目前我正在尝试学习 Haskell,但我偶然发现了一个我不明白的错误:
* Occurs check: cannot construct the infinite type: a ~ [a]
Expected type: [a]
Actual type: [[a]]
* In the expression: (addL x acc [])
In the first argument of `foldl', namely
`(\ x acc -> (addL x acc []))'
至于我实际尝试做的是,我试图转置一个矩阵(下面提供了代码)。奇怪的是,如果我在 Elm 中运行代码(稍作调整),它就可以正常工作。我需要一些帮助,因为我不明白我做错了什么。
榆木代码:
trans matrix =
List.foldl (\x acc -> addL x acc []) [] matrix
addL x matrix solution =
case x of
[] -> solution
h::t -> case matrix of
[] -> addL t matrix (solution++[[h]])
h2::t2 -> addL t t2 (solution++[h2++[h]])
haskell 代码:
trans matrix =
foldl (\x acc -> (addL x acc [])) [] matrix
addL x matrix solution =
case x of
[] -> solution
h:t -> case matrix of
[] -> (addL t matrix (solution++[[h]]))
h2:t2 -> (addL t t2 (solution++[h2++[h]]))
最佳答案
不同之处在于 foldl
函数的语义。在 Elm 中, foldl
[elm-doc] 函数的签名是:
foldl : (a -> b -> b) -> b -> List a -> b
而在 Haskell 中,
foldl
[haskell-doc] 的签名是:foldl :: (b -> a -> b) -> b -> [a] -> b
所以在 Haskell 中,累加器是第一个参数,第二个是列表的一个元素。在榆树中,情况正好相反。所以它可能应该与:
trans matrix =
foldl (\acc x -> (addL x acc [])) [] matrix
关于haskell - 代码在 Elm 中编译,但在 Haskell 中不编译,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/50856706/