New subject: How to manage typeclass hierarchies and instances?

21 Feb 2015

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-- in Semigroup.hs
class Semigroup a where
  (|.|) :: a -> a -> a
instance Semigroup Integer where
  (|.|) = (+)
-- In Monoid.hs
class (Semigroup a) => Monoid a where
  identity :: a
instance Monoid Integer where
  identity = 0
-- In Group.hs
class (Monoid a) => Group a where
  inverse :: a -> a
instance Group Integer where
  (|.|)    = (+)
  identity = 0
  inverse  = (-)
In Group.hs I'm trying to create an (additive) group instance for
Integer values but GHC complains that (|.|) and identity are not
"visible" typeclass methods.
You only get one instance per type, so the Semigroup/Monoid instances
for Integer are "set in stone." When you "import Semigroup" and "import
Monoid", those instances come into scope. So in Group.hs, '|.|' and
'identity' are already defined for Integer. All you need is,
instance Group Integer where
    inverse = negate
Thanks for that insight.  Ignoring the instance issue for a minute,
suppose I wanted to define a typeclass Additive where I'd like to set
default implementations for the methods:

class (Num a, Group a) => Additive a where
  (|.|) = (+)

  -- etc...

In this situation GHC says (|.|) is not a visible method. If I add a
type declaration for (|.|) am I effectively "shadowing" the method
from Semigroup with a new method rather than supplying a default
implementation?

Thanks,

Stu

Re: [Haskell-beginners] How to manage typeclass hierarchies and instances?

Stuart Hungerford

Michael Orlitzky

Brandon Allbery

PATRICK BROWNE

PATRICK BROWNE

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