Class Haskell,类中的函数声明
我正在尝试为一个变形类树创建一些实例,但我不明白 看,我的代码是:Class Haskell,类中的函数声明,class,haskell,polymorphism,instance,Class,Haskell,Polymorphism,Instance,我正在尝试为一个变形类树创建一些实例,但我不明白 看,我的代码是: data BTree a = BLeaf a | BBranch a (BTree a) (BTree a) deriving(Show) data TTree a = TLeaf a | TBranch a (TTree a) (TTree a) (TTree a) deriving(Show) class Tree a where getName :: a -> a -- How should i decl
data BTree a = BLeaf a | BBranch a (BTree a) (BTree a) deriving(Show)
data TTree a = TLeaf a | TBranch a (TTree a) (TTree a) (TTree a) deriving(Show)
class Tree a where
getName :: a -> a -- How should i declare this function?
instance Tree (BTree a) where
getName (BLeaf name) = name
getName (BBranch name lhs rhs) = name
instance Tree (TTree a) where
getName (TLeaf name) = name
getName (TBranch name lhs mhs rhs) = name
test1 = getName (BLeaf 1)
test2 = getName (TLeaf 1)
GHCI说:
Couldn't match expected type `a' with actual type `BTree a'
那么,我应该如何声明getName函数呢?对类型构造函数使用类型类参数
t
(比如BTree
或TTree
,与BTree a
和TTree a
不同):
如果需要根据元素类型a
改变实例,则需要多参数类:
{-# LANGUAGE MultiParamTypeClasses #-}
class Tree t a where
getName :: t a -> a
instance Tree BTree Int where
getName (BLeaf name) = name+1
getName (BBranch name lhs rhs) = name*2
instance Tree BTree Char where
getName (BLeaf name) = name
getName (BBranch name lhs rhs) = name
也许你不需要把它说得这么笼统
{-# LANGUAGE MultiParamTypeClasses #-}
class Tree t a where
getName :: t a -> a
instance Tree BTree Int where
getName (BLeaf name) = name+1
getName (BBranch name lhs rhs) = name*2
instance Tree BTree Char where
getName (BLeaf name) = name
getName (BBranch name lhs rhs) = name