MapDiff: A small library that computes the difference between two maps

Hello everyone,

I wrote a small library today called MapDiff.
It returns a map listing the (smallest amount of) changes to get from map_a to map_b:

iex> foo = %{a: 1, b: 2, c: %{d: 3, e: 4, f: 5}}
iex> bar = %{a: 1, b: 42, c: %{d: %{something_else: "entirely"}, f: 10}}
iex> MapDiff.diff(foo, bar)
%{changed: :map_change,
  value: %{a: %{changed: :equal, value: 1},
    b: %{added: 42, changed: :primitive_change, removed: 2},
    c: %{changed: :map_change,
      value: %{d: %{added: %{something_else: "entirely"},
      changed: :primitive_change, removed: 3},
    e: %{changed: :removed, value: 4},
    f: %{added: 10, changed: :primitive_change, removed: 5}}}}}

This is mainly useful if you have two maps describing the state of something, where
replacing all of the old state and then filling in the new is too slow, which means that only
replacing the things that actually are changed is better.

It is similar to what Elm does to represent its output to the HTML Domain Object Model. The reason I wrote MapDiff is that I am planning on writing a library that wraps Erlang’s wxWidgets bindings in a similar way that Elm wraps the HTML DOM, which would allow people to write GUIs in a Functional style (wxWidgets itself is Object-Oriented). Does this sound like a huge undertaking? Yes! Let’s see how far I can get with it …

Anyhow, MapDiff is a very small and basic library, but it might be useful for some other people as well, so I thought I’d publish it on Hex.pm . Feedback is of course greatly appreciated.

15 comments

#library #maps #mapdiff

36 8834 15

2018-02-04 21:52:57 UTC

Most Liked

Qqwy

TypeCheck Core Team

All right! MapDiff is updated to 1.1.0.
The single but important new feature: Supporting structs in a meaningful way.

comparing two structs of the same type returns a map of fields that were changed.
comparing two structs of a different type returns a primitive_change where the whole of the old struct was replaced for the whole of the new struct.

Examples:

iex> MapDiff.diff(%Foo{}, %Foo{a: 3})
%{changed: :map_change, struct_name: Foo,
  value: %{a: %{added: 3, changed: :primitive_change, removed: 1},
    b: %{changed: :equal, value: 2}, c: %{changed: :equal, value: 3}}}

When comparing two different kinds of structs, this of course results in a :primitive_change, as they are simply considered primitive data types:

iex> MapDiff.diff(%Foo{}, %Bar{})
%{added: %Bar{a: "foo", b: "bar", z: "baz"}, changed: :primitive_change,
  removed: %Foo{a: 1, b: 2, c: 3}}

Post #6

Qqwy

TypeCheck Core Team

By the way,

The library consists of one small module (or actually, one single public function, even).

The code still uses quite a few if/else statements inside this function definition. Is there a way to make this code even more Elixir-idiomatic?

Post #7

lambdadevfr

I was in a company where we had a huge usage of such a library. It was mostly used for tests.
Basically a test (in erlang) was written:
my_test() →
Expected = {this, that, [etc]}
Result = module:call(params…)
diff (Expected, Result).

When a test failed you could very quickly see the difference between what was expected and the result. Super usefull

Post #16