diff --git a/src/lenses/first.rs b/src/lenses/first.rs
index b029304..a89fc74 100644
--- a/src/lenses/first.rs
+++ b/src/lenses/first.rs
@@ -12,10 +12,13 @@ macro_rules! make_tuples {
             }
         }
         impl< $($t,)* > LensOver<( $($t,)* )> for _0 {
-            fn over(
+            fn over<F>(
                 mut tup: ($($t,)*),
-                f: &dyn Fn(Self::Field) -> Self::Field
-            ) -> ( $($t,)* ) {
+                f: F
+            ) -> ( $($t,)* )
+            where
+              F: FnOnce(Self::Field) -> Self::Field
+            {
                 tup.0 = f(tup.0);
                 tup
             }
diff --git a/src/lenses/id.rs b/src/lenses/identity.rs
similarity index 71%
rename from src/lenses/id.rs
rename to src/lenses/identity.rs
index c8ef6e1..d90c02f 100644
--- a/src/lenses/id.rs
+++ b/src/lenses/identity.rs
@@ -11,7 +11,10 @@ impl<T> LensView<T> for T {
     }
 }
 impl<T> LensOver<T> for T {
-    fn over(thing: T, f: &dyn Fn(Self::Field) -> Self::Field) -> T {
+    fn over<F>(thing: T, f: F) -> T
+    where
+        F: FnOnce(Self::Field) -> Self::Field,
+    {
         f(thing)
     }
 }
diff --git a/src/lenses/mod.rs b/src/lenses/mod.rs
index 509ae4a..8b1989a 100644
--- a/src/lenses/mod.rs
+++ b/src/lenses/mod.rs
@@ -1,4 +1,4 @@
-mod id;
-pub use id::id;
+mod identity;
+pub use identity::id;
 mod first;
 pub use first::_0;
diff --git a/src/lib.rs b/src/lib.rs
index 92cb510..5e8faa1 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -5,10 +5,12 @@ pub trait LensView<T> {
 }
 
 pub trait LensOver<T>: LensView<T> {
-    fn over(thing: T, f: &dyn Fn(Self::Field) -> Self::Field) -> T;
+    fn over<F>(thing: T, f: F) -> T
+    where
+        F: FnOnce(Self::Field) -> Self::Field;
 
     fn set(thing: T, v: Self::Field) -> T {
-        Self::over(thing, &|_| v)
+        Self::over(thing, |_| v)
     }
 }
 
@@ -18,7 +20,7 @@ pub fn view<T, L: LensView<T>>(_lens: L, thing: T) -> L::Field {
 pub fn set<T, L: LensOver<T>>(_lens: L, thing: T, v: L::Field) -> T {
     L::set(thing, v)
 }
-pub fn over<T, L: LensOver<T>>(_lens: L, thing: T, f: &dyn Fn(L::Field) -> L::Field) -> T {
+pub fn over<T, L: LensOver<T>>(_lens: L, thing: T, f: impl FnOnce(L::Field) -> L::Field) -> T {
     L::over(thing, f)
 }