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Comment by jsmeaton | Hacker News Reader

Looks like you're being downvoted, but I have to agree with you, especially given some of the examples right at the top.

  foldr (:) [] [1,2,3]

There is absolutely nothing natural about that line of code unless you happen to already have some background. I executed that in the sandbox and got:

  [1,2,3]:: Num a => [a]

And I still don't know what was achieved.

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For a function f and a value x,

    foldr f x [a,b,c]

gets turned into

    f(a, f(b, f(c, x)))

'(:)' is list concatenation, so the result is

    1 : (2 : (3 : []))

(Here we are writing list concatenation in infix notation, rather than the customary prefix notation). '[1,2,3]' is shorthand for 1 : (2 : (3: [])) in Haskell.

Thanks, I appreciate the walk through. So it's essentially just copying an array (in this particular example). I'd still argue that the syntax is not natural - since you have to have specific knowledge of the operator and the function.

Syntactic naturality seems like it's mostly a function of familiarity. That said, the example has, for a lot of mathematical reasons, a great deal of semantic naturality.

It's far from immediately obvious, but `foldr (:) []` is the way to copy singly-linked lists. In particular, if you look at a linked list as a degenerate tree then what `foldr f z` does is replace all of the nodes with `f` and the single leaf with `z`. Since cons, i.e. (:), creates a node and nil, i.e. [], is that single leaf then `foldr (:) []` is a very natural way to do nothing at all.

So it's kind of the most boring interesting example imaginable.

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