impl on type aliases

[calebmer]

To provide a nice interface to users I would like to be able to add some impl functions to a type alias. For example, I have the following code:

type Schema = Box<SchemaTrait + 'static>;

trait SchemaTrait {…}

struct SchemaNumber {…}
impl SchemaTrait for SchemaNumber {…}

I would like to add the following:

impl Schema {
  fn new_number() -> Schema {
    Box::new(SchemaNumber::new())
  }
}

So the following would work:

Schema::new_number()

But this would not:

Box::new_number() // Fails, method is only for the type alias.

Another use case is I have a map representing a JSON object (BTreeMap<Key, Value>) that I would like to add custom methods to without having to write a struct that redefines all of the BTreeMap methods. This currently cannot be done.

As a side note, this feels a little like classical inheritance…

[steveklabnik]

/cc @rust-lang/lang , is this a feature that's desired? And if so, would it require an RFC?

[jonas-schievink]

A type alias is just that: An alias, not a different type. If you want to create a different type, use the newtype pattern for that.

[calebmer]

@jonas-schievink but the newtype pattern doesn't inherit all of the aliased types methods. Is there a reason that type couldn't create a new type?

[KalitaAlexey]

@calebmer Yes, it is. Because everyone using type as alias.

[nrc]

This would require an RFC and would be better served as an issue on that repo rather than here. Having said that, I don't believe this is desirable. The preferred approach is to use newtypes (as @jonas-schievink mentions). A possible solution is 'newtype deriving' where a newtype inherits impls. That has been discussed a fair amount before, not sure if there is an issue open for it.

[calebmer]

I would like to see an issue on newtype deriving if it exists.
On Sun, Mar 20, 2016 at 17:34 Nick Cameron [email protected] wrote:

This would require an RFC and would be better served as an issue on that
repo rather than here. Having said that, I don't believe this is desirable.
The preferred approach is to use newtypes (as @jonas-schievink
https://github.com/jonas-schievink mentions). A possible solution is
'newtype deriving' where a newtype inherits impls. That has been discussed
a fair amount before, not sure if there is an issue open for it.

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#32357 (comment)

[nikomatsakis]

Indeed, as @nrc noted, this would most definitely require an RFC, but I think more importantly it's not really something we would want. Type aliases, indeed, are just aliases. Some kind of lighter-weight deriving for struct wrappers (newtypes) would be nice though and would... probably... fill the use-case here. Therefore, I'm going to close this issue in favor of rust-lang/rfcs#479.