On Wed, Feb 14, 2018 at 6:47 AM, Mario Blažević
I understand it's hard to grep, but can you guess how many of those 40-80 uses of (<>) with the union meaning actually depend on the left-biased semantics? I've looked at my code, and it turns out that wherever this operation appears, the two maps are guaranteed to have distinct keys.
I know that in a few cases I do explicitly want "left wins", and I do frequently use (x<>) as a way to accumulate new data. Sometimes I expect that new data to override old stuff, sometimes I don't expect them to overlap, but it's not a problem if they do. In the cases where it is a problem (e.g. warn about shadowed names or something), I have Util.Map.unique_union :: Ord k => Map.Map k a -> Map.Map k a -> (Map.Map k a, Map.Map k a) -- ^ (union, rejected) I can grep for that: 3 calls, plus 11 more for the version that takes [(k, a)]. However, in the absence of actual numbers for (<>), don't take me too seriously. These kinds of feelings are often shown wrong by the real numbers. But even if I could count them, it's not like it proves anything. It comes down to style and problem domain. That said though, is there a way to grep for methods overloaded at a certain type? One thing I thought of was recompile Data.Map with a TypeError instance, but it seems it might be a hassle for a base package like containers. Otherwise I imagine you could parse core, look for resolved methods, and map back to file and line. Is there a general purpose core analysis suite out there?
There is actually one use I'm not sure about, I'll have to make sure there's no bug hiding there. That is one reason I'm supporting the proposal, by the way: the existing behaviour that silently drops values is much more likely to cause bugs.
It's true it does, but since I think of (<>) as union, and union does that too, it doesn't surprise me. Actually the surprising thing was that Map.union is left-biased, while it seems like most languages go the other way. But that was just a general haskell oddity that I accustomed myself to many years ago.