I've also wanted this feature in the past. I think the DefaultSignatures language extension is the closest thing. There's a recent answer to your stackoverflow question that explains how it works:
https://stackoverflow.com/questions/3213490/how-do-i-write-if-typeclass-a-then-a-is-also-an-instance-of-b-by-this-definit/44456791#44456791 On Fri, Jun 9, 2017 at 3:15 AM, Saurabh Nanda <saurabhna...@gmail.com> wrote: > Hi, > > At the outset I would like to mention that I do not completely understand > how type-class instance resolution works, and why it works that way. I have > tried reading up, but I don't completely understand WHY it works that way. > So, please feel free to ask me to RTFM if what I'm about to say doesn't make > any sense, although TFM doesn't make sense to me! > > From a pragmatic standpoint is there any way to express the following idea > in Haskell (or a future version of Haskell), **with minimal amount of > boilerplate** > > If a type has an instance of type-class `a`, then here's how you can get > an instance of type-class `b` > > Without the ability to express this idea, you end up with a **lot** of > type-class boilerplate, (even if you use GHC Generics) eg: > > data State = data State = Inactive | Incomplete | Deleted | Trial | > Unpaid | Paid > deriving (Eq, Show, Generic) > > instance AppEnum State where > toString = gEnumToString > fromString = gEnumFromString > > -- You are still forced to do the following for **every single type** > even though, conceptually, this can be driven by a rule > > instance ToJSON State where > toJSON s = toJSON $ toString x > > instance FromJSON State where > parseJSON (Aeson.String s) = fromString s > parseJSON _ = fail "unexpected type" > > instance FromFIeld State where > fromField _ mBS = case mBS of > Nothing -> fail "not expecting a NULL/Nothing" > (Just bs) -> pure $ fromString bs > > instance ToField State where > toField s = toField $ toString s > > I tried talking to people on IRC and also trawling StackOverflow [1] and my > take-away is essentially, that Haskell cannot express this idea. However, my > submission is, that it is a very useful idea to express and probably the > next version of the language should do something to make the lives of > programmers a little easier. > > [1] > https://stackoverflow.com/questions/3213490/how-do-i-write-if-typeclass-a-then-a-is-also-an-instance-of-b-by-this-definit > > -- Saurabh. > > > _______________________________________________ > Haskell-prime mailing list > Haskell-prime@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-prime > _______________________________________________ Haskell-prime mailing list Haskell-prime@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-prime
