fun with forall

gistfile1.hs

{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ImpredicativeTypes #-}
module Forall where
import Control.Applicative
import Data.Aeson
import Control.Monad

type Foo = forall a. (FromJSON a) => a -> String

data Bar = Bar Int
data Baz = Baz Int Int
instance FromJSON Baz where
  parseJSON (Object v) = Baz <$> v .: "baz1"
                             <*> v .: "baz2"
  parseJSON _ = mzero
  
instance FromJSON Bar where
  parseJSON (Object v) = Bar <$> v .: "bar"
  parseJSON _ = mzero
                
showBaz_foo o = case fromJSON o of
  Error s -> s
  Success (Baz a b) ->  "Baz " ++ show a ++ "," ++ show b
  
showBar_foo o = case fromJSON o of
  Error s -> s
  Success (Bar a) ->  "Bar " ++ show a

fooList = [showBaz_foo,showBar_foo]
objects = [object ["baz1" .= Number 1, "baz2" .= Number 2],
           object ["bar" .= Number 3]]
          
-- res works fine
res = zipWith (\a b -> a b) fooList objects          

-- go fails miserably
go :: [(String, Foo)] -> Value -> String -> String
go table v key = case lookup key table of
  Nothing -> "not there"
  Just x -> case fromJSON v of
    Error s -> "broken: " ++ s
    Success thing -> x thing

*Forall> :r
[1 of 1] Compiling Forall ( forall.hs, interpreted )

forall.hs:43:22:
Couldn't match expected type t0 -> t1' with actual typeforall a. FromJSON a => a -> String'
The function x' is applied to one argument, but its typeFoo' has none
In the expression: x thing
In a case alternative: Success thing -> x thing
Failed, modules loaded: none.