Experiment with Nim Pattern Matching Syntax

case_types.nim

import macros

type
  EitherKind {.pure.} = enum
    Some, None

  Either[T] = object
    case kind: EitherKind
    of Some:
      val: T
    of None:
      discard

proc `==`*[T](a: Either[T]; b: Either[T]): bool =
  if a.kind == b.kind:
    case a.kind
    of Some:
      return a.val == b.val
    of None:
      return true
  else:
    return false

proc some*[T](val: T): Either[T] = Either[T](kind: Some, val: val)
proc none*[T](val: T): Either[T] = Either[T](kind: None)

macro `as`*(a: typed, b: untyped): untyped = 

  let
    tpinst = getTypeInst(a)
    tpimpl = getTypeImpl(a)
    kind = b[0]
    name = b[1]

  result = quote do:
    let res = `a`.kind == `kind`
    let `name` {.inject.}: int = if res: `a`.val
                                 else: default(`a`.val.type)
    res

block:
  let x = some 3

  if x as Some(y):
    echo "huzzah! x: ", x, " y: ", y
  else:
    echo "sad day!"

block:
  let x = none 3
  if x as Some(y):
    echo "huzzah! x: ", x, " y: ", y
  else:
    echo "sad day!"