[Haskell] rigid variables
droundy at darcs.net
Thu Jul 20 13:58:02 EDT 2006
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:
> 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".
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