Propoal: operators similar to .= for Maybe or Functor/Applicative types
#516 aberto em 3 de mar. de 2017
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Description
Oftentimes when manually writing ToJSON instances, we have fields with Maybe types and we want to just hide the field if its value is Nothing. I usually resort to something like this:
data A =
A
{ x :: Int
, y :: Maybe Int
}
instance ToJSON A where
toJSON (A x y) =
object $ catMaybes
[ Just ("x" .= x)
, ("y" .=) <$> y
]
In my opinion, that is not very elegant. I propose adding the following two operators to make this nicer:
(.=!) :: (Applicative f, ToJSON v, KeyValue kv) => Text -> v -> f kv
k .=! v = pure $ k .= v
(.=?) :: (Functor f, ToJSON v, KeyValue kv) => Text -> f v -> f kv
k .=? v = (k .=) <$> v
I envision these operators becoming the duals of .:! and .:?, which are very useful when parsing. Using them would look like this:
instance ToJSON A where
toJSON (A x y) =
object $ catMaybes
[ "x" .=! x
, "y" .=? y
]
I'm still undecided if the general Functor/Applicative constraints are that useful, or if we should just specialize these operators to Maybe:
(.=!) :: (ToJSON v, KeyValue kv) => Text -> v -> Maybe kv
k .=! v = Just $ k .= v
(.=?) :: (ToJSON v, KeyValue kv) => Text -> Maybe v -> Maybe kv
k .=? v = (k .=) <$> v
Let me know what you think. I'm still a little hesitant on this proposal, mainly because I want to avoid needless operator soup. I thought it was a decent idea though, so I decided to make open this issue to at least open up discussion.