haskell - 记录语法不合法。 haskell中的二叉树

Question

我用记录语法的方式定义了树类型。

data Tree a = Null | Node a {lTree:: Tree a, rTree :: Tree a}

但是，当我加载它时，它显示错误:

• Record syntax is illegal here: {lTree :: Tree a, rTree :: Tree a}
    • In the type ‘{lTree :: Tree a, rTree :: Tree a}’
      In the definition of data constructor ‘Node’
      In the data declaration for ‘Tree’
  |
2 | data Tree a = Null | Node a {lTree:: Tree a, rTree :: Tree a}
  |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Answer 1

a 也应该是记录语法的一部分，例如:

data Tree a = Null | Node {<b>valTree :: a</b>, lTree:: Tree a, rTree :: Tree a}

您不能将记录语法与 Haskell grammar in the Haskell '98 report 指定的位置语法“混合” :

constr  -> con [!] atype1 ... [!] atypek  (arity con = k, k>=0)
        |  (btype | ! atype) conop (btype | ! atype)  (infix conop)
        |  con { fielddecl1 , ... , fielddecln }  (n>=0)