[Cyther606]

As another illustrative example, let's say we want to create a library for working with mathematical expressions such as x^2 + 5*x. It's natural to define our data types like this:

    type
      FormulaKind = enum
        fkVar,        ## element is a variable like 'X'
        fkLit,        ## element is a literal like 0.1
        fkAdd,        ## element is an addition operation
        fkMul,        ## element is a multiplication operation
        fkExp         ## element is an exponentiation operation

    type
      Formula = ref object
        case kind: FormulaKind
        of fkVar: name: string
        of fkLit: value: float
        of fkAdd, fkMul, fkExp: left, right: Formula

Nim's enum [1] is an old-school typesafe enum, as in Ada, without any fields. To avoid name clashes, it's common to prefix the enum values with a two-letter abbreviation. The case part in the object declaration introduces a checked union. So the access of f.name will raise an exception if f.kind != fkVar.

[1] http://goran.krampe.se/2014/10/20/i-missed-nim/