There were a couple of places that took me a couple reads to figure out, like the fact that `(x:)` was "prepend". But overall, I followed the code pretty well. From the context of someone that wrote a small amount of Haskell a decade ago.
The : operator is the linked list data constructor. It takes an element and a list and creates a new linked list by linking the element to the existing list. It does the opposite when used in a pattern match: separates out the first element in a linked list.
It is also an operator, meaning it can be used with infix notation, as in (x : xs). Haskell has something called operator sections, where if one supplies only one of the arguments to an operator it will return a function expecting the other argument. In other words
(x:) == \xs -> (x:xs)
and
(:xs) == \x -> (x:xs)
This can be used as in this article, to create a function that prepends x to any list. Another common example is (1+) which increments any number it is given, or (:[]) which turns any value into a one-element list.
It can also be used much more cleverly -- especially considering that any two-argument function can be turned into an operator with backticks -- but then (in my opinion) readability starts to suffer.
It’s partial application of cons via the operator, admittedly a poor choice from Haskell, a language which likes operators a bit too much. I think eta-expansion makes the whole thing clearer: (\xs -> x:xs) but most Haskellers would disagree.
The article also features examples of point-free style, another unfortunate trend for readability.
As long as you use operators sparingly, don’t abuse partial application and prefer explicit lambdas to composition, Haskell is fairly readable. The issue is that approximately no Haskeller writes Haskell this way.
I don't use Haskell nearly enough to call myself a Haskeller but I will still disagree. Yes, operator sections are yet another thing to learn but I find them very intuitive, and actually easier to read than the equivalent lambda expression because I don't have to match up the bound variable.
(For example, (\x -> x ++ y) and (\y -> x ++ y) look pretty similar to me at first glance, but (++y) and (x++) are immediately distinguishable.)
Of course, this is reliant on knowing the operators but that seems like a mostly orthogonal issue to me: You still need to know the operator in the lambda expression. That said, the niceness of sections gives people yet another reason to introduce operators for their stuff when arguably they already are too prevalent.
It’s not that those language features are hard to understand. They’re all syntactic and don’t bring a ton of theory with them. It’s just that the tower of understanding for basic programs is very tall, and the tendency to introduce abstraction essentially never ends. I spent ten years with Haskell as my go-to language and there are still things I don’t understand and haven’t explored. It’s not like that with Python, or Go, or even Rust.
