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/html/Control-Monad-Exception-Synchronous.html
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
