On 10/03/2011 10:42 PM, Magicloud Magiclouds wrote:
Hi,
I have a function:
post :: (ToJson p, FromJson q) => String -> String -> String ->
Map.Map String p -> IO q
Now I'd like to call it like:
r<- post site token "user.addMedia" (Map.fromList [ ("users", users :: ToJson)
, ("medias", medias
:: ToJson) ])
So I got the problem. If I used things like "users :: ToJson", then
class used as a type error occurred. But if I did not use them, since
users and medias were actually different types, then fromList failed,
required the type of medias the same with users.
How to resolve the conflict?
If 'users' and 'medias' are actually of a general type (like "for all a
with ToJson a, users describes a value of type a"), use Jesse's
suggestion. Otherwise ("there is an a with ToJson a such that users
describes a value of type a"), you might want to use existentials:
{-# LANGUAGE ExistentialQuantification #-}
data SomeToJson = forall a. (ToJson a) => SomeToJson a
instance ToJson SomeToJson where
toJson (SomeToJson x) = toJson x -- I guess your class looks like
this?
And then:
r <- post site token "user.addMedia" $ Map.fromList
[("users", SomeToJson users), ("medias", SomeToJson medias)]
As a last remark, I needed this pattern exactly once, namely for dealing
with rank 2 types in rendering functions using takusen. I can conclude
that requiring it is often an indicator for a major design flaw in your
program. In this case:
Why not:
-- assuming that there is an
-- instance ToJson Json where toJson = id
r <- post site token "user.addMedia" $ Map.fromList
[("users", toJson users), ("medias", toJson medias)]
Cheers!
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