Num does not inherit Show and Eq in new versions of GHC

[Petar Kovačević]

I got the error "Could not deduce (Show a) arising from a use of ‘show’ from the context: Num a" and found the solution here (I'm on GHC 8.2.1)

I was wondering - is this just a GHC thing or is it a new language standard? 

What other type changes have there been (i.e. what else is different from this graph)?

The main reason for these questions is that on a lot of places online I still find "class (Eq a, Show a) => Num a" (including the preview when searching "Num" on Hoogle) but, if the Stack Overflow comment is correct, Num has been independent of Show and Eq in GHC since 2012.

[Luka Hadžiegrić]

Yes, there were some changes in GHC 8.x when compared to GHC 7.x. I don't know if there is a practical graph somewhere but you can usually find out all of useful information about a certain type class, as well as what it inherits from by typing `:i <name_of_typeclass>` in your REPL.

For example, if you check for `Real` type class, you can see that it "inherits" from `Num` and `Ord`, and if you check a concrete type like `Int` than you can see it has instance of `Show`.

Prelude> :i Real
class (Num a, Ord a) => Real a where
  toRational :: a -> Rational
  {-# MINIMAL toRational #-}
        -- Defined in `GHC.Real'
instance Real Word -- Defined in `GHC.Real'
instance Real Integer -- Defined in `GHC.Real'
instance Real Int -- Defined in `GHC.Real'
instance Real Float -- Defined in `GHC.Float'
instance Real Double -- Defined in `GHC.Float'

Prelude> :i Int
data Int = GHC.Types.I# GHC.Prim.Int#   -- Defined in `GHC.Types'
instance Bounded Int -- Defined in `GHC.Enum'
instance Enum Int -- Defined in `GHC.Enum'
instance Eq Int -- Defined in `GHC.Classes'
instance Integral Int -- Defined in `GHC.Real'
instance Num Int -- Defined in `GHC.Num'
instance Ord Int -- Defined in `GHC.Classes'
instance Read Int -- Defined in `GHC.Read'
instance Real Int -- Defined in `GHC.Real'
instance Show Int -- Defined in `GHC.Show'

[Luka Hadžiegrić]

Although, graph you found is from Haskell 98, current standard is Haskell 2010 so that might be it.

On Saturday, October 14, 2017 at 9:13:59 PM UTC+2, Petar Kovačević wrote: