Introduce Nullable trait to permit custom Option<T>

[nappa85]

As discussed in Issue 107, the introduction of a Nullable trait permits to retrive the correct Value variant for the Null type, therefore to add a generic impl From<Option<T>> for Value where T: Into<Value> + Nullable, permitting to use optional custom types inside Models

[tyt2y3]

Brilliant! I have stumbled upon the awkardness of impling traits for Option for quite some time...

[nappa85]

I've evolved the PR to remove also the various impl ValueType for Option<Something>, using the new trait Nullable it's possible to do a generic impl

[tyt2y3]

Sorry, not trying to nitpick, but can we use

impl<T> ValueType for Option<T>
where T:
    ValueType + Nullable {

inplace of

impl<T: ValueType + Nullable> ValueType for Option<T> {

[nappa85]

Sorry, not trying to nitpick, but can we use

impl<T> ValueType for Option<T>
where T:
    ValueType + Nullable {

inplace of

impl<T: ValueType + Nullable> ValueType for Option<T> {

Yes, not a problem, I personally prefer the compact form when there are few constraints

[tyt2y3]

Thank you )

[tyt2y3]

Um... so you think the type name shouldn't be Option<_>?

[nappa85]

Probably it should, but I failed to find a way to generate a static string from a method calling another method... And then I forgot about it

[nappa85]

maybe whit const trait fn stabilized it's possible, I don't know

[nappa85]

I was thinking https://doc.rust-lang.org/std/any/fn.type_name.html could have been a solution, but looking at the example I saw why it can't be

[tyt2y3]

So this type_name is mainly for diagnostic purpose. May be we wouldn't mind generating the string dynamically.

[nappa85]

If you change the return type to Cow<'static, str> you can still return static str in normal types and allocate String only in options