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in lots of use cases (parsing protocols), I end up with the side condition "there may be at most one extension of each type present". now, the standard Map.add replaces a potential existing value - same is done by Gmap.add.

It may be worth to explicitly provide add : k -> v -> t -> t option and replace/update : k -> v -> t -> t. the issue left is naming -- add in OCaml is already used for the destructive update in Hashtbl and Map, using it for a different purpose will confuse people. Any suggestions for a function name that does non-replacing addition to a map?

at the moment, every lookup (find, but also union, merge, equal) does two key comparisons: one to find the value in the map and the other to prove to the type system that the binding (a tuple of key and value) and the looked-up key are the same (see this comment).

this is inefficient since the GADT compare function is rather inefficient (afaict). solutions include Obj.magic, vendoring of the Map module from OCaml's stdlib, see https://github.com/cfcs/misc/blob/197ede0fc427d51bd52947e40c02c77350fc2980/gadt_comparison.ml for some more discussion. #3 reimplement equal in a type-safe way without the need to call key comparison twice.

ultimately, I'd like to understand better if there's a good (i.e. cheap and safe) way to do comparisons of GADTs. its also rather painful to write (and ppx_deriving doesn't help).