代码在Elm中编译,但在Haskell中不编译

Question

目前我正在尝试学习Haskell,但我偶然发现了一个我不理解的错误:

* 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 []))'

至于我实际上想做什么,是我试图转置矩阵(下面提供的代码).奇怪的是,如果我在Elm中运行代码(稍微调整一下),它就可以完美地运行.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]]))

Answer 1

所不同的是语义的foldl功能.在Elm中,foldl[elm-doc]功能的签名是:

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,第二个是列表的元素.在榆树中它恰恰相反.所以它可能适用于:

trans matrix =
  foldl (\acc x -> (addL x acc [])) [] matrix