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，只需比较每个构造函数中的Position字段。

我得到了它的工作：

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

但我觉得这段代码太重复了。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

Answer 2

我建议使用来自on的{{1}}组合器稍微清洁一点：

Data.Function

其中import Data.Function instance Eq CheckError where (==) = (==) `on` pos instance Ord CheckError where compare = compare `on` pos被定义为简单函数或记录字段（由@Xavier建议）。这可以简单地解读为“pos的比较是对他们立场的比较”。

根据数据的特定部分派生实例

2 个答案: