Custom deriving(Read,Show) for enum type

Question

Let's say I have this enumeration type:

data TVShow = BobsBurgers | MrRobot | BatmanTAS

and I want to define instances for Read and Show with the following behavior:

show BobsBurgers = "Bob's Burgers"
show MrRobot = "Mr. Robot"
show BatmanTAS = "Batman: The Animated Series"

read "Bob's Burgers" = BobsBurgers
read "Mr. Robot" = MrRobot
read "Batman: The Animated Series" = BatmanTAS

There is lots of repetition in these definitions, and so I'd like to associate each type constructor with a string and then generate Show and Read automatically from those associations. Is such a thing possible?

A side note: best practice is not to abuse `Show` and `Read` this way. Have separate pretty-printing/parsing classes if you must have a class; but leave `Show` and `Read` for emitting (and parsing) valid Haskell code representing the value in question. — Daniel Wagner, Jun 13 '15 at 07:10

Daniel Wagner · Accepted Answer · 2019-02-15T21:28:11.740

The paper Invertible Syntax Descriptions: Unifying Parsing and Pretty Printing describes one particularly idiomatic solution. Your example looks like this, using the invertible-syntax package based on that paper:

import Prelude hiding (Applicative(..), print)
import Data.Maybe (fromJust)
import Text.Syntax
import Text.Syntax.Parser.Naive
import Text.Syntax.Printer.Naive

data TVShow = BobsBurgers | MrRobot | BatmanTAS deriving (Eq, Ord)

tvShow :: Syntax f => f TVShow
tvShow =  pure BobsBurgers <* text "Bob's Burgers"
      <|> pure MrRobot     <* text "Mr. Robot"
      <|> pure BatmanTAS   <* text "Batman: The Animated Series"

runParser (Parser p) = p
instance Read TVShow where readsPrec _ = runParser tvShow
instance Show TVShow where show = fromJust . print tvShow

This is designed to be extensible to types more exciting than simple enumerations, as well.

`invertible-syntax` worked for me, thanks! I tried some similar packages as well -- `partial-isomorphisms` is undocumented, `roundtrip` errors with newer haskell/base, `aeson-roundtrip` depends on that, `JsonGrammar` focuses on json objects, `Boomerang` seems more complex... great to have one that actually works! — Kiara Grouwstra, May 21 '18 at 21:52

score 1 · Answer 2 · answered Jun 13 '15 at 04:50

Aha! I found some pre-existing code written by Simon Nicholls. This template haskell can be used to achieve what I wanted:

genData :: Name -> [Name] -> DecQ
genData name keys = dataD (cxt []) name [] cons [''Eq, ''Enum, ''Bounded]
  where cons = map (\n -> normalC n []) keys

genShow :: Name -> [(Name, String)] -> DecQ
genShow name pairs =
  instanceD (cxt [])
    (appT (conT ''Show) (conT name))
    [funD (mkName "show") $ map genClause pairs]
  where
    genClause (k, v) = clause [(conP k [])] (normalB [|v|]) []

mkEnum :: String -> [(String, String)] -> Q [Dec]
mkEnum name' pairs' =
  do
    ddec <- genData name (map fst pairs)
    sdec <- genShow name pairs
    rdec <- [d|instance Read $(conT name) where
                 readsPrec _ value =
                   case Map.lookup value m of
                     Just val -> [(val, [])]
                     Nothing  -> []
                   where
                     m = Map.fromList $ map (show &&& id) [minBound..maxBound]|]
    return $ ddec : sdec : rdec
  where name  = mkName name'
        pairs = map (\(k, v) -> (mkName k, v)) pairs'

Usage:

$(mkEnum "TVShow"
  [ ("BobsBurgers", "Bob's Burgers")
  , ("MrRobot", "Mr. Robot")
  , ("BatmanTAS", "Batman: The Animated Series")
  ])

(The QuasiQuotes weren't working, so I'll have to investigate that)

Please, don't use TH for problems that are more easily solved without it. — augustss, Jun 13 '15 at 08:45

score 1 · Answer 3 · answered Sep 22 '17 at 10:19

I came to this:

data FeedbackType
  = Abuse
  | AuthFailure
  | Fraud
  | NotSpam
  | Virus
  | Other
  deriving (Eq)

instance Show FeedbackType where
  show Abuse = "abuse"
  show AuthFailure = "auth-failure"
  show Fraud = "fraud"
  show NotSpam = "not-spam"
  show Virus = "virus"
  show Other = "other"

instance Read FeedbackType where
  readsPrec _ s
    | s == show Abuse = [(Abuse, "")]
    | s == show AuthFailure = [(AuthFailure, "")]
    | s == show Fraud = [(Fraud, "")]
    | s == show NotSpam = [(NotSpam, "")]
    | s == show Virus = [(Virus, "")]
    | s == show Other = [(Other, "")]
    | otherwise = []

Custom deriving(Read,Show) for enum type

3 Answers3