[Haskell-cafe] HList, hOccurs and MonadReader

[Marc Weber]

Having the module given below I can't see why using 
	printAndRerun l1 
	printAndRerun2 l2
but not 
	printAndRerun l1 
	printAndRerun l2
?
They only differ in their name.

Can you point me in the right direction?

------------------------------------------------
{-# OPTIONS -fglasgow-exts #-}
{-# OPTIONS -fallow-undecidable-instances #-}
{-# OPTIONS -fallow-incoherent-instances #-}
{-# OPTIONS -fallow-overlapping-instances #-}
module HListTest.MonadReader where
import HList hiding (liftIO)
import Monad
import Control.Monad.Reader

printAndRerun = do v <- liftM hOccursFst ask
		    liftIO $ print (v :: Int )
		    v2 <- liftM hOccursFst ask
		    return (v2 :: String)

printAndRerun2 = do v <- liftM hOccursFst ask
		     liftIO $ print (v :: Int )
		     v2 <- liftM hOccursFst ask
		     return (v2 :: String)

a = (2 :: Int)
b = "str"
l a b = HCons a (HCons b HNil)
--- l1 :: ( HCons Int (HCons String HNil))
l1 = l a b
l2 = l b a

printBoth f l = runReaderT f l >>= print

printBoth2 l = do print (hOccurs l :: Int)
		  print (hOccurs l :: String)

hlistMonadReaderTest = do
  print "hlistMonadReaderTest"
  printBoth printAndRerun l1
  -- printBoth printAndRerun l2  -- < this doesn't work but the next line ? Where is the difference between printAndRerun printAndRerun2 ?
  printBoth printAndRerun2 l2
  --                     ^

  printBoth2 l1 -- here is no trouble..
  printBoth2 l2

main = hlistMonadReaderTest
------------------------------------------------

when not commenting the line above I'm getting this error:

HListTest/MonadReader.hs|35| 26:
     Couldn't match expected type `Int' against inferred type `[Char]'
       Expected type: HCons Int (HCons [Char] HNil)
       Inferred type: HCons [Char] (HCons Int HNil)
     In the second argument of `printBoth', namely `l2'
     In the expression: printBoth printAndRerun l2

Thanks Marc