API doesn't work well with distinct owned / ref types

[epage]

EqPredicate stores T and its eval takes a &T. But what about Vec/slice, String/str, and PathBuf/path?

Exploration Summary

After making Predicate use generics rather than associated types (see #29)

Dead ends

• Predicate { eval<I: AsRef<Item>>(&self, item: I) }: not object-safe which means we can't Box<Predicate> or &Predicate.
• Predicate { eval(&self, item: Item) } (ie Item=&str rather than Item=str): violates borrow checker in boolean predicates
• Above with Item: Copy: Lifetime issues (branch)
• impl Predicate<AsRef<T>> for EqPredicate<T>: No AsRef implemented for numeric types
• impl Predicate<Borrow<T>> for EqPredicate<T>: Sized issues (branch)

Options:

• Create an AsRef adapter

[nastevens]

Commented in #18 but makes sense to add here. The Predicate trait could be changed to accept Borrow<T> instead of &T. Since Borrow<T> is implemented for &T, this probably won't even end up being a breaking change, and would allow things using String and Vec to be way more ergonomic.

[epage]

In looking at this, I also might need to change the Predicate trait. In researching, it seems the recommendation is to use associative types for return types (don't need to mention type on every call) and generics for parameters (allows multiple trait implementations per struct).

[epage]

Ok, got a branch with Predicate using generics.

• Can't make eval accept AsRef because that will make the function not object-safe which means we can't Box<Predicate> or &Predicate.
• Tried changing eval's signature to Item from &Item, putting the burden of & on the client. This causes borrow check violations in the boolean predicates. The only way to make this route work is if we required Item to be copyable.

So remaining areas to explore

• Item: Copy
• Just modifying EqPredicates impl to more easily allow the Predicate to borrow EqPredicates owned type

[epage]

You can see my Item: Copy work here

Ran into life time issues of all things.

[epage]

I tried just limited myself to modifying EqPredicate to have a Predicate<Borrow<T>>.

Example of issues I ran into

---- src/ord.rs - ord::eq (line 50) stdout ----
        error[E0277]: the trait bound `str: std::marker::Sized` is not satisfied
  --> src/ord.rs:58:31
   |
11 | assert_eq!(true, predicate_fn.eval("Hello World"));
   |                               ^^^^ `str` does not have a constant size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `str`
   = note: required because of the requirements on the impl of `predicates::Predicate<str>` for `predicates::ord::EqPredicate<&str>`

https://github.com/epage/predicates-rs/tree/borrow_borrow

I also tried AsRef since that would offer nicer conversions but I couldn't pass numbers in.

[epage]

Had some time to evaluate this. The work for #29 is probably the best we'll get.

[epage]

So while we need to address Predicate<str> for EqPredicate<String>, another angle on this is passing a &'static str to EqPredicate and being able to get a Predicate<str>.

[epage]

#49 is another step towards addressing this