Nim: algebraic data types (Maybe and Either), kinda Eq typeclass instances, kinda functors

algebraic_data_types_experiment.nim

type
  MaybeKind = enum Just, Nothing
  Maybe[T]  = object
    case kind: MaybeKind
      of Just: value: T
      of Nothing: discard

  EitherKind   = enum Left, Right
  Either[L, R] = object
    case kind: EitherKind
      of Left:  left:  L
      of Right: right: R

let
  a = Maybe[int32](kind: Nothing)
  b = Maybe[int32](kind: Just, value: 9000)
  c = Maybe[int32](kind: Just, value: 9000)
  d = Maybe[int32](kind: Just, value: 100)
  e = Maybe[int32](kind: Nothing)

  aa = Maybe[uint16](kind: Nothing)
  ab = Maybe[uint16](kind: Nothing)
  ac = Maybe[uint16](kind: Just, value: 10)
  ad = Maybe[uint16](kind: Just, value: 10)
  ae = Maybe[uint16](kind: Just, value: 20)

  ba = Either[uint, bool](kind: Left,  left: 5)
  bb = Either[uint, bool](kind: Left,  left: 5)
  bc = Either[uint, bool](kind: Right, right: true)
  bd = Either[uint, bool](kind: Right, right: true)
  be = Either[uint, bool](kind: Right, right: false)
  bf = Either[uint, bool](kind: Left,  left: 6)

proc `==`[T](a: Maybe[T], b: Maybe[T]): bool =
  case a.kind
    of Nothing: b.kind == Nothing
    of Just:    b.kind == Just and a.value == b.value

proc `==`[L, R](a: Either[L, R], b: Either[L, R]): bool =
  case a.kind
    of Left:  b.kind == Left  and a.left  == b.left
    of Right: b.kind == Right and a.right == b.right

proc `<$>`[A, B](f: (proc (x: A): B); a: Maybe[A]): Maybe[B] =
  case a.kind
    of Just: Maybe[B](kind: Just, value: a.value.f)
    of Nothing: Maybe[B](kind: Nothing)

proc `<$>`[L, R, N](f: (proc (x: R): N); a: Either[L, R]): Either[L, N] =
  case a.kind
    of Right: Either[L, N](kind: Right, right: a.right.f)
    of Left:  Either[L, N](kind: Left, left: a.left)

assert(a == e)
assert(b == c)
assert(b != d)
assert(c != d)
assert(a != b)
assert(b != e)

assert(aa == ab)
assert(ab != ac)
assert(ac == ad)
assert(ad != ae)
assert(ac != ae)

assert(ba == bb)
assert(bb != bc)
assert(bc == bd)
assert(bd != be)
assert(be != bf)
assert(ba != bf)

proc divBy2(x: int32): int32 = x div 2
proc divBy2x(x: int32): int16 = int16 x div 2
let b2: Maybe[int32] = divBy2 <$> b
let b3: Maybe[int16] = divBy2x <$> b
assert(b.kind == Just and b.value == 9000)
assert(b2.kind == Just and b2.value == 4500)
assert(b3.kind == Just and b3.value == 4500)

proc mult(x: int16): int32 = x * 2

let
  ca = Either[bool, int16](kind: Left,  left: true)
  cb = Either[bool, int16](kind: Right, right: 5)
  cc: Either[bool, int32] = mult <$> ca
  cd: Either[bool, int32] = mult <$> cb

assert(ca.kind == Left  and ca.left == true)
assert(cb.kind == Right and cb.right == 5)
assert(cc.kind == Left  and cc.left == true)
assert(cd.kind == Right and cd.right == 10)