haskell - 如何将文件拆分为 Haskell 中的模块？

Question

我对模块的语法有疑问。基本上我试图将我的代码分成两个单独的文件，一个用于我正在创建的对象 (AST)，另一个用于我的所有函数。

    --main.hs
    data AST = Add (AST) (AST)|
               Sub  (AST) (AST)|
               Mult (AST) (AST)|
               Ident Char|
               Num Int
               deriving Show

    aSTLeft (Num    l   ) = (Num l)
    aSTLeft (Ident  l   ) = (Ident l)
    aSTLeft (Add  l _   ) = l
    aSTLeft (Sub  l _   ) = l
    aSTLeft (Mult l _   ) = l
    aSTRight    (Num    r   ) = (Num r)
    aSTRight    (Ident  r   ) = (Ident r)
    aSTRight    (Add  _ r   ) = r
    aSTRight    (Sub  _ r   ) = r
    aSTRight    (Mult _ r   ) = r

    isNum (Num x) = True
    isNum (Ident x) = False
    isNum (Add  (x)(y)) = False
    isNum (Sub  (x)(y)) = False
    isNum (Mult (x)(y)) = False

    --a lot more functions here

这工作正常，但是当我尝试将 AST 数据类型拆分为一个单独的文件时，

    --ASTADT.hs
    module ASTADT (AST,aSTLeft,aSTRight) where
    data AST =    Add (AST) (AST)|
                  Sub  (AST) (AST)|
                  Mult (AST) (AST)|
                  Ident Char|
                  Num Int
                  deriving Show

    aSTLeft     (Num    l   ) = (Num l)
    aSTLeft     (Ident  l   ) = (Ident l)
    aSTLeft     (Add  l _   ) = l
    aSTLeft     (Sub  l _   ) = l
    aSTLeft     (Mult l _   ) = l
    aSTRight        (Num    r   ) = (Num r)
    aSTRight        (Ident  r   ) = (Ident r)
    aSTRight        (Add  _ r   ) = r
    aSTRight        (Sub  _ r   ) = r
    aSTRight        (Mult _ r   ) = r

和

    --main.hs
    import ASTADT
    isNum (Num x) = True
    isNum (Ident x) = False
    isNum (Add  (x)(y)) = False
    isNum (Sub  (x)(y)) = False
    isNum (Mult (x)(y)) = False
    --a lot more functions here

加载 main.hs 我收到错误信息

    Undefined data constructor "Num"

我很确定这只是我的模块语法的问题，但我已经完全被困了几个小时。我正在与 Hugs 合作。谢谢

Answer 1

模块 ASTADT 仅导出 AST 数据类型，但不导出其构造函数。如果您想对模块用户隐藏数据类型的实现细节，这将非常有用。

在您的情况下，您不希望这样，因此要导出构造函数，您可以在数据类型之后的 module ... where 行中指定它们，例如 AST (Num,标识)。要导出数据类型的所有构造函数，您可以使用 (..) 而不是显式命名它们:

module ASTADT (AST (..), aSTLeft, aSTRight) where
...