Partial type signature

Possible Duplicate:
Incomplete type signature

Consider the following:

import Network.HTTP.Conduit  

(parseUrl "http://stackoverflow.com") :: Maybe a

parseUrl returns Failure HttpException m => m (Request m')

It's documentation says:

Since this function uses Failure , the return monad can be anything that is an instance of Failure , such as IO or Maybe .

However, when I try to force parseUrl to use Maybe , I get the following error:

main.hs:9:11:
    Couldn't match type `a' with `Request m'0'
      `a' is a rigid type variable bound by
          an expression type signature: Maybe a at main.hs:9:10
    Expected type: Maybe a
      Actual type: Maybe (Request m'0)

Is there anyway to force the type to Maybe without specifying the full exact type? Answers including GHC extensions are fine.

Note that this works:

f :: Maybe a -> Maybe a
f x = x

f (parseUrl "http://stackoverflow.com")

But it seems ugly to me.

---

You can use asTypeOf ,

main = do
    print (parseUrl "http://stackoverflow.com" `asTypeOf` Nothing)

to force the monad to be Maybe . Not that that gains much over

main = do
    print (parseUrl "http://stackoverflow.com" :: Maybe (Request m))

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