CODEWITHSUNDEEP
haskelltypeclassexistential-type
highfly22
highfly22

Reputation: 1871

Existentially quantified types Could not deduce in the typeclass context

{-# LANGUAGE ExistentialQuantification, DeriveDataTypeable #-}
import Data.Typeable;

data EnumBox = forall s. (Enum s, Show s) => EB s
           deriving Typeable

instance Show EnumBox where
  show (EB s) = "EB " ++ show s

This works. But if I want to add a instance of Class Enum for EnumBox likes:

instance Enum EnumBox where
  succ (EB s) = succ s

It fails with the message:

Could not deduce (s ~ EnumBox)
from the context (Enum s, Show s)
  bound by a pattern with constructor
             EB :: forall s. (Enum s, Show s) => s -> EnumBox,
           in an equation for `succ'
  at typeclass.hs:11:9-12
  `s' is a rigid type variable bound by
      a pattern with constructor
        EB :: forall s. (Enum s, Show s) => s -> EnumBox,
      in an equation for `succ'
      at typeclass.hs:11:9
In the first argument of `succ', namely `s'
In the expression: succ s
In an equation for `succ': succ (EB s) = succ s

Why the first show can be deduced but the second succ cannot?

Upvotes: 3

Views: 304

Answers (1)

daniel gratzer
daniel gratzer

Reputation: 53911

You're only problem is that succ has the type

succ :: Enum a => a -> a

So you need

succ (EB s) = EB . succ $ s

Just boxing it up again.

Also you'll probably want

instance Enum EnumBox where
    toEnum = EB
    fromEnum (EB i) = fromEnum i

As this is the minimum definition of completeness, since

succ = toEnum . succ . fromEnum

Upvotes: 2

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