#2309: containers: specialize functions that fail in a Monad to Maybe

[Lennart Augustsson]

And here's another vote for Maybe.  That's almost always the return
type that you want anyway, so the inconvenience in minimal.
And anyone who wants the old behavior can easily make a utility
function that provides it.
The current behaviour encourages sloppy programming.

  -- Lennart

On Wed, May 28, 2008 at 7:51 PM, Duncan Coutts
<duncan.coutts at worc.ox.ac.uk> wrote:
>
> On Wed, 2008-05-28 at 11:11 +0100, Conor McBride wrote:
>> > Ross Paterson wrote:
>> >> Several functions on containers used to have types like
>> >>    lookup :: (Ord k) => k -> Map k a -> Maybe a
>> >> but these were generalized to
>> >>    lookup :: (Monad m, Ord k) => k -> Map k a -> m a
>> >> The proposal is to change these signatures back, specializing them
>> >> to Maybe.
>> >
>> > I'm all for Maybe in the question Monad VS Maybe.
>>
>> Me too. (+1) for Ross's proposal.
>
> If we need any more (+1) votes you can count mine.
>
>> > Also, it's not entirely clear whether MonadZero is the most general
>> > abstraction. Is ApplicativeZero better? Or something else?
>>
>> Maybe is the most general abstraction. Requiring
>> (>>=), or even (<*>) seems excessive. What we need
>> is "any f with zero and return", so why not pick
>> the canonical, initial, inductively defined such
>> thing?
>
> Good. I like that argument.
>
> As others have pointed out, it's easy to lift maybe into something else
> when that's needed.
>
> maybe :: a -> (b -> a) -> Maybe b -> a
>
> Duncan
>
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