On Fri, Dec 22, 2006 at 09:44:35PM -0500, Steve Downey wrote:
I suppose this is why Maybe is a monad rather than just another algebraic data type.
Maybe is an algebraic datatype. A monad is simply a structure defined on the datatype, reflecting a normal pattern of use.
This is exactly where I'm stuck. I've an eval1 function of ArithExpr -> m ArithExpr, where m started out as Maybe. Now only the calling eval function mentions Maybe. And I was wondering if I could keep eval1 generic.
That should be easy - anything you can do in Maybe or Either can be done in an arbitrary MonadError. f :: (MonadError e m, Error e) => Maybe a -> m a f Nothing = throwError noMsg f (Just x) = return x g :: (MonadError e m, Error e) => Either String a -> m a g (Left e) = throwError $ strMsg e g (Right x) = return x h :: (MonadError e m) => Either e a -> m a h (Left e) = throwError e h (Right x) = return x
OTOH, eval1 should really pass through information from the underlying parser...
If you have a type of errors, pattern 'h' above should be applicable?