Need to think how to bind C enums with aliases

[ian-ross]

Bug imported from C2HS Trac

Trac ticket created: 2011-08-26T02:01:18-0700

---

Consider this C enum:

typedef enum
{
  GTK_ANCHOR_CENTER,
  GTK_ANCHOR_NORTH,
  GTK_ANCHOR_NORTH_WEST,
  GTK_ANCHOR_NORTH_EAST,
  GTK_ANCHOR_SOUTH,
  GTK_ANCHOR_SOUTH_WEST,
  GTK_ANCHOR_SOUTH_EAST,
  GTK_ANCHOR_WEST,
  GTK_ANCHOR_EAST,
  GTK_ANCHOR_N          = GTK_ANCHOR_NORTH,
  GTK_ANCHOR_NW         = GTK_ANCHOR_NORTH_WEST,
  GTK_ANCHOR_NE         = GTK_ANCHOR_NORTH_EAST,
  GTK_ANCHOR_S          = GTK_ANCHOR_SOUTH,
  GTK_ANCHOR_SW         = GTK_ANCHOR_SOUTH_WEST,
  GTK_ANCHOR_SE         = GTK_ANCHOR_SOUTH_EAST,
  GTK_ANCHOR_W          = GTK_ANCHOR_WEST,
  GTK_ANCHOR_E          = GTK_ANCHOR_EAST
} GtkAnchorType;

Of course in C, enums are really ints, so it's ok to have aliases. It is the case that GTK_ANCHOR_E = GTK_ANCHOR_EAST since both enum names have the int value 9.

Currently c2hs binds this as:

data GtkAnchorType = GtkAnchorCenter
                   | GtkAnchorNorth
                   | GtkAnchorNorthWest
                   | GtkAnchorNorthEast
                   | GtkAnchorSouth
                   | GtkAnchorSouthWest
                   | GtkAnchorSouthEast
                   | GtkAnchorWest
                   | GtkAnchorEast
                   | GtkAnchorN
                   | GtkAnchorNw
                   | GtkAnchorNe
                   | GtkAnchorS
                   | GtkAnchorSw
                   | GtkAnchorSe
                   | GtkAnchorW
                   | GtkAnchorE
                   deriving (Eq,Show)

instance Enum GtkAnchorType where
  fromEnum GtkAnchorCenter = 0
  fromEnum GtkAnchorNorth = 1
  fromEnum GtkAnchorNorthWest = 2
  fromEnum GtkAnchorNorthEast = 3
  fromEnum GtkAnchorSouth = 4
  fromEnum GtkAnchorSouthWest = 5
  fromEnum GtkAnchorSouthEast = 6
  fromEnum GtkAnchorWest = 7
  fromEnum GtkAnchorEast = 8
  fromEnum GtkAnchorN = 1
  fromEnum GtkAnchorNw = 2
  fromEnum GtkAnchorNe = 3
  fromEnum GtkAnchorS = 4
  fromEnum GtkAnchorSw = 5
  fromEnum GtkAnchorSe = 6
  fromEnum GtkAnchorW = 7
  fromEnum GtkAnchorE = 8

  toEnum 0 = GtkAnchorCenter
  toEnum 1 = GtkAnchorNorth
  toEnum 2 = GtkAnchorNorthWest
  toEnum 3 = GtkAnchorNorthEast
  toEnum 4 = GtkAnchorSouth
  toEnum 5 = GtkAnchorSouthWest
  toEnum 6 = GtkAnchorSouthEast
  toEnum 7 = GtkAnchorWest
  toEnum 8 = GtkAnchorEast
  toEnum 1 = GtkAnchorN
  toEnum 2 = GtkAnchorNw
  toEnum 3 = GtkAnchorNe
  toEnum 4 = GtkAnchorS
  toEnum 5 = GtkAnchorSw
  toEnum 6 = GtkAnchorSe
  toEnum 7 = GtkAnchorW
  toEnum 8 = GtkAnchorE
  toEnum unmatched = error ("GtkAnchorType.toEnum: Cannot match " ++ show unmatched)

Note in particular that the toEnum has overlaps. It's also obviously not the case in Haskell that GtkAnchorE = GtkAnchorEast. Perhaps we should ignore aliases, that is pretend that the enum values GTK_ANCHOR_E etc are simply not present.