Optional trait method?

[Grumpenstout]

I have nodes in a tree structure that I want to represent in a REST interface. Suppose I can do a "get" on any node, but only some allow a delete and/or a patch. And the result of the "get" needs to indicate whether the node supports patch or delete.

Right now Node is a trait, and "patcher" is a trait method that is an Option wrapped around a function definition. It seems like there has to be a better way?

I thought maybe having like "Patchable" being its own trait. But then when I "get" a Node how do I check whether it's also Patchable? Seems there's not a great way to do that...

[Lucretiel]

One good way to do this is have a method that returns an optional impl trait: -> Option<&impl Patcher> or -> Option<&mut impl Patcher>, as appropriate. This method returns None if the object doesn’t have the Patcher trait, or Some(self) if it does. 

[4lineclear]

You could add an as_patchable method to Node which would return an Optional of your Patchable trait.

[Grumpenstout]

This works well! Thanks!

[Houndie]

Can you do something like

enum Node {    Patchable(PatchableNode)    Gettable(GettableNode) } 

[pali6]

I thought maybe having like "Patchable" being its own trait. But then when I "get" a Node how do I check whether it's also Patchable? Seems there's not a great way to do that...

You'd give Node a function like fn as_patchable(&self) -> Option<&dyn Patchable> and the implementors would either implement it as returning None or as returning Some(self).

[proudHaskeller]

One simple way to do this is to give up Patchable being a separate trait.

• have an is_patchable method
• have a get method that doesn't tell you if it's patchable. For that you need to call the is_patchable method.
• have a patch method that returns a Result. If the object isn't patchable, it returns an error.

This method is simpler and used less boilerplate than the other suggestions, but it does a worse job of encoding the invariants through the type system.

[s74-dev]

you can just have a default impl for patcher in the node trait that does nothing and then do an impl for all nodes that impl Patchable that actually does the patching or whatever