Auto merge of #11924 - emilio:interpolate-cleanup, r=nox,Ms2ger

style: Cleanup `Interpolate` code.

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- [x] These changes do not require tests because refactoring.

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components/style/properties/helpers/animated_properties.mako.rs

@@ -25,9 +25,8 @@ use properties::longhands::vertical_align::computed_value::T as VerticalAlign;
 use properties::longhands::visibility::computed_value::T as Visibility;
 use properties::longhands::z_index::computed_value::T as ZIndex;
 use properties::style_struct_traits::*;
-use std::cmp::{self, Ordering};
+use std::cmp;
 use std::fmt;
-use std::iter::repeat;
 use super::ComputedValues;
 use values::computed::{Angle, LengthOrPercentageOrAuto, LengthOrPercentageOrNone};
 use values::computed::{BorderRadiusSize, LengthOrNone};
@@ -119,7 +118,7 @@ impl AnimatedProperty {
             % for prop in data.longhands:
                 % if prop.animatable:
                     AnimatedProperty::${prop.camel_case}(ref from, ref to) => {
-                        if let Some(value) = from.interpolate(to, progress) {
+                        if let Ok(value) = from.interpolate(to, progress) {
                             style.mutate_${prop.style_struct.ident.strip("_")}().set_${prop.ident}(value);
                         }
                     }
@@ -155,27 +154,25 @@ impl AnimatedProperty {
 ///
 /// [interpolated-types]: https://drafts.csswg.org/css-transitions/#interpolated-types
 pub trait Interpolate: Sized {
-    fn interpolate(&self, other: &Self, time: f64) -> Option<Self>;
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()>;
 }
 
 /// https://drafts.csswg.org/css-transitions/#animtype-number
 impl Interpolate for Au {
     #[inline]
-    fn interpolate(&self, other: &Au, time: f64) -> Option<Au> {
-        Some(Au((self.0 as f64 + (other.0 as f64 - self.0 as f64) * time).round() as i32))
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
+        Ok(Au((self.0 as f64 + (other.0 as f64 - self.0 as f64) * time).round() as i32))
     }
 }
 
 impl <T> Interpolate for Option<T> where T: Interpolate {
     #[inline]
-    fn interpolate(&self, other: &Option<T>, time: f64) -> Option<Option<T>> {
+    fn interpolate(&self, other: &Option<T>, time: f64) -> Result<Option<T>, ()> {
         match (self, other) {
             (&Some(ref this), &Some(ref other)) => {
-                this.interpolate(other, time).and_then(|value| {
-                    Some(Some(value))
-                })
+                Ok(this.interpolate(other, time).ok())
             }
-            (_, _) => None
+            _ => Err(()),
         }
     }
 }
@@ -183,33 +180,33 @@ impl <T> Interpolate for Option<T> where T: Interpolate {
 /// https://drafts.csswg.org/css-transitions/#animtype-number
 impl Interpolate for f32 {
     #[inline]
-    fn interpolate(&self, other: &f32, time: f64) -> Option<f32> {
-        Some(((*self as f64) + ((*other as f64) - (*self as f64)) * time) as f32)
+    fn interpolate(&self, other: &f32, time: f64) -> Result<Self, ()> {
+        Ok(((*self as f64) + ((*other as f64) - (*self as f64)) * time) as f32)
     }
 }
 
 /// https://drafts.csswg.org/css-transitions/#animtype-number
 impl Interpolate for f64 {
     #[inline]
-    fn interpolate(&self, other: &f64, time: f64) -> Option<f64> {
-        Some(*self + (*other - *self) * time)
+    fn interpolate(&self, other: &f64, time: f64) -> Result<Self, ()> {
+        Ok(*self + (*other - *self) * time)
     }
 }
 
 /// https://drafts.csswg.org/css-transitions/#animtype-number
 impl Interpolate for i32 {
     #[inline]
-    fn interpolate(&self, other: &i32, time: f64) -> Option<i32> {
+    fn interpolate(&self, other: &i32, time: f64) -> Result<Self, ()> {
         let a = *self as f64;
         let b = *other as f64;
-        Some((a + (b - a) * time).round() as i32)
+        Ok((a + (b - a) * time).round() as i32)
     }
 }
 
 /// https://drafts.csswg.org/css-transitions/#animtype-number
 impl Interpolate for Angle {
     #[inline]
-    fn interpolate(&self, other: &Angle, time: f64) -> Option<Angle> {
+    fn interpolate(&self, other: &Angle, time: f64) -> Result<Self, ()> {
         self.radians().interpolate(&other.radians(), time).map(Angle)
     }
 }
@@ -217,19 +214,18 @@ impl Interpolate for Angle {
 /// https://drafts.csswg.org/css-transitions/#animtype-visibility
 impl Interpolate for Visibility {
     #[inline]
-    fn interpolate(&self, other: &Visibility, time: f64)
-                   -> Option<Visibility> {
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
         match (*self, *other) {
             (Visibility::visible, _) | (_, Visibility::visible) => {
-                if time >= 0.0 && time <= 1.0 {
-                    Some(Visibility::visible)
+                Ok(if time >= 0.0 && time <= 1.0 {
+                    Visibility::visible
                 } else if time < 0.0 {
-                    Some(*self)
+                    *self
                 } else {
-                    Some(*other)
-                }
+                    *other
+                })
             }
-            (_, _) => None,
+            _ => Err(()),
         }
     }
 }
@@ -237,56 +233,40 @@ impl Interpolate for Visibility {
 /// https://drafts.csswg.org/css-transitions/#animtype-integer
 impl Interpolate for ZIndex {
     #[inline]
-    fn interpolate(&self, other: &ZIndex, time: f64)
-                   -> Option<ZIndex> {
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
         match (*self, *other) {
             (ZIndex::Number(ref this),
              ZIndex::Number(ref other)) => {
-                this.interpolate(other, time).and_then(|value| {
-                    Some(ZIndex::Number(value))
-                })
+                this.interpolate(other, time).map(ZIndex::Number)
             }
-            (_, _) => None,
+            _ => Err(()),
         }
     }
 }
 
 impl<T: Interpolate + Clone> Interpolate for Size2D<T> {
     #[inline]
-    fn interpolate(&self, other: &Self, time: f64) -> Option<Self> {
-        let width = match self.width.interpolate(&other.width, time) {
-            Some(width) => width,
-            None => return None,
-        };
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
+        let width = try!(self.width.interpolate(&other.width, time));
+        let height = try!(self.height.interpolate(&other.height, time));
 
-        let height = match self.height.interpolate(&other.height, time) {
-            Some(height) => height,
-            None => return None,
-        };
-        Some(Size2D::new(width, height))
+        Ok(Size2D::new(width, height))
     }
 }
 
 impl<T: Interpolate + Clone> Interpolate for Point2D<T> {
     #[inline]
-    fn interpolate(&self, other: &Self, time: f64) -> Option<Self> {
-        let x = match self.x.interpolate(&other.x, time) {
-            Some(x) => x,
-            None => return None,
-        };
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
+        let x = try!(self.x.interpolate(&other.x, time));
+        let y = try!(self.y.interpolate(&other.y, time));
 
-        let y = match self.y.interpolate(&other.y, time) {
-            Some(y) => y,
-            None => return None,
-        };
-
-        Some(Point2D::new(x, y))
+        Ok(Point2D::new(x, y))
     }
 }
 
 impl Interpolate for BorderRadiusSize {
     #[inline]
-    fn interpolate(&self, other: &Self, time: f64) -> Option<Self> {
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
         self.0.interpolate(&other.0, time).map(BorderRadiusSize)
     }
 }
@@ -294,16 +274,15 @@ impl Interpolate for BorderRadiusSize {
 /// https://drafts.csswg.org/css-transitions/#animtype-length
 impl Interpolate for VerticalAlign {
     #[inline]
-    fn interpolate(&self, other: &VerticalAlign, time: f64)
-                   -> Option<VerticalAlign> {
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
         match (*self, *other) {
             (VerticalAlign::LengthOrPercentage(LengthOrPercentage::Length(ref this)),
              VerticalAlign::LengthOrPercentage(LengthOrPercentage::Length(ref other))) => {
-                this.interpolate(other, time).and_then(|value| {
-                    Some(VerticalAlign::LengthOrPercentage(LengthOrPercentage::Length(value)))
+                this.interpolate(other, time).map(|value| {
+                    VerticalAlign::LengthOrPercentage(LengthOrPercentage::Length(value))
                 })
             }
-            (_, _) => None,
+            _ => Err(()),
         }
     }
 }
@@ -311,12 +290,10 @@ impl Interpolate for VerticalAlign {
 /// https://drafts.csswg.org/css-transitions/#animtype-simple-list
 impl Interpolate for BorderSpacing {
     #[inline]
-    fn interpolate(&self, other: &BorderSpacing, time: f64)
-                   -> Option<BorderSpacing> {
-        self.horizontal.interpolate(&other.horizontal, time).and_then(|horizontal| {
-            self.vertical.interpolate(&other.vertical, time).and_then(|vertical| {
-                Some(BorderSpacing { horizontal: horizontal, vertical: vertical })
-            })
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
+        Ok(BorderSpacing {
+            horizontal: try!(self.horizontal.interpolate(&other.horizontal, time)),
+            vertical: try!(self.vertical.interpolate(&other.vertical, time)),
         })
     }
 }
@@ -324,30 +301,25 @@ impl Interpolate for BorderSpacing {
 /// https://drafts.csswg.org/css-transitions/#animtype-color
 impl Interpolate for RGBA {
     #[inline]
-    fn interpolate(&self, other: &RGBA, time: f64) -> Option<RGBA> {
-        match (self.red.interpolate(&other.red, time),
-               self.green.interpolate(&other.green, time),
-               self.blue.interpolate(&other.blue, time),
-               self.alpha.interpolate(&other.alpha, time)) {
-            (Some(red), Some(green), Some(blue), Some(alpha)) => {
-                Some(RGBA { red: red, green: green, blue: blue, alpha: alpha })
-            }
-            (_, _, _, _) => None
-        }
+    fn interpolate(&self, other: &RGBA, time: f64) -> Result<Self, ()> {
+        Ok(RGBA {
+            red: try!(self.red.interpolate(&other.red, time)),
+            green: try!(self.green.interpolate(&other.green, time)),
+            blue: try!(self.blue.interpolate(&other.blue, time)),
+            alpha: try!(self.alpha.interpolate(&other.alpha, time)),
+        })
     }
 }
 
 /// https://drafts.csswg.org/css-transitions/#animtype-color
 impl Interpolate for CSSParserColor {
     #[inline]
-    fn interpolate(&self, other: &Self, time: f64) -> Option<Self> {
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
         match (*self, *other) {
             (CSSParserColor::RGBA(ref this), CSSParserColor::RGBA(ref other)) => {
-                this.interpolate(other, time).and_then(|value| {
-                    Some(CSSParserColor::RGBA(value))
-                })
+                this.interpolate(other, time).map(CSSParserColor::RGBA)
             }
-            (_, _) => None,
+            _ => Err(()),
         }
     }
 }
@@ -355,11 +327,10 @@ impl Interpolate for CSSParserColor {
 /// https://drafts.csswg.org/css-transitions/#animtype-lpcalc
 impl Interpolate for CalcLengthOrPercentage {
     #[inline]
-    fn interpolate(&self, other: &CalcLengthOrPercentage, time: f64)
-                   -> Option<CalcLengthOrPercentage> {
-        Some(CalcLengthOrPercentage {
-            length: self.length().interpolate(&other.length(), time),
-            percentage: self.percentage().interpolate(&other.percentage(), time),
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
+        Ok(CalcLengthOrPercentage {
+            length: try!(self.length.interpolate(&other.length, time)),
+            percentage: try!(self.percentage.interpolate(&other.percentage, time)),
         })
     }
 }
@@ -367,27 +338,21 @@ impl Interpolate for CalcLengthOrPercentage {
 /// https://drafts.csswg.org/css-transitions/#animtype-lpcalc
 impl Interpolate for LengthOrPercentage {
     #[inline]
-    fn interpolate(&self, other: &LengthOrPercentage, time: f64)
-                   -> Option<LengthOrPercentage> {
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
         match (*self, *other) {
             (LengthOrPercentage::Length(ref this),
              LengthOrPercentage::Length(ref other)) => {
-                this.interpolate(other, time).and_then(|value| {
-                    Some(LengthOrPercentage::Length(value))
-                })
+                this.interpolate(other, time).map(LengthOrPercentage::Length)
             }
             (LengthOrPercentage::Percentage(ref this),
              LengthOrPercentage::Percentage(ref other)) => {
-                this.interpolate(other, time).and_then(|value| {
-                    Some(LengthOrPercentage::Percentage(value))
-                })
+                this.interpolate(other, time).map(LengthOrPercentage::Percentage)
             }
             (this, other) => {
                 let this: CalcLengthOrPercentage = From::from(this);
                 let other: CalcLengthOrPercentage = From::from(other);
-                this.interpolate(&other, time).and_then(|value| {
-                    Some(LengthOrPercentage::Calc(value))
-                })
+                this.interpolate(&other, time)
+                    .map(LengthOrPercentage::Calc)
             }
         }
     }
@@ -396,30 +361,26 @@ impl Interpolate for LengthOrPercentage {
 /// https://drafts.csswg.org/css-transitions/#animtype-lpcalc
 impl Interpolate for LengthOrPercentageOrAuto {
     #[inline]
-    fn interpolate(&self, other: &LengthOrPercentageOrAuto, time: f64)
-                   -> Option<LengthOrPercentageOrAuto> {
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
         match (*self, *other) {
             (LengthOrPercentageOrAuto::Length(ref this),
              LengthOrPercentageOrAuto::Length(ref other)) => {
-                this.interpolate(other, time).and_then(|value| {
-                    Some(LengthOrPercentageOrAuto::Length(value))
-                })
+                this.interpolate(other, time).map(LengthOrPercentageOrAuto::Length)
             }
             (LengthOrPercentageOrAuto::Percentage(ref this),
              LengthOrPercentageOrAuto::Percentage(ref other)) => {
-                this.interpolate(other, time).and_then(|value| {
-                    Some(LengthOrPercentageOrAuto::Percentage(value))
-                })
+                this.interpolate(other, time).map(LengthOrPercentageOrAuto::Percentage)
             }
             (LengthOrPercentageOrAuto::Auto, LengthOrPercentageOrAuto::Auto) => {
-                Some(LengthOrPercentageOrAuto::Auto)
+                Ok(LengthOrPercentageOrAuto::Auto)
             }
             (this, other) => {
                 let this: Option<CalcLengthOrPercentage> = From::from(this);
                 let other: Option<CalcLengthOrPercentage> = From::from(other);
-                this.interpolate(&other, time).unwrap_or(None).and_then(|value| {
-                    Some(LengthOrPercentageOrAuto::Calc(value))
-                })
+                match this.interpolate(&other, time) {
+                    Ok(Some(result)) => Ok(LengthOrPercentageOrAuto::Calc(result)),
+                    _ => Err(()),
+                }
             }
         }
     }
@@ -428,25 +389,20 @@ impl Interpolate for LengthOrPercentageOrAuto {
 /// https://drafts.csswg.org/css-transitions/#animtype-lpcalc
 impl Interpolate for LengthOrPercentageOrNone {
     #[inline]
-    fn interpolate(&self, other: &LengthOrPercentageOrNone, time: f64)
-                   -> Option<LengthOrPercentageOrNone> {
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
         match (*self, *other) {
             (LengthOrPercentageOrNone::Length(ref this),
              LengthOrPercentageOrNone::Length(ref other)) => {
-                this.interpolate(other, time).and_then(|value| {
-                    Some(LengthOrPercentageOrNone::Length(value))
-                })
+                this.interpolate(other, time).map(LengthOrPercentageOrNone::Length)
             }
             (LengthOrPercentageOrNone::Percentage(ref this),
              LengthOrPercentageOrNone::Percentage(ref other)) => {
-                this.interpolate(other, time).and_then(|value| {
-                    Some(LengthOrPercentageOrNone::Percentage(value))
-                })
+                this.interpolate(other, time).map(LengthOrPercentageOrNone::Percentage)
             }
             (LengthOrPercentageOrNone::None, LengthOrPercentageOrNone::None) => {
-                Some(LengthOrPercentageOrNone::None)
+                Ok(LengthOrPercentageOrNone::None)
             }
-            (_, _) => None,
+            _ => Err(())
         }
     }
 }
@@ -455,25 +411,20 @@ impl Interpolate for LengthOrPercentageOrNone {
 /// https://drafts.csswg.org/css-transitions/#animtype-length
 impl Interpolate for LineHeight {
     #[inline]
-    fn interpolate(&self, other: &LineHeight, time: f64)
-                   -> Option<LineHeight> {
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
         match (*self, *other) {
             (LineHeight::Length(ref this),
              LineHeight::Length(ref other)) => {
-                this.interpolate(other, time).and_then(|value| {
-                    Some(LineHeight::Length(value))
-                })
+                this.interpolate(other, time).map(LineHeight::Length)
             }
             (LineHeight::Number(ref this),
              LineHeight::Number(ref other)) => {
-                this.interpolate(other, time).and_then(|value| {
-                    Some(LineHeight::Number(value))
-                })
+                this.interpolate(other, time).map(LineHeight::Number)
             }
             (LineHeight::Normal, LineHeight::Normal) => {
-                Some(LineHeight::Normal)
+                Ok(LineHeight::Normal)
             }
-            (_, _) => None,
+            _ => Err(()),
         }
     }
 }
@@ -481,12 +432,11 @@ impl Interpolate for LineHeight {
 /// http://dev.w3.org/csswg/css-transitions/#animtype-font-weight
 impl Interpolate for FontWeight {
     #[inline]
-    fn interpolate(&self, other: &FontWeight, time: f64)
-                   -> Option<FontWeight> {
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
         let a = (*self as u32) as f64;
         let b = (*other as u32) as f64;
         let weight = a + (b - a) * time;
-        Some(if weight < 150. {
+        Ok(if weight < 150. {
             FontWeight::Weight100
         } else if weight < 250. {
             FontWeight::Weight200
@@ -511,48 +461,38 @@ impl Interpolate for FontWeight {
 /// https://drafts.csswg.org/css-transitions/#animtype-rect
 impl Interpolate for ClipRect {
     #[inline]
-    fn interpolate(&self, other: &ClipRect, time: f64)
-                   -> Option<ClipRect> {
-        match (self.top.interpolate(&other.top, time),
-               self.right.interpolate(&other.right, time),
-               self.bottom.interpolate(&other.bottom, time),
-               self.left.interpolate(&other.left, time)) {
-            (Some(top), Some(right), Some(bottom), Some(left)) => {
-                Some(ClipRect { top: top, right: right, bottom: bottom, left: left })
-            },
-            (_, _, _, _) => None,
-        }
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
+        Ok(ClipRect {
+            top: try!(self.top.interpolate(&other.top, time)),
+            right: try!(self.right.interpolate(&other.right, time)),
+            bottom: try!(self.bottom.interpolate(&other.bottom, time)),
+            left: try!(self.left.interpolate(&other.left, time)),
+        })
     }
 }
 
 /// https://drafts.csswg.org/css-transitions/#animtype-simple-list
 impl Interpolate for BackgroundPosition {
     #[inline]
-    fn interpolate(&self, other: &BackgroundPosition, time: f64)
-                   -> Option<BackgroundPosition> {
-        match (self.horizontal.interpolate(&other.horizontal, time),
-               self.vertical.interpolate(&other.vertical, time)) {
-            (Some(horizontal), Some(vertical)) => {
-                Some(BackgroundPosition { horizontal: horizontal, vertical: vertical })
-            },
-            (_, _) => None,
-        }
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
+        Ok(BackgroundPosition {
+            horizontal: try!(self.horizontal.interpolate(&other.horizontal, time)),
+            vertical: try!(self.vertical.interpolate(&other.vertical, time)),
+        })
     }
 }
 
 impl Interpolate for BackgroundSize {
-    fn interpolate(&self, other: &Self, time: f64) -> Option<Self> {
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
         use properties::longhands::background_size::computed_value::ExplicitSize;
         match (self, other) {
-            (&BackgroundSize::Explicit(ref me), &BackgroundSize::Explicit(ref other))
-                => match (me.width.interpolate(&other.width, time),
-                          me.height.interpolate(&other.height, time)) {
-                       (Some(width), Some(height))
-                           => Some(BackgroundSize::Explicit(
-                               ExplicitSize { width: width, height: height })),
-                        _ => None,
-                   },
-            _ => None
+            (&BackgroundSize::Explicit(ref me), &BackgroundSize::Explicit(ref other)) => {
+                Ok(BackgroundSize::Explicit(ExplicitSize {
+                    width: try!(me.width.interpolate(&other.width, time)),
+                    height: try!(me.height.interpolate(&other.height, time)),
+                }))
+            }
+            _ => Err(()),
         }
     }
 }
@@ -560,25 +500,20 @@ impl Interpolate for BackgroundSize {
 /// https://drafts.csswg.org/css-transitions/#animtype-shadow-list
 impl Interpolate for TextShadow {
     #[inline]
-    fn interpolate(&self, other: &TextShadow, time: f64)
-                   -> Option<TextShadow> {
-        match (self.offset_x.interpolate(&other.offset_x, time),
-               self.offset_y.interpolate(&other.offset_y, time),
-               self.blur_radius.interpolate(&other.blur_radius, time),
-               self.color.interpolate(&other.color, time)) {
-            (Some(offset_x), Some(offset_y), Some(blur_radius), Some(color)) => {
-                Some(TextShadow { offset_x: offset_x, offset_y: offset_y, blur_radius: blur_radius, color: color })
-            },
-            _ => None,
-        }
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
+        Ok(TextShadow {
+            offset_x: try!(self.offset_x.interpolate(&other.offset_x, time)),
+            offset_y: try!(self.offset_y.interpolate(&other.offset_y, time)),
+            blur_radius: try!(self.blur_radius.interpolate(&other.blur_radius, time)),
+            color: try!(self.color.interpolate(&other.color, time)),
+        })
     }
 }
 
 /// https://drafts.csswg.org/css-transitions/#animtype-shadow-list
 impl Interpolate for TextShadowList {
     #[inline]
-    fn interpolate(&self, other: &TextShadowList, time: f64)
-                   -> Option<TextShadowList> {
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
         let zero = TextShadow {
             offset_x: Au(0),
             offset_y: Au(0),
@@ -588,14 +523,25 @@ impl Interpolate for TextShadowList {
             })
         };
 
-        let interpolate_each = |(a, b): (&TextShadow, &TextShadow)| {
-            a.interpolate(b, time).unwrap()
-        };
+        let max_len = cmp::max(self.0.len(), other.0.len());
+        let mut result = Vec::with_capacity(max_len);
 
-        Some(TextShadowList(match self.0.len().cmp(&other.0.len()) {
-            Ordering::Less => other.0.iter().chain(repeat(&zero)).zip(other.0.iter()).map(interpolate_each).collect(),
-            _ => self.0.iter().zip(other.0.iter().chain(repeat(&zero))).map(interpolate_each).collect(),
-        }))
+        for i in 0..max_len {
+            let shadow = match (self.0.get(i), other.0.get(i)) {
+                (Some(shadow), Some(other))
+                    => try!(shadow.interpolate(other, time)),
+                (Some(shadow), None) => {
+                    shadow.interpolate(&zero, time).unwrap()
+                }
+                (None, Some(shadow)) => {
+                    zero.interpolate(&shadow, time).unwrap()
+                }
+                (None, None) => unreachable!(),
+            };
+            result.push(shadow);
+        }
+
+        Ok(TextShadowList(result))
     }
 }
 
@@ -725,8 +671,7 @@ fn build_identity_transform_list(list: &[TransformOperation]) -> Vec<TransformOp
 
 impl Interpolate for BoxShadowList {
     #[inline]
-    fn interpolate(&self, other: &Self, time: f64)
-                   -> Option<Self> {
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
         // The inset value must change
         let mut zero = BoxShadow {
             offset_x: Au(0),
@@ -744,12 +689,8 @@ impl Interpolate for BoxShadowList {
 
         for i in 0..max_len {
             let shadow = match (self.0.get(i), other.0.get(i)) {
-                (Some(shadow), Some(other)) => {
-                    match shadow.interpolate(other, time) {
-                        Some(shadow) => shadow,
-                        None => return None,
-                    }
-                }
+                (Some(shadow), Some(other))
+                    => try!(shadow.interpolate(other, time)),
                 (Some(shadow), None) => {
                     zero.inset = shadow.inset;
                     shadow.interpolate(&zero, time).unwrap()
@@ -763,45 +704,25 @@ impl Interpolate for BoxShadowList {
             result.push(shadow);
         }
 
-        Some(BoxShadowList(result))
+        Ok(BoxShadowList(result))
     }
 }
 
 /// https://drafts.csswg.org/css-transitions/#animtype-shadow-list
 impl Interpolate for BoxShadow {
     #[inline]
-    fn interpolate(&self, other: &Self, time: f64)
-                  -> Option<Self> {
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
         if self.inset != other.inset {
-            return None;
+            return Err(());
         }
 
-        let x = match self.offset_x.interpolate(&other.offset_x, time) {
-            Some(x) => x,
-            None => return None,
-        };
+        let x = try!(self.offset_x.interpolate(&other.offset_x, time));
+        let y = try!(self.offset_y.interpolate(&other.offset_y, time));
+        let color = try!(self.color.interpolate(&other.color, time));
+        let spread = try!(self.spread_radius.interpolate(&other.spread_radius, time));
+        let blur = try!(self.blur_radius.interpolate(&other.blur_radius, time));
 
-        let y = match self.offset_y.interpolate(&other.offset_y, time) {
-            Some(y) => y,
-            None => return None,
-        };
-
-        let color = match self.color.interpolate(&other.color, time) {
-            Some(c) => c,
-            None => return None,
-        };
-
-        let spread = match self.spread_radius.interpolate(&other.spread_radius, time) {
-            Some(s) => s,
-            None => return None,
-        };
-
-        let blur = match self.blur_radius.interpolate(&other.blur_radius, time) {
-            Some(r) => r,
-            None => return None,
-        };
-
-        Some(BoxShadow {
+        Ok(BoxShadow {
             offset_x: x,
             offset_y: y,
             blur_radius: blur,
@@ -813,36 +734,21 @@ impl Interpolate for BoxShadow {
 }
 
 impl Interpolate for LengthOrNone {
-    fn interpolate(&self, other: &Self, time: f64) -> Option<Self> {
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
         match (*self, *other) {
             (LengthOrNone::Length(ref len), LengthOrNone::Length(ref other)) =>
                 len.interpolate(&other, time).map(LengthOrNone::Length),
-            _ => None,
+            _ => Err(()),
         }
     }
 }
 
 impl Interpolate for TransformOrigin {
-    fn interpolate(&self, other: &Self, time: f64) -> Option<Self> {
-        let horizontal = match self.horizontal.interpolate(&other.horizontal, time) {
-            Some(h) => h,
-            None => return None,
-        };
-
-        let vertical = match self.vertical.interpolate(&other.vertical, time) {
-            Some(v) => v,
-            None => return None,
-        };
-
-        let depth = match self.depth.interpolate(&other.depth, time) {
-            Some(d) => d,
-            None => return None,
-        };
-
-        Some(TransformOrigin {
-            horizontal: horizontal,
-            vertical: vertical,
-            depth: depth,
+    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
+        Ok(TransformOrigin {
+            horizontal: try!(self.horizontal.interpolate(&other.horizontal, time)),
+            vertical: try!(self.vertical.interpolate(&other.vertical, time)),
+            depth: try!(self.depth.interpolate(&other.depth, time)),
         })
     }
 }
@@ -850,7 +756,7 @@ impl Interpolate for TransformOrigin {
 /// https://drafts.csswg.org/css-transforms/#interpolation-of-transforms
 impl Interpolate for TransformList {
     #[inline]
-    fn interpolate(&self, other: &TransformList, time: f64) -> Option<TransformList> {
+    fn interpolate(&self, other: &TransformList, time: f64) -> Result<Self, ()> {
         // http://dev.w3.org/csswg/css-transforms/#interpolation-of-transforms
         let result = match (&self.0, &other.0) {
             (&Some(ref from_list), &Some(ref to_list)) => {
@@ -873,6 +779,6 @@ impl Interpolate for TransformList {
             }
         };
 
-        Some(result)
+        Ok(result)
     }
 }