Add get_value/2 and get_value/3.

[ewanmellor]

These work the same as proplists:get_value but should be faster because they use the fact that the dictionary is ordered.

[alavrik]

I don't know about your use case, but I found that it is usually more convenient to define get_value as a macro like this:

-define(get_value(X, Default),
    try X
    catch error:'erlson_not_found' ->
        Default
    end
).
-define(get_value(X), ?get_value(X, 'undefined')).

This way, you can use get_value with Erlson syntax. For example:

?get_value(X.foo.bar),
?get_value(X.foo.bar, 1)

As for the functional interface for accessing nested fields, I'm still feeling reluctant about adding it. There are at least two different behaviors one can choose depending on a particular use case:

• return the provided default value only for missing leafs
• return the provided default value even if a subtree is missing

Your addition (and my macro) implements the second behavior but I'm not sure it is a good default for a library function.

[ewanmellor]

I was thinking about the case where the key is a variable, so you can't use the dot syntax:

Received = erlson:from_proplist([{foo, bar}, {baz, buzz}]),
lists:map(fun (K) -> erlson:get_value(K, Received) end, [foo, baz]).

In that case, you couldn't write Received.K.

I like the idea of a macro for the three-arg case -- I hadn't thought of that. Maybe if you gave them more descriptive names, it wouldn't seem too bad having them in a library header, and you could support both cases:

?get_value_def_leaf(X.foo.bar, 1).
?get_value_def_any(X.foo.bar, 1).

[alavrik]

I would really like to avoid implementing any field access functions as a part of the Erlson library. Erlson has always been more about syntax rather than comprehensive features. Besides, as you may know, key-value lookup interfaces is a highly debatable topic in the Erlang community and I wanted to refrain from that.

There are some external libraries that can do that. For example, this one looks pretty good: https://github.com/etrepum/kvc

As for the macro, let me think about better names and I'll add them to the header.

[ewanmellor]

kvc wouldn't be aware that erlson lists are ordered though, would it? That was the only reason for this change in the first place -- to use the fact that the lists are ordered to get a performance boost. Otherwise, I could just use proplists:get_value.

[alavrik]

Good point about optimized access! Let's take another shot at it. With the above macros, your example can be rewritten as

lists:map(fun (K) -> ?get_value(erlson:fetch(K, Received)) end, [foo, baz]).

This would become possible as well:

lists:map(fun (K) -> ?get_value(erlson:fetch(K, Received), Default) end, [foo, baz]).

Hmm... These examples got me thinking about adding both: get_value functions as you suggested and also ?get_value macros. Let me think a little bit more about details and I'll probably do it in the next couple of days.

[alavrik]

@ewanmellor, I decided to merge it "as is" and deal with everything else separately. Distinguishing between missing leafs and missing sub-trees at Erlson syntax level is not as simple as I thought. It will take some extra work.

Also, I will add the macros shortly and update the documentation a little bit later.

Thank you for the contribution!