Hi On 28 May 2008, at 10:21, apfelmus 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. [..]
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?
In any case, I prefer Maybe here as well because it's straightforward and simple. Every other abstraction is just a lift away.
And that's no coincidence, of course. The whole point of inductive definition is to, er, induce such lifts to other settings with /at least/ the same structure. The point was well made to me by James McKinna, years ago, when he suggested that we mediate Epigram's system of "views" via datatypes: the cheap way of abstracting over "all predicates closed under this and that" is to consider the initial one, ie the inductively defined thing with this and that. A whole bunch of higher-order jiggery-pokery with control operators became plain first-order code at a stroke. So it's not a question of concrete-vs-abstract here. The concrete is by inductive (morally anyway) definition, the most abstract thing you can have. All the best Conor