Document implementation for HashMap<String, usize>

[dbrgn]

It would probably be useful to have a default implementation of Document and ProcessedDocument for HashMap<String, usize>.

I did a first attempt, but somehow can't get it to compile:

impl<T> Document for HashMap<T, usize> {
    type Term = T;
}

impl<T> ProcessedDocument for HashMap<String, usize> where T : PartialEq {
    fn term_frequency<K>(&self, term: K) -> usize where K : Borrow<T> {
        &self.get(&term).unwrap_or(0)
    }

    fn max(&self) -> Option<&T> {
        match self.iter().max_by_key(|&(_, v)| v) {
            Some(&(ref k, _)) => Some(k),
            None => None
        }
    }
}

The error message I'm getting:

src/lib.rs:75:6: 75:7 error: the type parameter `T` is not constrained by the impl trait, self type, or predicates [E0207]
src/lib.rs:75 impl<T> ProcessedDocument for HashMap<String, usize> where T : PartialEq {
                   ^
src/lib.rs:75:6: 75:7 help: run `rustc --explain E0207` to see a detailed explanation
error: aborting due to previous error

Any ideas?

[ferristseng]

Hey,

Thanks for using my library! The issue is you actually don't need the T type parameter in the declaration of the implementation, since your HashMap isn't using it.

Here is a generic version of what you're trying to do (compiles in rustc 1.8.0-nightly):

use std::collections::HashMap;
use std::cmp::Eq;
use std::hash::Hash;

impl<T> Document for HashMap<T, usize> {
  type Term = T;
}

impl<T> ProcessedDocument for HashMap<T, usize> where T : Hash + Eq {
  fn term_frequency<K>(&self, term: K) -> usize where K : Borrow<T> {
    *self.get(term.borrow()).unwrap_or(&0)
  }

  fn max(&self) -> Option<&T> {
    match self.iter().max_by_key(|&(_, v)| v) {
      Some((ref k, _)) => Some(k),
      None => None
    }
  }
}

[dbrgn]

Ah, thanks for that! :)