fix missing imports and wrong type declaration

src/Classes/Physics/Collidible.hs

@@ -2,6 +2,10 @@ module Classes.Physics.Collidible where
 
 import Linear
 
+-- internal imports
+
+import Classes.Physics.Mass
+
 -- | Typeclass for implementing collision results on objects.
 class Mass c => Collidible c where
 
@@ -14,6 +18,6 @@ class Mass c => Collidible c where
        )
   -- | This Function is called for every collision for both colliding objects.
   collide
-    -> c -- ^ Original object
+    :: c -- ^ Original object
     -> c -- ^ Collision partner
     -> c -- ^ Updated original object

src/Classes/Physics/Mass.hs

@@ -1,5 +1,7 @@
 module Classes.Physics.Mass where
 
+import Linear
+
 -- | This typeclass is a centerpiece of the physics implementation and is used
 -- to implement the basic mass properties of a (very simplified) physical body.
 class Mass m where