feat(str): Trimming predicate decorator

Part of #25

Author: epage

src/prelude.rs

@@ -11,6 +11,7 @@
 pub use core::Predicate;
 pub use boolean::PredicateBooleanExt;
 pub use boxed::PredicateBoxExt;
+pub use str::PredicateStrExt;
 
 /// Predicate factories
 pub mod predicate {

src/str/adapters.rs

@@ -0,0 +1,49 @@
+use Predicate;
+
+/// Predicate adaper that trims the variable being tested.
+///
+/// This is created by `pred.trim()`.
+#[derive(Copy, Clone, Debug)]
+pub struct TrimPedicate<P>
+where
+    P: Predicate<str>,
+{
+    p: P,
+}
+
+impl<P> Predicate<str> for TrimPedicate<P>
+where
+    P: Predicate<str>,
+{
+    fn eval(&self, variable: &str) -> bool {
+        self.p.eval(variable.trim())
+    }
+}
+
+/// `Predicate` extension adapting a `str` Predicate.
+pub trait PredicateStrExt
+where
+    Self: Predicate<str>,
+    Self: Sized,
+{
+    /// Returns a `TrimPedicate` that ensures the data passed to `Self` is trimmed.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use predicates::prelude::*;
+    ///
+    /// let predicate_fn = predicate::str::is_empty().trim();
+    /// assert_eq!(true, predicate_fn.eval("    "));
+    /// assert_eq!(false, predicate_fn.eval("    Hello    "));
+    /// ```
+    fn trim(self) -> TrimPedicate<Self> {
+        TrimPedicate { p: self }
+    }
+}
+
+impl<P> PredicateStrExt for P
+where
+    P: Predicate<str>,
+{
+}

src/str/mod.rs

@@ -12,6 +12,8 @@
 
 mod basics;
 pub use self::basics::*;
+mod adapters;
+pub use self::adapters::*;
 
 #[cfg(feature = "difference")]
 mod difference;