Add URL value type class.

Author: alexmingoia

src/CSS/Common.purs

@@ -28,6 +28,8 @@ class Hidden   a where hidden   :: a
 class Initial  a where initial  :: a
 class Unset    a where unset    :: a
 
+class URL      a where url      :: String -> a
+
 -- | The other type class is used to escape from the type safety introduced by
 -- embedding CSS properties into the typed world of purescript-css.
 -- `Other` allows you to cast any `Value` to a specific value type.
@@ -47,6 +49,8 @@ instance otherValue    :: Other    Value where other    = id
 instance initialValue  :: Initial  Value where initial  = fromString "initial"
 instance unsetValue    :: Unset    Value where unset    = fromString "unset"
 
+instance urlValue :: URL Value where url s = fromString ("url(\"" <> s <> "\")")
+
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 -- | Common list browser prefixes to make experimental properties work in