Haskell, Reverse Polish Notation, and Parsing

Summary

My Side Quest into Haskell, Reverse Polish Notation, and Parsing 26 Jun, 2025 My Journey into Haskell: Building a Reverse Polish Notation Calculator Introduction: A Side Quest In my attempt to get my first paycheck, aka get a job, I have led myself down a fascinating rabbit hole into functional programming, mathematical notation, and parsing theory. This is the story of how I discovered Haskell, tackled reverse Polish notation, and learned about monadic parsing along the way. My journey began when my friend Jackson Slipock encouraged me to explore functional programming. After a quick Google search, I landed on Haskell as my first functional language. Around the same time, during a networking call about job opportunities, someone mentioned reverse Polish notation. The idea struck me: why not build a reverse Polish notation calculator in Haskell? What could have been a simple weekend project became weeks of deep learning. This blog chronicles that journey, drawing heavily from Graham Hutton's excellent book Programming in Haskell, which I highly recommend. What you're reading is my attempt to learn something deeply, implement it myself, and then teach it to others. From Imperative to Functional: A Mental Shift Coming from a background in Python and C—languages I'm comfortable with after a couple years of experience—Haskell represented a fundamental shift in thinking. Both Python and C are imperative languages that share similar concepts: loops, mutable data, global state, and variables that can be modified. The transition between them never required a major mental adjustment. Functional programming throws all of that out the window. In Haskell, functions are mathematical abstractions. Given the same input, a function will always produce the same output. There are no global variables, and data is immutable once created. Instead of writing a series of statements that accomplish a task, functional programming encourages building expressions that naturally compose togeth...