On 16/12/2013 16:50, Herbert Valerio Riedel wrote:
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)
I'm not sure I follow the logic of this. If it does continue to exist, what are the arguments against using it explicitly? Cheers, Ganesh