Question

我正在尝试书中的代码示例＆＃34; The Craft of Functional Programming＆＃34;。

我想创建一个包含Int s的树，但我似乎最终在其中创建了一个带有整数的树（请参阅下面的GHCi中的执行）。

如何创建一个包含Int s的树？有没有办法在Haskell中编写Int文字？

*Chapter18> sumTree myTree

<interactive>:35:9:
    Couldn't match type `Integer' with `Int'
    Expected type: Tree Int
      Actual type: Tree Integer
    In the first argument of `sumTree', namely `myTree'
    In the expression: sumTree myTree
    In an equation for `it': it = sumTree myTree

以下是相应的代码：

-- A type of binary trees.
myTree = Node 2 (Nil) (Nil)

data Tree a = Nil | Node a (Tree a) (Tree a)

-- Summing a tree of integers

-- A direct solution:

sTree :: Tree Int -> Int

sTree Nil            = 0
sTree (Node n t1 t2) = n + sTree t1 + sTree t2

-- A monadic solution: first giving a value of type Id Int ...

sumTree :: Tree Int -> Id Int

sumTree Nil = return 0

--sumTree (Node n t1 t2)
--  = do num <- return n
--       s1  <- sumTree t1
--       s2  <- sumTree t2
--       return (num + s1 + s2)
sumTree (Node n t1 t2) =
    return n >>=(\num ->
      sumTree t1 >>= (\s1 ->
        sumTree t2 >>= (\s2 ->
          return (num + s1 + s2))))
-- ... then adapted to give an Int solution



sTree' :: Tree Int -> Int

sTree' = extract . sumTree

-- where the value is extracted from the Id monad thus:

extract :: Id a -> a
extract (Id x) = x

Answer 1

Monomorphism restriction再次罢工！因为myTree没有任何参数，所以编译器避免使其变为多态。但是数字文字是多态的（没有Int文字，只有整数Num文字！），所以编译器需要决定一些Num类型。好吧，如果你处理大量数字，Int可能是一个问题，所以选择Integer。

给予myTree明确的签名会阻止这种情况;要么使用

myTree :: Num a => Tree a

或者

myTree :: Tree Int

在Haskell中的Int literal

1 个答案: