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Comment by soegaard | Hacker News Reader

It's not magic. It's pattern matching.

The `[Posn(x, y), ...]` is a pattern that matches a list of positions. The `[Posn(y, x), ...]` is a template that produces a list of positions.

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That syntax suggests the first pair will be flipped, and the rest will just be as they were, i.e. more like flip_first

When the pattern `[Posn(x, y), ...]` matches a list of positions, since `Posn(x, y)` is followed by `...` the `x` is bound to a sequence of first coordinates and `y` is bound to a sequence of second coordinates.

In the template the `Posn(y, x)` is followed by `...` so the same number of positions is produced as the common length of x and y.

What would be the corresponding syntax for matching and flipping only the first Posn?

If I had to guess:

  fun flip_all([Posn(x, y), rest]):
    [Posn(y, x), rest]

If I am correct then the only difference between flipping all and flipping just the first is `...` vs `rest`

    fun flip_first([Posn(x, y), more, ...]):
      [Posn(y, x), more, ...]

Your version would expect a list of two elements, you use & to indicate that you're collecting the remainder of the list as in:

  fun flip_all([Posn(x, y), & rest]):

You'd need to amend the logic of the function body to iterate or recurse over the remainder. You'd also ended up with a nested list using your version but you can use & again to splice in the result, so if you wanted a recursive version of that it would be something like (skipping the base case):

  fun
    | flip_all([]): []
    | flip_all([Posn(x, y), & rest]):
        [Posn(y, x), & flip_all(rest)]

(not tested, don't have Rhombus available on this system but that should be correct)

If you really want flip_first it'd be:

  fun flip_first([Posn(x, y), & rest]):
    [Posn(y, x), & rest]

In Haskell:

  flip_one Posn(a,b) = Posn(b,a)
  flip_first p:ps = flip one : ps
  flip_all ps = map flip ps

To my taste that's a lot nicer

Typical pattern matching like in Haskell or Prolog wouldn't do this.

It makes perfect sense if you're used to the pattern matching of Scheme and Racket syntax macros.

I guess so, though that might be counterproductive to the goals of Rhombus.

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