haskell - 根据数据的特定部分派(dispatch)生实例

Question

我有以下代码

type Position = (Int,Int)

data CheckError where
    LError :: Position -> LexerError  -> CheckError
    PError :: Position -> ParseError  -> CheckError
    SError :: Position -> StaticError -> CheckError

我需要它来实例 Ord，但只需比较每个构造函数中的 Position 字段即可。

我让它与这个一起工作:

data CheckError where
    LError :: Position -> LexerError  -> CheckError
    PError :: Position -> ParseError  -> CheckError
    SError :: Position -> StaticError -> CheckError
    deriving (Show)

instance Eq CheckError where
    (LError l _) == (LError r _) = l == r
    (LError l _) == (PError r _) = l == r
    (LError l _) == (SError r _) = l == r
    (PError l _) == (LError r _) = l == r
    (PError l _) == (PError r _) = l == r
    (PError l _) == (SError r _) = l == r
    (SError l _) == (LError r _) = l == r
    (SError l _) == (PError r _) = l == r
    (SError l _) == (SError r _) = l == r

instance Ord CheckError where
    compare (LError l _) (LError r _) = l `compare` r
    compare (LError l _) (PError r _) = l `compare` r
    compare (LError l _) (SError r _) = l `compare` r
    compare (PError l _) (LError r _) = l `compare` r
    compare (PError l _) (PError r _) = l `compare` r
    compare (PError l _) (SError r _) = l `compare` r
    compare (SError l _) (LError r _) = l `compare` r
    compare (SError l _) (PError r _) = l `compare` r
    compare (SError l _) (SError r _) = l `compare` r

但是我发现这段代码太重复了。有没有办法告诉 Haskell 直接与 Position 字段进行比较？

Answer 1

也许是这样？

 instance Eq CheckError where
    a == b = pos a == pos b 
              where 
                pos (LError p _) = p
                pos (PError p _) = p
                pos (SError p _) = p