[Haskell-cafe] Automatically generate sum type members -> [String]

[Cody Goodman]

Can I use either template Haskell, GHC Generics, or something else to
get the list of Constructors in Codes? Then I could do `map show
codeSumTypeMembers` and get `["A0100A","A0500A"]`?

Here's an example of solving this problem manually:

data Codes = A0100A | A0500A deriving Show

codeExists "A0100A" = True
codeExists "A0500A" = True
codeExists _ = False


main = print $ codeExists xmlElementName
       where xmlElementName = "A0500A"

-- λ> main
-- True