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

19 May 2015

      On Tue, May 19, 2015 at 12:03 PM, Anatoly Zaretsky
 wrote:
...
data Codes = A0100A | A0500A deriving Read
codeExists code =
  case reads code of
    [(_, "")] -> True
    _ -> False
Or with -XPatternGuards:
codeExists code | [(_, "")] <- reads code = True
                | otherwise = False
Oops, should have checked the types before posting:

codeExists code =
  case reads code :: [(Codes, String)] of
    [(_, "")] -> True
    _ -> False

Otherwise the compiler could not guess the right Read instance.