haskell - (Re)-defining (==) for class Eq -

haskell - (Re)-defining (==) for class Eq -

in next example:

data colourname = white | gray | grayness | black | bluish -- ... -- hundreds more of colours -- ... | lastcolor deriving (read, show, eq)

i'd redefine (==) grey , gray evaluate equal.

obviously, 1 way not include eq in deriving, however, i'd have define

(==) :: colourname (==) white white = true (==) grayness gray = true (==) gray grey = true (==) grayness gray = true (==) gray grayness = true (==) black black = true -- frickin' log of other colors, hundreds of lines of typing (==) lastcolor lastcolor = true (==) b = false

which nil plan do.

i can't do

instance eq colourname (==) :: colourname -> colourname -> bool (==) grayness gray = true (==) gray grayness = true (==) b = (a == b)

because leads infinite recursion, underdefined.

is there way out?

(no, don't want utilize data colour = colour string or similar. want valid colours represented enumeration, such providing automatic validation, want allow spelling variation end users of module!)

you can utilize derived enum instance :

data colourname = grayness | gray | ... deriving (read, show, enum) instance eq colourname grayness == gray = true gray == grayness = true == b = fromenum == fromenum b

edit: can utilize patternsynonyms ghc 7.8+. works smart constructor, can used in pattern matches.

pattern grayness = gray

haskell