Adrian Hey wrote:
I've never been very keen on alter myself (on efficiency grounds) and was wondering whether of not to include it. If the "altering" results in an unchanged map it would be nice to just return the unchanged map (rather than duplicate all nodes on the search path). There are other possible alternatives to alter that are more efficient in this respect.
There's something else that's always bugged me about alter. I'm sure I can't be the only one who thinks requiring me to define a function like this is a very weird thing to do... f :: Maybe a -> Maybe a f Nothing = maybea -- A constant, either Nothing or (Just somea) f (Just a) = f' a I've never felt comfortable with this. In any case I realised that alter can be easily implemented using other class primitives alter f k map = case f Nothing of Just somea -> insertMaybe f' k somea map Nothing -> deleteMaybe f' k map where f' a = f (Just a) but since I know what maybea and f' are, it seems to me it's a lot easier to just use f' directly in either insertMaybe or deleteMaybe as appropriate (depending on maybea). The type of the proposed merge now seems similarly strange to me.. merge :: (k -> Maybe a -> Maybe b -> Maybe c) -> map a -> map b -> map c This requres users to define a function argument that is presumably of form.. f k Nothing Nothing = undefined f k (Just a) Nothing = fa k a f k (Just a) (Just b) = fab k a b f k Nothing (Just b) = fb k b Why not just pass fa,fab and fb directly, which will be more convenient for both users and implementors I think.. merge :: (k -> a -> Maybe c) -> (k -> a -> b -> Maybe c) -> (k -> b -> Maybe c) -> map a -> map b -> map c Though I actually think if this is to be included perhaps it should be called mergeMaybeWithKey, and we also have.. mergeMaybe :: (a -> Maybe c) -> (a -> b -> Maybe c) -> ( b -> Maybe c) -> map a -> map b -> map c merge :: (a -> c) -> (a -> b -> c) -> ( b -> c) -> map a -> map b -> map c mergeWithKey :: (k -> a -> c) -> (k -> a -> b -> c) -> (k -> b -> c) -> map a -> map b -> map c Regards -- Adrian Hey