CODEWITHSUNDEEP
haskelltypeclass
FrankBr
FrankBr

Reputation: 896

Haskell ordering function

What's wrong with this? I tried to write an ordering function defined as this:

let orderFunction:: Num b => (a, [b]) -> (a, [b]) -> Ordering; 
orderFunction a1 a2 = if sum $ snd a1 > sum $ snd a2 then GT else LT

but I get an error:

Could not deduce (Ord b) arising from a use of `>'
    from the context (Num b)
      bound by the type signature for
                 orderFunction :: Num b => (a, [b]) -> (a, [b]) -> Ordering
      at <interactive>:110:21-61
    Possible fix:
      add (Ord b) to the context of
        the type signature for
          orderFunction :: Num b => (a, [b]) -> (a, [b]) -> Ordering
    In the expression: sum (snd a1) > sum (snd a2)
    In the expression: if sum (snd a1) > sum (snd a2) then GT else LT
    In an equation for `orderFunction':
        orderFunction a1 a2
          = if sum (snd a1) > sum (snd a2) then GT else LT

Is there a more Haskell oriented way to write the function?

Thanks, FB

Upvotes: 1

Views: 901

Answers (1)

FrankBr
FrankBr

Reputation: 896

As pointed by @ymonad, Num typeclass does not imply that ordering feature is supported. So I added another constraint to function definition as follows:

let orderFunction::(Ord b, Num b) => (a, [b]) -> (a, [b]) -> Ordering; 
orderFunction a1 a2 = if sum (snd a1) > sum (snd a2) then GT else LT

Upvotes: 3

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