[GHC] #11650: Default definitions for Alternative(some, many) can easily blow up

26 Feb 2016

      #11650: Default definitions for Alternative(some, many) can easily blow up
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           Reporter:  bgamari        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Documentation  |           Version:  7.10.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Consider this case (taken from #11649)
 {{{#!hs
 import Control.Applicative

 data U1 p = U1

 instance Functor U1 where
     fmap f U1 = U1

 instance Applicative U1 where
     pure _ = U1
     U1 <*> U1 = U1

 instance Alternative U1 where
     empty = U1
     U1 <|> U1 = U1

 instance Show (U1 p) where
     show U1 = "U1"

 main = print (some U1)
 }}}

 This looks fine, right? Wrong,
 {{{
 $ ./Main
 Main: <<loop>>
 }}}

 It seems that the default definitions of `some` and `many` will produce
 looping code in these cases (although it's not entirely clear to me which
 cases "these" cases are).

 I suppose this is due to the fact that `U1` will always "succeed". We
 should ensure that this is noted in the documentation to ensure that users
 don't end up with accidentally bottoming instances.
 Well, consider what happens

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11650
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