#12372: bug: documentation for Control.Monad.guard not useful after AMP -------------------------------------+------------------------------------- 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: | -------------------------------------+------------------------------------- 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 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler