预计有一个类型,但是TYPENAME有点'* - >*'

Question

我遇到了一个我不明白的错误问题

代码:

mergHufffman::(String,Int) -> (String,Int) -> (String,Int)
mergHufffman x y = (fst x ++ fst y, snd x + snd y)


data HTree a = Leaf a | Branch (HTree a) (HTree a) deriving Show


treeHuff::[(String,Int)] -> HTree
treeHuff (x:[]) = Leaf x
treeHuff (x:y:[])
        | snd x < snd y = Branch Leaf x Leaf y
        | snd x > snd y = Branch Leaf y Leaf x
treeHuff (x:y:z:list)
        | snd x > snd merged = Branch (Leaf x) (treeHuff y:z:list)
        | otherwise = Branch (treeHuff y:z:[]) (treeHuff x:list)
        where merged = mergHufffman y z 

错误:

Expecting one more argument to ‘HTree’
    Expected a type, but ‘HTree’ has kind ‘* -> *’
    In the type signature for ‘treeHuff’:
      treeHuff :: [(String, Int)] -> HTree

如果可能的话,你可以解释错误,以及我做错的地方.

方案:

感谢Sepp2k和Carsten,我解决了这个问题.

mergHufffman::(String,Int) -> (String,Int) -> (String,Int)
mergHufffman x y =  (fst x ++ fst y, snd x + snd y)

data HTree a = Leaf a | Branch (HTree a) (HTree a) deriving Show

treeHuff::[(String,Int)] -> HTree (String,Int)
treeHuff (x:[]) = Leaf x
treeHuff (x:y:[])
        | snd x < snd y = Branch (Leaf x) (Leaf y)
        | snd x > snd y = Branch (Leaf y) (Leaf x)
treeHuff (x:y:z:list)
        | snd x > snd merged = Branch (Leaf x) (treeHuff $ sortFirst $ y:z:list)
        | otherwise = Branch (treeHuff $ y:z:[]) (treeHuff $ sortFirst $ x:list)
        where merged = mergHufffman y z 

sortFirst::[(String,Int)]->[(String,Int)]
sortFirst freq = reverse $ sortBy (comparing snd) freq

readHuffTree :: HTree (String,Int)-> String -> [(String, String)]
readHuffTree (Branch x y) code = f1 ++ f2
                          where 
                          f1 = readHuffTree x (code ++ "0")
                          f2 = readHuffTree y (code ++ "1")
readHuffTree (Leaf x) code = ((fst x, code):[])

Answer 1

在HTree您再给它一个参数之前,错误消息告诉您这不是类型.那是HTree String或HTree Int将是类型,但HTree本身不是.

另一种说法:当你说"我想要归还一棵树"时,哈斯克尔问"一棵树是什么？".