An Either monad(ish) implementation for Nim v0.14

either.nim

## A right-biased Either[L, R] implementation in Nim

import optional_t

type
  EitherType* {.pure.} = enum
    Left, Right
  Either*[L, R] = object
    kind: EitherType
    right: Option[R]
    left: Option[L]

proc unsafeGetR*[R](self: Either[any, R]): R =
  self.right.unsafeGet()

proc unsafeGetL*[L](self: Either[L, any]): L =
  self.left.unsafeGet()

proc Right*[L, R](val: R): Either[L, R] =
  result.right = Some[R](val)
  result.left = None[L]()
  result.kind = EitherType.Right

proc Left*[L, R](val: L): Either[L, R] =
  result.left = Some[L](val)
  result.right = None[R]()
  result.kind = EitherType.Left

proc isLeft*[L, R](self: Either[L, R]): bool = ( self.kind == EitherType.Left )
proc isRight*[L, R](self: Either[L, R]): bool = ( self.kind == EitherType.Right )

proc map*[L, R, V](self: Either[L, R], oper: proc (input: R): V): Either[L, R] =
  if self.isRight:
    return Right[L, R](oper(self.unsafeGetR))
  else:
    return self

proc `$`*[L, R](self: Either[L, R]): string =
  if self.isLeft:
    result = "Left(" & $self.unsafeGetL & ")"
  else:
    result = "Right(" & $self.unsafeGetR & ")"

## Slightly dodgy usage examples...

proc aRight*() : Either[string, int] =
  var test = Right[string, int](1234)
  echo test.unsafeGetR
  test = test.map do (val: int) -> int:
    echo val
    val + 1
  test.map do (val: int) -> int:
    echo val
    val

proc aLeft*() : Either[string, int] =
  result = Left[string, int]("This is an error!")

echo aRight()