On Thu, Dec 15, 2011 at 1:23 AM, Gregory Crosswhite
On Dec 15, 2011, at 3:36 PM, Antoine Latter wrote:
There are a lot of combinators you can build from (<|>) and empty that go terribly wrong for Maybe and List but are still quite useful.
Yes, you *could* do that, but the whole point is that you shouldn't. Typeclasses generally come with informal laws that must be obeyed. If your instance does not obey those laws, then it should not be an instance.
I said 'combinators', not 'instances'. A lot of popular parsers combinators can be written exclusively from (<|>) and empty, but make little sense for List and Maybe, and may not even function properly. The 'trifecta' package includes a nice reference: http://hackage.haskell.org/packages/archive/trifecta/0.49.1/doc/html/Text-Tr... See 'skipSome' through 'chainr1' - I wouldn't be surprised if most of these lead to the same infinite loop behavior for Maybe as the stock 'many' and 'some' in base. These sorts of functions are what Alternative is for. Maybe I'm missing something fundamental here. Antoine