merge should be a setter

[pranaysashank]

merge is trivially a Setter. The current signature of merge

merge :: (IsMergeable a b c) => a -> b -> c can be changed to

merge :: (IsMergeable a b c) => Setter a c a b.

If this seems too much of a change, we can export a new setter mergeS that is defined in terms of the existing merge function as

-- This is a Setter with an additional IsMergeable constraint
mergeS :: (IsMergeable a b c, Settable f) => (a -> f b) -> a -> f c
mergeS  = sets (\f s -> merge (f s) s)

Additionally, the order in which we merge shouldn't matter. So changing the class definition slightly as

class (IsMergeable b a c) => (IsMergeable a b c) | a b -> c where
    merge :: a -> b -> c
    merge a b = merge b a -- flip merge

Example:

tn   :: TagNew String Validated
tnd :: TagNewDelta Validated
t     :: Tag String Validated

tn & mergeS .~ tnd == tnd & mergeS .~ tn  == t -- Tag String Validated

These should be simple fixes and can be incorporated into the library pretty easily.

Possible Future Improvements:
This also seems to indicate that the HasSplice class and IsMergeable class can be mixed into one. In a future improvement it can probably be developed in such a way as to say HasSplice c a b means c can be decomposed into a and b, and a and b can be merged to form a c, but this seems too much like sub typing in Java .