On Sat, Apr 17, 2010 at 2:52 PM, Henning Thielemann
Ryan Ingram schrieb:
It's used in the implementation of "fail" for those monads.
class Monad m where ... fail :: String -> m a fail = error -- default implementation
which is then used to desugar do-notation when pattern matching fails:
do Left x <- something return x => something >>= \v -> case v of { Left x -> return x ; _ -> fail "Pattern match failure ..." }
You can argue about whether "fail" belongs in Monad (and many people have), but that's why it is how it is.
I also prefered to not support the fail method in this way and wrote: http://hackage.haskell.org/packages/archive/explicit-exception/0.1.4/doc/htm...
I find that having pattern match desugar to "fail", and supporting fail, can lead to extremely concise, clear code. For example, an excerpt from some type checking/inference code I wrote: data Equal a b = (a ~ b) => Refl data Typ a where TInt :: Typ Int TBool :: Typ Bool TList :: Typ a -> Typ [a] TArrow :: Typ a -> Typ b -> Typ (a -> b) eqT :: Typ a -> Typ b -> Maybe (Equal a b) eqT TInt TInt = return Refl eqT TBool TBool = return Refl eqT (TList a) (TList b) = do Refl <- eqT a b return Refl eqT (TArrow a1 a2) (TArrow b1 b2) = do Refl <- eqT a1 b1 Refl <- eqT a2 b2 return Refl eqT _ _ = fail "not equal" This relies heavily on the pattern match desugaring to "case", which brings the type equality (a ~ b) into scope, and "fail" returning Nothing. For example, the list case: desugaring eqT (TList TInt) (TList TInt), with all operations on the Maybe monad inlined and simplified we have ta = TList TInt :: Typ a, tb :: TList TInt :: Typ b, for some a, b eqT = case ta of ... (TList ta1) -> -- now we have a ~ [a1] for some a1 case tb of ... (TList tb1) -> -- now we have b ~ [b1] for some b1 case eqT ta1 tb1 of Just Refl -> -- now we have a1 ~ b1, which gives us [a1] ~ [b1], which gives us a ~ b Just Refl _ -> Nothing -- ryan