Function 组合，带有类型默认值的类型注释

Question

Let's say I have the following type:

data ImageSize = ImageSize {height :: Int, width :: Int}

I now want to convert that to a JSON array (for legacy API surface reasons):

instance ToJSON ImageSize where
  toJSON ImageSize{..} = Array $ fromList $ Number <$> map (fromFloatDigits . fromIntegral) [height, width]

This fails to compile with:

 error: [-Wtype-defaults, -Werror=type-defaults]
    • Defaulting the following constraints to type ‘Double’
        (RealFloat a0)
          arising from a use of ‘fromFloatDigits’
          at lib/Filler/Filler/Filler/Filler/Filler/ImageSize.hs:14:63-77
        (Num a0)
          arising from a use of ‘fromIntegral’
          at lib/Filler/Filler/Filler/Filler/Filler/ImageSize.hs:14:80-98
    • In the first argument of ‘map’, namely ‘fromFloatDigits’
      In the second argument of ‘(<$>)’, namely
        ‘map (fromFloatDigits . fromIntegral) [height, width]’
      In the second argument of ‘($)’, namely
        ‘Number
           <$> map (fromFloatDigits . fromIntegral) [height, width]’

This problem is (trivially) resolved by:

toJSON ImageSize{..} = Array $ fromList $ Number <$> map (fromFloatDigits) [(fromIntegral height) :: Double, (fromIntegral width)]

But that feels very verbose and ugly. Is there a way to attach the typecast into the composition? Something like (fromFloatDigits. :: Double. fromIntegral) , but actually functional?

Answer 1

Just use fromIntegral directly, no need to add an extra intermediate type.

toJSON ImageSize{..} = Array $ fromList $ Number <$> map fromIntegral [height, width]

But even simpler is to use toJSON ...

toJSON ImageSize{..} = toJSON [height, width]