[moved to cafe] On Jul 20, 2006, at 12:48 PM, 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?

Thanks,

-Rod

-- module Store where

import Control.Monad.Error import Control.Concurrent.STM

data StoreError = Default String

instance Error StoreError where noMsg = Default "Store error" strMsg = Default

instance MonadError StoreError STM where throwError (Default msg) = return msg

Lets take a look here at the definition of MonadError from Control.Monad.Error: class Monad m => MonadError e m | m -> e where throwError :: e -> m a catchError :: m a -> (e -> m a) -> m a In the signature for 'throwError' there are three type variables: e, m and a. e and m are bound by the instance declaration, but a is free. In Haskell the rule is that free variables are implicitly bound with a universal quantifier. So, the type for throwError can be regarded as, throwError :: forall a. e -> m a, for some concrete choices of e and m which are determined by the instance. The forall means that a user of this function can put any type there that he likes. In other words, the monadic action created by 'throwError', when executed, evaluates to _a value of any type at all_ (yes, I know the terminology is a little loose here). That means you can't just 'return msg', which has type 'm String' because a user might have used throwError to create an action of type 'm Int', for example. In fact, you won't really be able to return anything at all, because there isn't any way to write a program that can generate a value of any unknown type. This should hopefully correspond to your intuition about what throwing an exception does. The error generated by the typechecker basically tells you that the function you have written is not polymorphic enough. It has type 'StoreError -> STM String' rather than 'forall a. StoreError -> STM a' as it ought. I think perhaps you have misunderstood how MonadError is used. The idea is to expose to users a particular non-local control flow construct (throw/catch style exceptions) by hiding all the stuff necessary for that inside the monad plumbing. Usually, whoever writes the monad itself will provide the necessary instances. It's often not possible to write instances like this by using the external API of the monad. This is particularly the case for the abstract monads available in GHC (IO, ST, and STM). In short, I don't think you'll be successful in writing a 'MonadError' instance for STM that has the customary semantics. What you may be looking for is the ErrorT monad transformer, which will let you layer error handling over STM. It's hard to know with what info you've provided here. If you give a few more details on what you're trying to accomplish, someone may be able to give you a push in the correct direction. Rob Dockins Speak softly and drive a Sherman tank. Laugh hard; it's a long way to the bank. -- TMBG