I'm interested in similar proposals that can provide the basis of some common syntax that could be transpiled to other functional programming languages https://github.com/adsharma/py2many

I'm less interested in the syntax (will take anything accepted by a modified ast module), more in the semantics.

Here's a syntax I played with in the past:

  def area(s: Shape):
    pmatch(s):
        Circle(c):
            pi * c.r * c.r
        Rectangle(w, h):
            w * h
    ;;

  def times10(i):
    x = pmatch(i):
        1:
            10
        2:
            20
    ;;
    return x

Two notes:

  * Had to be pmatch, since match is more likely to run into namespace collision with existing code.
  * The delimiter `;;` at the end was necessary to avoid ambiguity in the grammar for the old parser. Perhaps things are different with the new PEG parser.

Code is on my github.