Sounds like a job for...Type Inference!
In Olegs post where he gave examples of how to make exceptions explicit in type signatures: oleg said:
{-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-}
module TER where
class CERROR a c where darn:: a -> c-> String -> b
instance Show a => CERROR a c where darn label _ msg = error $ "Error: " ++ (show label) ++ " " ++ msg
data EHead = EHead deriving Show data ETail = ETail deriving Show data EJust = EJust deriving Show
myhead x@([]) = darn EHead x "head of an empty list" myhead (x:_) = x
mytail x@([]) = darn ETail x "tail of an empty list" mytail (_:xs) = xs
Now, if we ask GHCi for the type of myhead, we get *TER> :t myhead myhead :: forall a. (CERROR EHead [a]) => [a] -> a ---------------------------------------------------------- As you can see you get the advantage of explicit exceptions, but with type inference. Keean.