Question

there’s an error I come across all the time but can’t understand how to make it right. An example of code that gives me this error is:

class Someclass a where
    somefunc :: (Num b) => b -> a -> a

data Sometype = Somecons Int

instance Someclass Sometype where
    somefunc x (Somecons y) = Somecons (x+y)

The error message is:

Couldn’t match expected type ‘b’ against inferred type ‘Int’
‘b’ is a rigid type variable bound by
the type signature for ‘somefunc’ at error.hs:3:21
In the second argument of ‘(+)’, namely ‘y’
In the first argument of ‘Somecons’, namely ‘(x + y)’
In the expression: Somecons (x + y)

I understand that the error message is trying to tell me that I used a name of type Int where he was expecting something with type (Num b) => b. What I can’t understand is that Int fits in (Num b)=>b. Shouldn’t the compiler understand what I’m telling him (that for this specific instance b should be an integer? How can I make this fit?

Coment:
Of course in this specific example I could have made somefunc with type signature:

somefunc :: a -> a-> a 

but supose I wanted something like:

data Newtype = Newcons (Int, Int) 

instance Someclass Newtype where
    somefunc x (Newtype (y,z) ) = Newtype (y+x, z)

Things like that recurrently happens when I’m trying to do something in haskell.

Answer 1

Well, you can make the point clearer when thinking of the generics notation using universal quantification.

somefunc :: (Num b) => b -> a -> a

therefore means nothing but

somefunc :: forall a b . Num b => b -> a -> a

This means your class function must be defined for any numeric b.

The code

Data Sometype = Somecons Int

instance Someclass Sometype where
    somefunc x (Somecons y) = Somecons (x+y)

forces b to have one concrete type – Int, which doesn’t conform with the requirement to work for any numeric type.

You might want to have something like this

class Num b => SomeClass a b where
    somefunc :: b -> a -> a

instance Someclass Somecons Int where
    -- ...