In Haskell, why isn't there a TypeClass for things that can act like lists?

Question

I\'m reading Learn You a Haskell and I\'m wondering why so many things are acting like a list, and nothing in the Prelude is using the native facility of type classes to set

Answer 1

The main problem with such a class is that even if it existed it would only offer a superficial similarity.

The asymptotics of the same algorithm built using different structures would vary tremendously.

In the case of strict bytestrings building them up with cons is terrible, because you wind up copying the entire string every time you add another Char. This O(1) operation on a list turns it into an O(n) operation on a Bytestring.

This leads to O(n^2) behavior when you implement the first algorithm that might come to mind, map, whereas building up a list or Data.Sequence.Seq with cons is linear time and it can be implemented in O(n) for bytestrings or vectors as well with a little bit of thought.

It turns out the utility of such a class in light of this is more superficial than actual.

I'm not saying that a good design can't be found, but such a design would be difficult to use and to optimize for and likely a usable version of the design would not wind up being Haskell 98.

I've eked out portions of this design space in my keys package, which provides a lot of functions for indexing into containers, etc, but I've deliberately avoided providing a list-like API a.) because it has been done before to little success and b.) because of the asymptotic concerns above.

tl;dr You typically want to implement algorithms very differently when the asymptotics of the underlying operations change.