类型的实例是什么？

Question

Monoidmempty函数的类型签名是：

*Exercises Data.Monoid> :t mempty
mempty :: Monoid a => a

mempty :: Monoid b => a -> b

mempty :: Monoid a => a
mempty :: a -> b

newtype Combine a b = 
  Combine { unCombine :: a -> b }

instance (Semigroup b) 
  => Semigroup (Combine a b) where 
  Combine {unCombine=f} <> Combine {unCombine=g} = Combine (f <> g)

instance (Semigroup b, Monoid b)
  => Monoid (Combine a b) where
  mempty = Combine mempty
  mappend = (<>)

mempty :: Monoid a => a

mempty :: Combine a b

Answer 1

为什么不问问GHCI？您有mempty=combined mempty，因此：

> :t Combine mempty
Combine mempty :: Monoid b => Combine a b

因此，combine的mempty的类型必须至少与此相同。