Building a JSON Parser in Haskell

I decided to learn Haskell by building something I understood well: a JSON parser. JSON has a simple, well-defined spec, which meant I could focus on the language rather than the domain.

Parser combinators felt foreign at first. Instead of manually tracking position and managing a stack, you compose small parsers together. The type system forces you to handle failure explicitly - parse results are wrapped in Either or Maybe types, which means you can’t pretend errors don’t exist.

The JSON grammar has nested structures: objects contain key-value pairs, arrays contain elements, and values can be any of these types. Modeling this in Haskell’s algebraic data types felt natural. The parser for an object returns a value of type Object, the parser for an array returns Array, and so on. The type system guides you toward correct implementations.

What surprised me was how little code it took to get a working parser. The combinators handle the tedious parts - whitespace skipping, error positioning, backtracking. I wrote parsers for primitives (strings, numbers, booleans) and composed them into parsers for complex types.

This was my first real project in Haskell. I still struggle with monads and type classes sometimes, but parser combinators gave me something concrete to work with. The code I write in other languages now tends to be more explicit about error handling, and I think about data types differently.