重载类型类 function 时出现多个声明错误

Question

The following code is a code example for overloading the functions of the partial ordered typeclass (POrd) from a lecture slide.

When trying to run this code, I get the following multiple declarations errors for the function 'pcompare' and for the operators.

> class Eq a => POrd a where
>    pcompare :: a -> a -> Maybe Ordering
>    (~<), (~>), (~<=), (~>=) :: a -> a -> Bool
>
>       -- Minimal complete definition:
>       --      (~<)& (~>) | pcompare
> pcompare x y | x == y       = Just EQ
>              | x ~< y       = Just LT
>              | x ~> y       = Just GT
>              | otherwise    = Nothing
>
> x ~<= y = pcompare x y ==  Just LT || x == y
> x ~<  y = pcompare x y ==  Just LT
> x ~>= y = pcompare x y ==  Just GT || x == y
> x ~>  y = pcompare x y ==  Just GT

I get the following error messages:

lecture9.lhs:5:1: error:
    Multiple declarations of `pcompare'
    Declared at: lecture9.hs:2:4
                 lecture9.hs:5:1
  |
5 | pcompare x y | x == y       = Just EQ
  | ^^^^^^^^

lecture9.lhs:10:3: error:
    Multiple declarations of `~<='
    Declared at: lecture9.lhs:3:4
                 lecture9.lhs:10:3
   |
10 | x ~<= y = pcompare x y ==  Just LT || x == y
   |   ^^^

lecture9.lhs:11:3: error:
    Multiple declarations of `~<'
    Declared at: lecture9.lhs:3:4
                 lecture9.lhs:11:3
   |
11 | x ~<  y = pcompare x y ==  Just LT
   |   ^^

lecture9.lhs:12:3: error:
    Multiple declarations of `~>='
    Declared at: lecture9.lhs:3:4
                 lecture9.lhs:12:3
   |
12 | x ~>= y = pcompare x y ==  Just GT || x == y
   |   ^^^

lecture9.lhs:13:3: error:
    Multiple declarations of `~>'
    Declared at: lecture9.lhs:3:4
                 lecture9.lhs:13:3
   |
13 | x ~>  y = pcompare x y ==  Just GT
   |   ^^

I don't understand what causes the multiple declarations error in this example, as it looks very similar to the official PartialOrd typeclass implementation.

Answer 1

As the commenters on your question point out, the issue is one of indentation. For what it's worth, the > symbols are appropriate considering you're trying to compile a literate Haskell file (.lhs) rather than a regular Haskell file. (That said, in a regular Haskell file (.hs), they would cause parsing errors.)

In Haskell, "top-level" declarations are not indented at all. For instance, the definition of the POrd class is not indented. On the flip side, non -top-level declarations must be indented. The type declarations of pcompare and the other operators are part of the POrd class and, as such, are indented.

The definitions of pcompare and the other operators must also be indented, but they can technically go in two different places. They could be defined for particular instances of POrd , or they could be defined as defaults for the POrd class. Based on the definitions themselves, it seems these are intended as defaults.

Long story short, if you indent the definitions of pcompare and the other operators, you should be good to go. (Keep in mind that the > still needs to be at the start of the line, so add the extra spaces just after the > , as in > pcompare xy | x == y... .)