Ask HN: Best approach to designing a parser / creating your programming language

I've been writing a parser in Haskell Parsec to parse a subset of Haskell;

I didnt stick to anything like BNF and it turns out to be much more daunting task than expected.

When designing a parser for a (subset of) known language (or creating your own programming language), are things like BNF fundemental to a good design? What toolkits or frameworks can be employed in the planning process?

Any tips or approaches (or readings) on the best way to strike a balance between theoretical formalization v.s. implementing something that actually works and have good performance and easy to debug?