Enums

Enums in Nim are like enums in C, but are type-checked. There are no anonymous enums in Nim.

type
  CompassDirections = enum
    cdNorth, cdEast, cdSouth, cdWest

  Colors {.pure.} = enum
    Red = "FF0000", Green = (1, "00FF00"), Blue = "0000FF"

  OtherColors {.pure.} = enum
    Red = 0xFF0000, Orange = 0xFFA500, Yellow = 0xFFFF00

  Signals = enum
    sigQuit = 3, sigAbort = 6, sigKill = 9

# echo Red  # Error: ambiguous identifier: 'Red'
echo Colors.Red
echo OtherColors.Red
echo OtherColors.Orange, " ", Orange

$ nim c -r enumdefs.nim
FF0000
Red
Orange Orange

Notice that each element in CompassDirections is prepended with cd to avoid name conflicts since references to the enum value do not need to be qualified.

The {.pure.} pragma that Colors and OtherColors have requires that all ambigious references to the enums’ values be qualified. This mean that Orange can be referred to without OtherColors.Orange (although OtherColors.Orange is still allowed).

Enums can be given custom values and stringify values, as shown by Colors and Signals.

Ordinals

While enums can also have disjoint values, it should not be used for any other reason than compatibility with C because it breaks the idea that enums are ordinal.

for direction in ord(low(CompassDirections))..
                 ord(high(CompassDirections)):
  echo CompassDirections(direction), " ord: ", direction

var ordinal = low(int)
inc ordinal
dec ordinal
echo high(char)

$ nim c -r enums.nim
cdNorth ord: 0
cdEast ord: 1
cdSouth ord: 2
cdWest ord: 3
3
�

Because enums are ordinals, they have the low, high, inc, dec, and ord methods defined, where

• low gives the lowest possible value
• high give the highest possible value
• inc increments
• dec decrements
• ord gives the integer value of the enum
• CompassDirections is a cast that gives an enum from an integer

It is also possible to iterate through all possible values of ordinal enums, either as shown above, or cdNorth..cdWest, which is equivalent.

when false:
  var nonOrdinal = sigQuit
  inc nonOrdinal
  dec nonOrdinal

Signals is not an ordinal type, and so doesn’t have the inc and dec procedures.