On Thu, Jul 20, 2006 at 10:48:02AM -0600, Rodney D Price wrote:
I've gotten this sort of error several times, which mysteriously disappears when I add more functions to the code:
storeError.hs:13:38: Couldn't match expected type `a' (a rigid variable) against inferred type `String' `a' is bound by the type signature for `throwError' at <no location info> Expected type: a Inferred type: String In the first argument of `return', namely `msg' In the call (return msg)
(This is GHCi.) The code is below. The type variable a can't be bound to String, obviously, but a relative novice like myself has no idea why. Can someone tell me?
instance MonadError StoreError STM where throwError (Default msg) = return msg
throwError :: e -> m a The return type is wrong. throwError needs to be able to return an arbitrary type, which is more than a bit tricky. It looks like in STM you might only be able to do this using "check". -- David Roundy