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[英]Haskell Couldn't match expected type `Integer' with actual type `[Integer]'
[英]Couldn't match expected type `a` with actual type `Integer`
我有一个treeBuild
函数没有得到编译,因为where子句中的签名:
unfold :: (a -> Maybe (a,b,a)) -> a -> BinaryTree b
unfold f x = case f x of Nothing -> Leaf
Just (s,t,u) -> Node (unfold f s) t (unfold f u)
treeBuild :: Integer -> BinaryTree Integer
treeBuild n = unfold f 0
where f :: a -> Maybe (a,b,a)
f x
| x == n = Nothing
| otherwise = Just (x+1, x, x+1)
而且我有以下编译器错误:
* Couldn't match expected type `a' with actual type `Integer'
`a' is a rigid type variable bound by
the type signature for:
f :: forall a b. a -> Maybe (a, b, a)
at D:\haskell\chapter12\src\Small.hs:85:16
* In the second argument of `(==)', namely `n'
In the expression: x == n
In a stmt of a pattern guard for
an equation for `f':
x == n
* Relevant bindings include
x :: a (bound at D:\haskell\chapter12\src\Small.hs:86:13)
f :: a -> Maybe (a, b, a)
(bound at D:\haskell\chapter12\src\Small.hs:86:11)
f
签名有什么问题?
在您的程序中,您将编写:
treeBuild :: Integer -> BinaryTree Integer
treeBuild n = unfold f 0
where f :: a -> Maybe (a,b,a)
f x
| x == n = Nothing
| otherwise = Just (x+1, x, x+1)
因此,这意味着您要检查Integer
和a
之间的相等性。 但是(==)
具有类型签名: (==) :: Eq a => a -> a -> Bool
。 因此,这意味着在Haskell中,两个操作数应具有相同的类型 。
因此,您有两个选择:(1)指定f
函数,或者(2)概括treeBuild
函数。
f
功能 treeBuild :: Integer -> BinaryTree Integer
treeBuild n = unfold f 0
where f :: Integer -> Maybe (Integer,Integer,Integer)
f x
| x == n = Nothing
| otherwise = Just (x+1, x, x+1)
在这里,我们简单地使f
为函数f :: Integer -> Maybe (Integer,Integer,Integer)
。
treeBuild
函数 我们可以-并建议这样做-泛化treeBuild
函数(并稍微泛化f
函数):
treeBuild :: (Num a, Eq a) => a -> BinaryTree a
treeBuild n = unfold f 0
where f x
| x == n = Nothing
| otherwise = Just (x+1, x, x+1)
然后f
将具有类型f :: (Num a, Eq a) => a -> Maybe ( a,a,a )
。
从现在开始,我们可以为数字类型的任何类型构建树并支持相等性。
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