Haskell, Instance, Type Constraints

Question

I created in Haskell a new class Eqa

class Eqa a where

   (=~) :: a -> a -> Bool

   (/~) :: a -> a -> Bool

and want to define (=~) same as (==) from the Prelude. So I tried

instance Eqa Int where            
   x=~y = x==y

   x/~y = x/=y

but this only works for Int (of course). How do I have to change my code that this works with all numerical types?

How do I have to change my code that this works with all numerical types? What does "this" refer to in your sentence, exactly? — jub0bs
– jub0bs, Commented Jun 21, 2015 at 10:29
With my defined instance I am only able to compare types of Int but I want to compare all numerical Types i.e. float, Integer, ... — Chango
– Chango, Commented Jun 21, 2015 at 12:02
This isn't something you can really do. The Num typeclass doesn't provide (==). You can convince GHC to accept a definition of instance Num a => Eqa a with the right language pragmas but it's nonsensical and in practice you won't be able to use it. — Rein Henrichs
– Rein Henrichs, Commented Jun 21, 2015 at 17:53

MathematicalOrchid · Accepted Answer · 2015-06-22 12:02:07Z

2

Why not just write

(=~) :: Num a => a -> a -> Bool
x=~y = x==y

If you don't actually need the code to be different for different types, why do you need a class at all?

answered Jun 22, 2015 at 12:02

MathematicalOrchid

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Guiraldelli · Accepted Answer · 2015-06-21 10:51:29Z

-1

All you have to do is to bind a to Num a or

instance Num a => Eqa a where            
    x=~y = x==y
    x/~y = x/=y

For more information, take a look at Numeric Types subsection of Real World Haskell to understand which class is related to each numerical types.

answered Jun 21, 2015 at 10:51

Guiraldelli

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3 Comments

Antal Spector-Zabusky Over a year ago

This isn't a great solution. First of all, it doesn't work as written: it requires FlexibleInstances and UndecidableInstances; plus, in modern GHCs, Eq is no longer a superclass of Num, so you need an Eq a constraint too. More importantly, though, it makes defining other instances of Eqa rather fraught: if we add instance Eqa () where { _ =~ _ = True ; _ /~ _ = False }, then () =~ () gives us an "Overlapping instances" error, because () could have a Num instance!

jub0bs Over a year ago

@AntalS-Z Do you happen to know when (i.e., for which version of GHC) Eq ceased to be a superclass of Num?

Antal Spector-Zabusky Over a year ago

@Jubobs: Since GHC 7.4.1. Here are the GHC 7.4.1 release notes, which says "The Num class no longer has Eq or Show superclasses"; compare the GHC 7.2.1 release notes, which say nothing of the sort. This is from Ben James's answer to exactly this question.

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