now supported by almost all languages. JSON has a simple syntax specification with only four scalar data types and two composite data types. So, writing a parser for JSON is a great exercise for learning the basics of parsing. Let’s write one from scratch in Haskell.

Haskell—with its powerful type system—has a great support for type-level programming and it has gotten much better in the recent times with the new releases of the GHC compiler. But type-level programming remains a daunting topic even with seasoned haskellers. Thinking with Types: Type-level Programming in Haskell by Sandy Maguire is a book which attempts to fix that. I’ve taken some notes to summarize my understanding of the same.