ADT Values should support named fields

[macrologist]

(define-struct Point
    (x F32)
    (y F32))

(define-type (Shape F32)
   (Circle (@radius F32) (@center Point))
   (Square (@side  F32) (@center Point))
   (Triangle Point Point Point))

These named fields would be available for pattern matching

(define (centroid shape)
    (match shape
       ((Circle @center) center)
       ((Square @center) center)
       ((Triangle a b c)
         (let (Pt x y) = (sum-points (make-list a b c)))
         (Pt (/ x 3) (/ y 3))))

For the above Shape type, this extension does not seem very useful. But in real applications ADT fields may number greater than the 7 plus-or-minus 2 mark, at which point remembering orderings becomes tedious for we pea brained humans.

This is an attempt to start a discussion

[fukamachi]

I can relate to the motivation for adding named fields. However, regarding the @ syntax, I feel it might be a bit much.

In Common Lisp it only appears in ,@ for splicing unquote, and in Clojure, @ is used for deref (which is a completely different operation and can take even a form).

While we could certainly add a new meaning to @ in Coalton, it feels a bit unidiomatic for the Lisp ecosystem.

Here are alternatives that might feel more natural to me:

;; Keyword prefix:
(define-type Shape
  (Circle (:radius F32) (:center Point))
  (Triangle Point Point Point))

(match shape
  ((Circle :center center) center)
  ((Square :center center) center))
  ...)

;; Dot prefix:
(define-type Shape
  (Circle (.radius F32) (.center Point))
  (Square (.side F32) (.center Point))
  (Triangle Point Point Point))

(match shape
  ((Circle .center) center)
  ((Square .center) center))
  ...)

What do you think?

[macrologist]

Dot prefix is probably preferable to colon prefix.

For example, colon prefix names may become visually uncomfortable when type variables enter the picture.

(define (Shape :f)
   (Circle (:radius :f) (:center (Point :f)))
   (Square (:side :f)   (:center (Point :f))))

So with that in mind, I think a dot syntax would be good