fix(API): Switch from Associated Types to Generics

The hope is that generics will also make implementing support for assert-rs#20
easier.

So when to use each according to [The
Book](https://doc.rust-lang.org/book/second-edition/ch19-03-advanced-traits.html#associated-types-versus-generics):

Generics
- Used when type is going to be function input
- Can have multiple implementations of trait

Associated types
- Used when type is going to be a function output, like iterator
  - Avoids need to annotate type
- Can only have one implementation of trait

BREAKING CHANGE: The `Predicate` trait's API has changed.

Author: epage

src/boolean.rs

@@ -8,40 +8,48 @@
 
 //! Definition of boolean logic combinators over `Predicate`s.
 
+use std::marker::PhantomData;
+
 use Predicate;
 
 /// Predicate that combines two `Predicate`s, returning the AND of the results.
 ///
 /// This is created by the `Predicate::and` function.
 #[derive(Debug)]
-pub struct AndPredicate<M1, M2>
+pub struct AndPredicate<M1, M2, Item>
 where
-    M1: Predicate,
-    M2: Predicate<Item = M1::Item>,
+    M1: Predicate<Item>,
+    M2: Predicate<Item>,
+    Item: ?Sized,
 {
     a: M1,
     b: M2,
+    _phantom: PhantomData<Item>,
 }
 
-impl<M1, M2> AndPredicate<M1, M2>
+impl<M1, M2, Item> AndPredicate<M1, M2, Item>
 where
-    M1: Predicate,
-    M2: Predicate<Item = M1::Item>,
+    M1: Predicate<Item>,
+    M2: Predicate<Item>,
+    Item: ?Sized,
 {
     /// Create a new `AndPredicate` over predicates `a` and `b`.
-    pub fn new(a: M1, b: M2) -> AndPredicate<M1, M2> {
-        AndPredicate { a: a, b: b }
+    pub fn new(a: M1, b: M2) -> AndPredicate<M1, M2, Item> {
+        AndPredicate {
+            a: a,
+            b: b,
+            _phantom: PhantomData,
+        }
     }
 }
 
-impl<M1, M2> Predicate for AndPredicate<M1, M2>
+impl<M1, M2, Item> Predicate<Item> for AndPredicate<M1, M2, Item>
 where
-    M1: Predicate,
-    M2: Predicate<Item = M1::Item>,
+    M1: Predicate<Item>,
+    M2: Predicate<Item>,
+    Item: ?Sized,
 {
-    type Item = M1::Item;
-
-    fn eval(&self, item: &Self::Item) -> bool {
+    fn eval(&self, item: &Item) -> bool {
         self.a.eval(item) && self.b.eval(item)
     }
 }
@@ -50,34 +58,40 @@ where
 ///
 /// This is created by the `Predicate::or` function.
 #[derive(Debug)]
-pub struct OrPredicate<M1, M2>
+pub struct OrPredicate<M1, M2, Item>
 where
-    M1: Predicate,
-    M2: Predicate<Item = M1::Item>,
+    M1: Predicate<Item>,
+    M2: Predicate<Item>,
+    Item: ?Sized,
 {
     a: M1,
     b: M2,
+    _phantom: PhantomData<Item>,
 }
 
-impl<M1, M2> OrPredicate<M1, M2>
+impl<M1, M2, Item> OrPredicate<M1, M2, Item>
 where
-    M1: Predicate,
-    M2: Predicate<Item = M1::Item>,
+    M1: Predicate<Item>,
+    M2: Predicate<Item>,
+    Item: ?Sized,
 {
     /// Create a new `OrPredicate` over predicates `a` and `b`.
-    pub fn new(a: M1, b: M2) -> OrPredicate<M1, M2> {
-        OrPredicate { a: a, b: b }
+    pub fn new(a: M1, b: M2) -> OrPredicate<M1, M2, Item> {
+        OrPredicate {
+            a: a,
+            b: b,
+            _phantom: PhantomData,
+        }
     }
 }
 
-impl<M1, M2> Predicate for OrPredicate<M1, M2>
+impl<M1, M2, Item> Predicate<Item> for OrPredicate<M1, M2, Item>
 where
-    M1: Predicate,
-    M2: Predicate<Item = M1::Item>,
+    M1: Predicate<Item>,
+    M2: Predicate<Item>,
+    Item: ?Sized,
 {
-    type Item = M1::Item;
-
-    fn eval(&self, item: &Self::Item) -> bool {
+    fn eval(&self, item: &Item) -> bool {
         self.a.eval(item) || self.b.eval(item)
     }
 }
@@ -86,30 +100,35 @@ where
 ///
 /// This is created by the `Predicate::not` function.
 #[derive(Debug)]
-pub struct NotPredicate<M>
+pub struct NotPredicate<M, Item>
 where
-    M: Predicate,
+    M: Predicate<Item>,
+    Item: ?Sized,
 {
     inner: M,
+    _phantom: PhantomData<Item>,
 }
 
-impl<M> NotPredicate<M>
+impl<M, Item> NotPredicate<M, Item>
 where
-    M: Predicate,
+    M: Predicate<Item>,
+    Item: ?Sized,
 {
     /// Create a new `NotPredicate` over predicate `inner`.
-    pub fn new(inner: M) -> NotPredicate<M> {
-        NotPredicate { inner: inner }
+    pub fn new(inner: M) -> NotPredicate<M, Item> {
+        NotPredicate {
+            inner: inner,
+            _phantom: PhantomData,
+        }
     }
 }
 
-impl<M> Predicate for NotPredicate<M>
+impl<M, Item> Predicate<Item> for NotPredicate<M, Item>
 where
-    M: Predicate,
+    M: Predicate<Item>,
+    Item: ?Sized,
 {
-    type Item = M::Item;
-
-    fn eval(&self, item: &Self::Item) -> bool {
+    fn eval(&self, item: &Item) -> bool {
         !self.inner.eval(item)
     }
 }

src/boxed.rs

@@ -15,35 +15,45 @@ use Predicate;
 
 /// `Predicate` that wraps another `Predicate` as a trait object, allowing
 /// sized storage of predicate types.
-pub struct BoxPredicate<T: ?Sized>(Box<Predicate<Item = T> + Send + Sync>);
+pub struct BoxPredicate<Item: ?Sized>(Box<Predicate<Item> + Send + Sync>);
 
-impl<T> fmt::Debug for BoxPredicate<T> {
+impl<Item> BoxPredicate<Item>
+where
+    Item: ?Sized,
+{
+    /// Creates a new `BoxPredicate`, a wrapper around a dynamically-dispatched
+    /// `Predicate` type with useful trait impls.
+    pub fn new<P: Predicate<Item>>(inner: P) -> BoxPredicate<Item>
+    where
+        P: Send + Sync + 'static,
+    {
+        BoxPredicate(Box::new(inner))
+    }
+}
+
+impl<Item> fmt::Debug for BoxPredicate<Item>
+where
+    Item: ?Sized,
+{
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         f.debug_struct("BoxPredicate").finish()
     }
 }
 
-impl<T> fmt::Display for BoxPredicate<T> {
+impl<Item> fmt::Display for BoxPredicate<Item>
+where
+    Item: ?Sized,
+{
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         write!(f, "BoxPredicate")
     }
 }
 
-impl<T: ?Sized> Predicate for BoxPredicate<T> {
-    type Item = T;
-
-    fn eval(&self, variable: &Self::Item) -> bool {
+impl<Item> Predicate<Item> for BoxPredicate<Item>
+where
+    Item: ?Sized,
+{
+    fn eval(&self, variable: &Item) -> bool {
         self.0.eval(variable)
     }
 }
-
-impl<T: ?Sized> BoxPredicate<T> {
-    /// Creates a new `BoxPredicate`, a wrapper around a dynamically-dispatched
-    /// `Predicate` type with useful trait impls.
-    pub fn new<P: Predicate<Item = T>>(inner: P) -> BoxPredicate<T>
-    where
-        P: Send + Sync + 'static,
-    {
-        BoxPredicate(Box::new(inner))
-    }
-}

src/constant.rs

@@ -16,15 +16,13 @@ use Predicate;
 ///
 /// This is created by the `predicate::always` and `predicate::never` functions.
 #[derive(Debug)]
-pub struct BooleanPredicate<T> {
+pub struct BooleanPredicate<Item> {
     retval: bool,
-    _phantom: PhantomData<T>,
+    _phantom: PhantomData<Item>,
 }
 
-impl<T> Predicate for BooleanPredicate<T> {
-    type Item = T;
-
-    fn eval(&self, _variable: &T) -> bool {
+impl<Item> Predicate<Item> for BooleanPredicate<Item> {
+    fn eval(&self, _variable: &Item) -> bool {
         self.retval
     }
 }
@@ -43,7 +41,7 @@ impl<T> Predicate for BooleanPredicate<T> {
 /// // Won't work - Predicates can only operate on a single type
 /// // assert_eq!(true, predicate_fn.eval("hello"))
 /// ```
-pub fn always<T>() -> BooleanPredicate<T> {
+pub fn always<Item>() -> BooleanPredicate<Item> {
     BooleanPredicate {
         retval: true,
         _phantom: PhantomData,
@@ -64,7 +62,7 @@ pub fn always<T>() -> BooleanPredicate<T> {
 /// // Won't work - Predicates can only operate on a single type
 /// // assert_eq!(false, predicate_fn.eval("hello"))
 /// ```
-pub fn never<T>() -> BooleanPredicate<T> {
+pub fn never<Item>() -> BooleanPredicate<Item> {
     BooleanPredicate {
         retval: false,
         _phantom: PhantomData,

src/core.rs

@@ -16,13 +16,10 @@ use boxed::BoxPredicate;
 /// mean that the evaluated item is in some sort of pre-defined set.  This is
 /// different from `Ord` and `Eq` in that an `item` will almost never be the
 /// same type as the implementing `Predicate` type.
-pub trait Predicate {
-    /// The type that this `Predicate` will accept for evaluating.
-    type Item: ?Sized;
-
+pub trait Predicate<Item: ?Sized> {
     /// Execute this `Predicate` against `variable`, returning the resulting
     /// boolean.
-    fn eval(&self, variable: &Self::Item) -> bool;
+    fn eval(&self, variable: &Item) -> bool;
 
     /// Compute the logical AND of two `Predicate` results, returning the result.
     ///
@@ -35,9 +32,9 @@ pub trait Predicate {
     /// let predicate_fn2 = predicate::always().and(predicate::never());
     /// assert_eq!(true, predicate_fn1.eval(&4));
     /// assert_eq!(false, predicate_fn2.eval(&4));
-    fn and<B>(self, other: B) -> AndPredicate<Self, B>
+    fn and<B>(self, other: B) -> AndPredicate<Self, B, Item>
     where
-        B: Predicate<Item = Self::Item>,
+        B: Predicate<Item>,
         Self: Sized,
     {
         AndPredicate::new(self, other)
@@ -56,9 +53,9 @@ pub trait Predicate {
     /// assert_eq!(true, predicate_fn1.eval(&4));
     /// assert_eq!(true, predicate_fn2.eval(&4));
     /// assert_eq!(false, predicate_fn3.eval(&4));
-    fn or<B>(self, other: B) -> OrPredicate<Self, B>
+    fn or<B>(self, other: B) -> OrPredicate<Self, B, Item>
     where
-        B: Predicate<Item = Self::Item>,
+        B: Predicate<Item>,
         Self: Sized,
     {
         OrPredicate::new(self, other)
@@ -75,7 +72,7 @@ pub trait Predicate {
     /// let predicate_fn2 = predicate::never().not();
     /// assert_eq!(false, predicate_fn1.eval(&4));
     /// assert_eq!(true, predicate_fn2.eval(&4));
-    fn not(self) -> NotPredicate<Self>
+    fn not(self) -> NotPredicate<Self, Item>
     where
         Self: Sized,
     {
@@ -106,7 +103,7 @@ pub trait Predicate {
     /// assert_eq!(true, predicates[0].eval(&4));
     /// assert_eq!(false, predicates[1].eval(&4));
     /// ```
-    fn boxed(self) -> BoxPredicate<Self::Item>
+    fn boxed(self) -> BoxPredicate<Item>
     where
         Self: Sized + Send + Sync + 'static,
     {

src/float/close.rs

@@ -78,9 +78,7 @@ impl IsClosePredicate {
     }
 }
 
-impl Predicate for IsClosePredicate {
-    type Item = f64;
-
+impl Predicate<f64> for IsClosePredicate {
     fn eval(&self, variable: &f64) -> bool {
         variable.approx_eq(&self.target, self.epsilon, self.ulps)
     }

src/function.rs

@@ -23,12 +23,10 @@ where
     _phantom: PhantomData<T>,
 }
 
-impl<F, T> Predicate for FnPredicate<F, T>
+impl<F, T> Predicate<T> for FnPredicate<F, T>
 where
     F: Fn(&T) -> bool,
 {
-    type Item = T;
-
     fn eval(&self, variable: &T) -> bool {
         (self.function)(variable)
     }

src/lib.rs

@@ -62,9 +62,8 @@
 //! // returns a `bool`. Implementing a custom `Predicate` still allows all the
 //! // usual combinators of the `Predicate` trait to work!
 //! struct IsTheAnswer;
-//! impl Predicate for IsTheAnswer {
-//!     type Item = i32;
-//!     fn eval(&self, variable: &Self::Item) -> bool {
+//! impl Predicate<i32> for IsTheAnswer {
+//!     fn eval(&self, variable: &i32) -> bool {
 //!         *variable == 42
 //!     }
 //! }

src/ord.rs

@@ -26,13 +26,11 @@ pub struct EqPredicate<T> {
     op: EqOps,
 }
 
-impl<T> Predicate for EqPredicate<T>
+impl<T> Predicate<T> for EqPredicate<T>
 where
     T: PartialEq,
 {
-    type Item = T;
-
-    fn eval(&self, variable: &Self::Item) -> bool {
+    fn eval(&self, variable: &T) -> bool {
         match self.op {
             EqOps::Equal => variable.eq(&self.constant),
             EqOps::NotEqual => variable.ne(&self.constant),
@@ -102,13 +100,11 @@ pub struct OrdPredicate<T> {
     op: OrdOps,
 }
 
-impl<T> Predicate for OrdPredicate<T>
+impl<T> Predicate<T> for OrdPredicate<T>
 where
     T: PartialOrd,
 {
-    type Item = T;
-
-    fn eval(&self, variable: &Self::Item) -> bool {
+    fn eval(&self, variable: &T) -> bool {
         match self.op {
             OrdOps::LessThan => variable.lt(&self.constant),
             OrdOps::LessThanOrEqual => variable.le(&self.constant),