[GHC] #12372: bug: documentation for Control.Monad.guard not useful after AMP

[GHC]

#12372: bug: documentation for Control.Monad.guard not useful after AMP
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        Reporter:  ntc2              |                Owner:
            Type:  bug               |               Status:  infoneeded
        Priority:  normal            |            Milestone:
       Component:  libraries/base    |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Documentation     |  Unknown/Multiple
  bug                                |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ntc2):

 Replying to [comment:4 bgamari]:
 > ntc2, any word on this?

 So, I'm only familiar with `MonadPlus` examples. E.g.
 {{{
 safeDivision :: (Eq a, Fractional a, MonadPlus m) => a -> a -> m a
 safeDivison n d = do
   guard (d /= 0)
   return (n / d)

 ghci> safeDivision 3 0 :: Maybe Double
 Nothing
 ghci> safeDivision 3 2 :: Maybe Double
 Just 1.5
 }}}

 Now, it turns out I can also write that example using `Alternative`, i.e.
 {{{
 safeDivisionA :: (Eq a, Fractional a, Alternative f) => a -> a -> f a
 safeDivisionA n d = guard (d /= 0) *> pure (n / d)

 ghci> safeDivisionA 3 0 :: Maybe Double
 Nothing
 ghci> safeDivisionA 3 2 :: Maybe Double
 Just 1.5
 }}}
 but the default `Alternatives` I know are exactly the `MonadPlus` types,
 i.e. `[]` and `Maybe`, so I don't see what I've gained here (and in
 practice I think I'd just use `Maybe` here).

 So, I think a `MonadPlus` example like `safeDivision` would be useful.

 An example that only makes use of the weaker `Alternative` assumption
 might also be useful, but I don't have one.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12372#comment:5>
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