[erlang-questions] adding maps:merge_nested/2

Jesper Louis Andersen jesper.louis.andersen@REDACTED
Tue Nov 15 13:53:54 CET 2016

On Mon, Nov 14, 2016 at 9:20 PM Michael Truog <mjtruog@REDACTED> wrote:

> The main merge function you are missing in the maps module, is the merge/3
> (instead of merge/2) that is common in other modules (like the dict/orddict
> modules).  I added an implementation of merge/3 at
> https://github.com/okeuday/mapsd due to needing the dict API elsewhere.
> With the merge/3 function it should be easier to merge nested maps, in
> whatever way is required, since one size shouldn't fit all.

In OCaml, you often have a merge-function like this one:

val merge : ('k, 'v1, 'cmp) t
-> ('k, 'v2, 'cmp) t
-> f:(key:'k -> [
| `Left of 'v1
| `Right of 'v2
| `Both of 'v1 * 'v2
] -> 'v3 option) -> ('k, 'v3, 'cmp) t

which means that you have to supply a function of the form

  (K, {left, VL}) -> Res;
  (K, {right, VR}) -> Res;
  (K, {both, VL, VR}) -> Res

where Res is either undefined | {ok, Result} for some result value. The
semantics are that left,right, and both encodes on which side the value was
in the input maps. And the Res encodes if a new value should be produced in
the new map.

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