On Mon, Dec 16, 2013 at 5:50 PM, Herbert Valerio Riedel
On 2013-12-16 at 14:54:53 +0100, Andreas Abel wrote:
On 16.12.2013 14:12, Roman Cheplyaka wrote:
The purpose of 'fail' is to handle monadic pattern-match failures. I consider explicit use of 'fail' in user code a hack.
I treat 'fail' as a semi-private method of the Monad class. I may override it, but I should never call it. It only exists for the compiler.
This is useful information. Please add it to the documentation. That would definitely help newcomers to get some orientation in the design space of exceptions...
[...]
If this is really the intent of `fail`, shouldn't we then also attach a
{-# WARNING fail "Monad(fail) is not supposed to be called directly" #-}
to the `fail` method definition to help catch illegal uses of `fail` in user code?
(Fwiw, I've tested attaching such a warning to `fail` in GHC HEAD's libraries/base/GHC/Base.lhs and it seems to work just fine so far... so if there's consensus, we could add that for GHC 7.8)
Cheers, hvr _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries
+1! I think `fail` is a huge wart anyway. I'm still partial to the Haskell 1.4 solution to this problem, where the translation of refutable patterns in do-notation would involve `mzero` rather than `fail "Here, have some debugging info."`. :), Stijn