I am testing out some of the ideas in this article.
I want to derive an instance of Eq for the type Term:
{-# LANGUAGE DeriveFunctor #-}
data Tree a = Branch Int [a] | Leaf Int deriving (Eq, Functor, Show)
data Term f = Term (f (Term f)) deriving (Eq)
But get this error:
No instance for (Eq (f (Term f)))
arising from the first field of ‘Term’ (type ‘f (Term f)’)
Possible fix:
use a standalone 'deriving instance' declaration,
so you can specify the instance context yourself
When deriving the instance for (Eq (Term f))
I tried adding a standalone deriving instance:
{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE StandaloneDeriving #-}
data Tree a = Branch Int [a] | Leaf Int deriving (Eq, Functor, Show)
data Term f = Term (f (Term f))
deriving instance (Eq f) => Eq (Term f)
But get this error:
The first argument of ‘Term’ should have kind ‘* -> *’,
but ‘f’ has kind ‘*’
In the stand-alone deriving instance for ‘(Eq f) => Eq (Term f)’
Now I'm stuck. How do I show that f has a kind * -> *? And why do I need a standalone deriving instance for Term but not Tree? Both have type variables that are not necessarily going to be instances of Eq? (a and f)
There are two solutions:
Using some GHC extensions StandaloneDeriving, UndecidableInstances and some others you can write:
deriving instance (Eq (f (Term f))) => Eq (Term f)
This is what recursion-schemes does at the moment
--
Alternatively, you can use Eq1 from transformers or base-4.9.0.0
class Eq1 f where
liftEq :: (a -> b -> Bool) -> f a -> f b -> Bool
eq1 :: (Eq1 f, Eq a) -> f a -> f a -> Bool
eq1 = liftEq (==)
instance Eq1 f => Eq (Term f) where
Term a == Term b = eq1 a b