Polymorphism is a powerful programming language feature. In polymorphism, we have generic functions that don’t know exactly what type of data they will be operating on. Often, the data types won’t even all have been designed yet when the generic function is written. The generic function provides the general outline of the work, but the details of some parts of the work, some specific operations, must be tailored to the specific types being used. The generic code needs some way of accessing these specific operations, and the users of the generic code need some way of specifying them.

The Error Message

I’ve written before about just how befuddling Haskell error messages can be, especially for beginners. And now, even though I have some professional Haskell development under my belt, I ran across a Haskell error message that confused me for a bit, where I had to get help. It’s clear to me now when I look at the error message what it’s trying to say, but I legitimately was stumped by it, and so, even though it’s embarrassing for me now, I feel the need to write about how this error message could have been easier to understand:

I am a big fan of strongly typed languages, and my favorite GC’d language is Haskell. And I want you, the reader, to keep that in mind today. What I am writing is some commentary about a language I deeply love, some loving criticism.

So here’s what happened: A few days ago, I was showing off some Haskell for a friend who primarily programs in Python. The stakes were high – could I demonstrate that this strange language was worth some investigation?